perl(1) DG/UX R4.11 perl(1)
NAME
perl, taintperl - Practical Extraction and Report Language
SYNOPSIS
perl [options] filename args
taintperl [options] filename args
DESCRIPTION
Perl is an interpreted language optimized for scanning arbitrary text
files, extracting information from those text files, and printing
reports based on that information. It's also a good language for
many system management tasks. The language is intended to be
practical (easy to use, efficient, complete) rather than beautiful
(tiny, elegant, minimal). It combines (in the author's opinion,
anyway) some of the best features of C, sed, awk, and sh, so people
familiar with those languages should have little difficulty with it.
(Language historians will also note some vestiges of csh, Pascal, and
even BASIC-PLUS.) Expression syntax corresponds quite closely to C
expression syntax. Unlike most Unix utilities, perl does not
arbitrarily limit the size of your data--if you've got the memory,
perl can slurp in your whole file as a single string. Recursion is
of unlimited depth. And the hash tables used by associative arrays
grow as necessary to prevent degraded performance. Perl uses
sophisticated pattern matching techniques to scan large amounts of
data very quickly. Although optimized for scanning text, perl can
also deal with binary data, and can make dbm files look like
associative arrays (where dbm is available). Setuid perl scripts are
safer than C programs through a dataflow tracing mechanism which
prevents many stupid security holes. If you have a problem that
would ordinarily use sed or awk or sh, but it exceeds their
capabilities or must run a little faster, and you don't want to write
the silly thing in C, then perl may be for you. There are also
translators to turn your sed and awk scripts into perl scripts. OK,
enough hype.
Upon startup, perl looks for your script in one of the following
places:
1. Specified line by line via -e switches on the command line.
2. Contained in the file specified by the first filename on the
command line. (Note that systems supporting the #! notation
invoke interpreters this way.)
3. Passed in implicitly via standard input. This only works if
there are no filename arguments--to pass arguments to a stdin
script you must explicitly specify a - for the script name.
After locating your script, perl compiles it to an internal form. If
the script is syntactically correct, it is executed.
Options
Note: on first reading this section may not make much sense to you.
It's here at the front for easy reference.
A single-character option may be combined with the following option,
if any. This is particularly useful when invoking a script using the
#! construct which only allows one argument. Example:
#!/usr/bin/perl -spi.bak # same as -s -p -i.bak
...
Options include:
-0digits
specifies the record separator ($/) as an octal number. If
there are no digits, the null character is the separator. Other
switches may precede or follow the digits. For example, if you
have a version of find which can print filenames terminated by
the null character, you can say this:
find . -name '*.bak' -print0 | perl -n0e unlink
The special value 00 will cause Perl to slurp files in paragraph
mode. The value 0777 will cause Perl to slurp files whole since
there is no legal character with that value.
-a turns on autosplit mode when used with a -n or -p. An implicit
split command to the @F array is done as the first thing inside
the implicit while loop produced by the -n or -p.
perl -ane 'print pop(@F), "\n";'
is equivalent to
while (<>) {
@F = split(' ');
print pop(@F), "\n";
}
-c causes perl to check the syntax of the script and then exit
without executing it.
-d runs the script under the perl debugger. See the section on
Debugging.
-Dnumber
sets debugging flags. To watch how it executes your script, use
-D14. (This only works if debugging is compiled into your
perl.) Another nice value is -D1024, which lists your compiled
syntax tree. And -D512 displays compiled regular expressions.
-e commandline
may be used to enter one line of script. Multiple -e commands
may be given to build up a multi-line script. If -e is given,
perl will not look for a script filename in the argument list.
-iextension
specifies that files processed by the <> construct are to be
edited in-place. It does this by renaming the input file,
opening the output file by the same name, and selecting that
output file as the default for print statements. The extension,
if supplied, is added to the name of the old file to make a
backup copy. If no extension is supplied, no backup is made.
Saying "perl -p -i.bak -e "s/foo/bar/;" ... " is the same as
using the script:
#!/usr/bin/perl -pi.bak
s/foo/bar/;
which is equivalent to
#!/usr/bin/perl
while (<>) {
if ($ARGV ne $oldargv) {
rename($ARGV, $ARGV . '.bak');
open(ARGVOUT, ">$ARGV");
select(ARGVOUT);
$oldargv = $ARGV;
}
s/foo/bar/;
}
continue {
print; # this prints to original filename
}
select(STDOUT);
except that the -i form doesn't need to compare $ARGV to
$oldargv to know when the filename has changed. It does,
however, use ARGVOUT for the selected filehandle. Note that
STDOUT is restored as the default output filehandle after the
loop.
You can use eof to locate the end of each input file, in case
you want to append to each file, or reset line numbering (see
example under eof).
-Idirectory
may be used in conjunction with -P to tell the C preprocessor
where to look for include files. By default /usr/include and
/usr/lib/perl are searched.
-loctnum
enables automatic line-ending processing. It has two effects:
first, it automatically chops the line terminator when used with
-n or -p, and second, it assigns $\ to have the value of octnum
so that any print statements will have that line terminator
added back on. If octnum is omitted, sets $\ to the current
value of $/. For instance, to trim lines to 80 columns:
perl -lpe 'substr($_, 80) = ""'
Note that the assignment $\ = $/ is done when the switch is
processed, so the input record separator can be different than
the output record separator if the -l switch is followed by a -0
switch:
gnufind / -print0 | perl -ln0e 'print "found $_" if -p'
This sets $\ to newline and then sets $/ to the null character.
-n causes perl to assume the following loop around your script,
which makes it iterate over filename arguments somewhat like
"sed -n" or awk:
while (<>) {
... # your script goes here
}
Note that the lines are not printed by default. See -p to have
lines printed. Here is an efficient way to delete all files
older than a week:
find . -mtime +7 -print | perl -nle 'unlink;'
This is faster than using the -exec switch of find because you
don't have to start a process on every filename found.
-p causes perl to assume the following loop around your script,
which makes it iterate over filename arguments somewhat like
sed:
while (<>) {
... # your script goes here
} continue {
print;
}
Note that the lines are printed automatically. To suppress
printing use the -n switch. A -p overrides a -n switch.
-P causes your script to be run through the C preprocessor before
compilation by perl. (Since both comments and cpp directives
begin with the # character, you should avoid starting comments
with any words recognized by the C preprocessor such as "if",
"else" or "define".)
-s enables some rudimentary switch parsing for switches on the
command line after the script name but before any filename
arguments (or before a --). Any switch found there is removed
from @ARGV and sets the corresponding variable in the perl
script. The following script prints "true" if and only if the
script is invoked with a -xyz switch.
#!/usr/bin/perl -s
if ($xyz) { print "true\n"; }
-S makes perl use the PATH environment variable to search for the
script (unless the name of the script starts with a slash).
Typically this is used to emulate #! startup on machines that
don't support #!, in the following manner:
#!/usr/bin/perl
eval "exec /usr/bin/perl -S $0 $*"
if $running_under_some_shell;
The system ignores the first line and feeds the script to
/bin/sh, which proceeds to try to execute the perl script as a
shell script. The shell executes the second line as a normal
shell command, and thus starts up the perl interpreter. On some
systems $0 doesn't always contain the full pathname, so the -S
tells perl to search for the script if necessary. After perl
locates the script, it parses the lines and ignores them because
the variable $running_under_some_shell is never true. A better
construct than $* would be ${1+"$@"}, which handles embedded
spaces and such in the filenames, but doesn't work if the script
is being interpreted by csh. In order to start up sh rather
than csh, some systems may have to replace the #! line with a
line containing just a colon, which will be politely ignored by
perl. Other systems can't control that, and need a totally
devious construct that will work under any of csh, sh or perl,
such as the following:
eval '(exit $?0)' && eval 'exec /usr/bin/perl -S $0 ${1+"$@"}'
& eval 'exec /usr/bin/perl -S $0 $argv:q'
if 0;
-u causes perl to dump core after compiling your script. You can
then take this core dump and turn it into an executable file by
using the undump program (not supplied). This speeds startup at
the expense of some disk space (which you can minimize by
stripping the executable). (Still, a "hello world" executable
comes out to about 200K on my machine.) If you are going to run
your executable as a set-id program then you should probably
compile it using taintperl rather than normal perl. If you want
to execute a portion of your script before dumping, use the dump
operator instead. Note: availability of undump is platform
specific and may not be available for a specific port of perl.
-U lets perl do unsafe operations. Currently the only "unsafe"
operations are the unlinking of directories while running with
appropriate privilege, and running setuid programs with fatal
taint checks turned into warnings.
On a generic DG/UX system, appropriate privilege is granted by having
an effective UID of 0 (root). See the appropriate_privilege(5) man
page for more information.
On a system with DG/UX information security, appropriate privilege is
granted by having one or more specific capabilities enabled in the
effective capability set of the user. See cap_defaults(5) for the
default capabilities for this command.
-v prints the version and patchlevel of your perl executable.
-w prints warnings about identifiers that are mentioned only once,
and scalar variables that are used before being set. Also warns
about redefined subroutines, and references to undefined
filehandles or filehandles opened readonly that you are
attempting to write on. Also warns you if you use == on values
that don't look like numbers, and if your subroutines recurse
more than 100 deep.
-xdirectory
tells perl that the script is embedded in a message. Leading
garbage will be discarded until the first line that starts with
#! and contains the string "perl". Any meaningful switches on
that line will be applied (but only one group of switches, as
with normal #! processing). If a directory name is specified,
Perl will switch to that directory before running the script.
The -x switch only controls the the disposal of leading garbage.
The script must be terminated with __END__ if there is trailing
garbage to be ignored (the script can process any or all of the
trailing garbage via the DATA filehandle if desired).
Data Types and Objects
Perl has three data types: scalars, arrays of scalars, and
associative arrays of scalars. Normal arrays are indexed by number,
and associative arrays by string.
The interpretation of operations and values in perl sometimes depends
on the requirements of the context around the operation or value.
There are three major contexts: string, numeric and array. Certain
operations return array values in contexts wanting an array, and
scalar values otherwise. (If this is true of an operation it will be
mentioned in the documentation for that operation.) Operations which
return scalars don't care whether the context is looking for a string
or a number, but scalar variables and values are interpreted as
strings or numbers as appropriate to the context. A scalar is
interpreted as TRUE in the boolean sense if it is not the null string
or 0. Booleans returned by operators are 1 for true and 0 or '' (the
null string) for false.
There are actually two varieties of null string: defined and
undefined. Undefined null strings are returned when there is no real
value for something, such as when there was an error, or at end of
file, or when you refer to an uninitialized variable or element of an
array. An undefined null string may become defined the first time
you access it, but prior to that you can use the defined() operator
to determine whether the value is defined or not.
References to scalar variables always begin with '$', even when
referring to a scalar that is part of an array. Thus:
$days # a simple scalar variable
$days[28] # 29th element of array @days
$days{'Feb'} # one value from an associative array
$#days # last index of array @days
but entire arrays or array slices are denoted by '@':
@days # ($days[0], $days[1],... $days[n])
@days[3,4,5] # same as @days[3..5]
@days{'a','c'} # same as ($days{'a'},$days{'c'})
and entire associative arrays are denoted by '%':
%days # (key1, val1, key2, val2 ...)
Any of these eight constructs may serve as an lvalue, that is, may be
assigned to. (It also turns out that an assignment is itself an
lvalue in certain contexts--see examples under s, tr and chop.)
Assignment to a scalar evaluates the righthand side in a scalar
context, while assignment to an array or array slice evaluates the
righthand side in an array context.
You may find the length of array @days by evaluating "$#days", as in
csh. (Actually, it's not the length of the array, it's the subscript
of the last element, since there is (ordinarily) a 0th element.)
Assigning to $#days changes the length of the array. Shortening an
array by this method does not actually destroy any values.
Lengthening an array that was previously shortened recovers the
values that were in those elements. You can also gain some measure
of efficiency by preextending an array that is going to get big.
(You can also extend an array by assigning to an element that is off
the end of the array. This differs from assigning to $#whatever in
that intervening values are set to null rather than recovered.) You
can truncate an array down to nothing by assigning the null list ()
to it. The following are exactly equivalent
@whatever = ();
$#whatever = $[ - 1;
If you evaluate an array in a scalar context, it returns the length
of the array. The following is always true:
scalar(@whatever) == $#whatever - $[ + 1;
If you evaluate an associative array in a scalar context, it returns
a value which is true if and only if the array contains any elements.
(If there are any elements, the value returned is a string consisting
of the number of used buckets and the number of allocated buckets,
separated by a slash.)
Multi-dimensional arrays are not directly supported, but see the
discussion of the $; variable later for a means of emulating multiple
subscripts with an associative array. You could also write a
subroutine to turn multiple subscripts into a single subscript.
Every data type has its own namespace. You can, without fear of
conflict, use the same name for a scalar variable, an array, an
associative array, a filehandle, a subroutine name, and/or a label.
Since variable and array references always start with '$', '@', or
'%', the "reserved" words aren't in fact reserved with respect to
variable names. (They ARE reserved with respect to labels and
filehandles, however, which don't have an initial special character.
Hint: you could say open(LOG,'logfile') rather than
open(log,'logfile'). Using uppercase filehandles also improves
readability and protects you from conflict with future reserved
words.) Case IS significant--"FOO", "Foo" and "foo" are all
different names. Names which start with a letter may also contain
digits and underscores. Names which do not start with a letter are
limited to one character, e.g. "$%" or "$$". (Most of the one
character names have a predefined significance to perl. More later.)
Numeric literals are specified in any of the usual floating point or
integer formats:
12345
12345.67
.23E-10
0xffff # hex
0377 # octal
4_294_967_296
String literals are delimited by either single or double quotes.
They work much like shell quotes: double-quoted string literals are
subject to backslash and variable substitution; single-quoted strings
are not (except for \' and \\). The usual backslash rules apply for
making characters such as newline, tab, etc., as well as some more
exotic forms:
\t tab
\n newline
\r return
\f form feed
\b backspace
\a alarm (bell)
\e escape
\033 octal char
\x1b hex char
\c[ control char
\l lowercase next char
\u uppercase next char
\L lowercase till \E
\U uppercase till \E
\E end case modification
You can also embed newlines directly in your strings, i.e. they can
end on a different line than they begin. This is nice, but if you
forget your trailing quote, the error will not be reported until perl
finds another line containing the quote character, which may be much
further on in the script. Variable substitution inside strings is
limited to scalar variables, normal array values, and array slices.
(In other words, identifiers beginning with $ or @, followed by an
optional bracketed expression as a subscript.) The following code
segment prints out "The price is $100."
$Price = '$100'; # not interpreted
print "The price is $Price.\n";# interpreted
Note that you can put curly brackets around the identifier to delimit
it from following alphanumerics. Also note that a single quoted
string must be separated from a preceding word by a space, since
single quote is a valid character in an identifier (see Packages).
Two special literals are __LINE__ and __FILE__, which represent the
current line number and filename at that point in your program. They
may only be used as separate tokens; they will not be interpolated
into strings. In addition, the token __END__ may be used to indicate
the logical end of the script before the actual end of file. Any
following text is ignored, but may be read via the DATA filehandle.
(The DATA filehandle may read data only from the main script, but not
from any required file or evaluated string.) The two control
characters ^D and ^Z are synonyms for __END__.
A word that doesn't have any other interpretation in the grammar will
be treated as if it had single quotes around it. For this purpose, a
word consists only of alphanumeric characters and underline, and must
start with an alphabetic character. As with filehandles and labels,
a bare word that consists entirely of lowercase letters risks
conflict with future reserved words, and if you use the -w switch,
Perl will warn you about any such words.
Array values are interpolated into double-quoted strings by joining
all the elements of the array with the delimiter specified in the $"
variable, space by default. (Since in versions of perl prior to 3.0
the @ character was not a metacharacter in double-quoted strings, the
interpolation of @array, $array[EXPR], @array[LIST], $array{EXPR}, or
@array{LIST} only happens if array is referenced elsewhere in the
program or is predefined.) The following are equivalent:
$temp = join($",@ARGV);
system "echo $temp";
system "echo @ARGV";
Within search patterns (which also undergo double-quotish
substitution) there is a bad ambiguity: Is /$foo[bar]/ to be
interpreted as /${foo}[bar]/ (where [bar] is a character class for
the regular expression) or as /${foo[bar]}/ (where [bar] is the
subscript to array @foo)? If @foo doesn't otherwise exist, then it's
obviously a character class. If @foo exists, perl takes a good guess
about [bar], and is almost always right. If it does guess wrong, or
if you're just plain paranoid, you can force the correct
interpretation with curly brackets as above.
A line-oriented form of quoting is based on the shell here-is syntax.
Following a << you specify a string to terminate the quoted material,
and all lines following the current line down to the terminating
string are the value of the item. The terminating string may be
either an identifier (a word), or some quoted text. If quoted, the
type of quotes you use determines the treatment of the text, just as
in regular quoting. An unquoted identifier works like double quotes.
There must be no space between the << and the identifier. (If you
put a space it will be treated as a null identifier, which is valid,
and matches the first blank line--see Merry Christmas example below.)
The terminating string must appear by itself (unquoted and with no
surrounding whitespace) on the terminating line.
print <<EOF; # same as above
The price is $Price.
EOF
print <<"EOF"; # same as above
The price is $Price.
EOF
print << x 10; # null identifier is delimiter
Merry Christmas!
print <<`EOC`; # execute commands
echo hi there
echo lo there
EOC
print <<foo, <<bar; # you can stack them
I said foo.
foo
I said bar.
bar
Array literals are denoted by separating individual values by commas,
and enclosing the list in parentheses:
(LIST)
In a context not requiring an array value, the value of the array
literal is the value of the final element, as in the C comma
operator. For example,
@foo = ('cc', '-E', $bar);
assigns the entire array value to array foo, but
$foo = ('cc', '-E', $bar);
assigns the value of variable bar to variable foo. Note that the
value of an actual array in a scalar context is the length of the
array; the following assigns to $foo the value 3:
@foo = ('cc', '-E', $bar);
$foo = @foo; # $foo gets 3
You may have an optional comma before the closing parenthesis of an
array literal, so that you can say:
@foo = (
1,
2,
3,
);
When a LIST is evaluated, each element of the list is evaluated in an
array context, and the resulting array value is interpolated into
LIST just as if each individual element were a member of LIST. Thus
arrays lose their identity in a LIST--the list
(@foo,@bar,&SomeSub)
contains all the elements of @foo followed by all the elements of
@bar, followed by all the elements returned by the subroutine named
SomeSub.
A list value may also be subscripted like a normal array. Examples:
$time = (stat($file))[8]; # stat returns array value
$digit = ('a','b','c','d','e','f')[$digit-10];
return (pop(@foo),pop(@foo))[0];
Array lists may be assigned to if and only if each element of the
list is an lvalue:
($a, $b, $c) = (1, 2, 3);
($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00);
The final element may be an array or an associative array:
($a, $b, @rest) = split;
local($a, $b, %rest) = @_;
You can actually put an array anywhere in the list, but the first
array in the list will soak up all the values, and anything after it
will get a null value. This may be useful in a local().
An associative array literal contains pairs of values to be
interpreted as a key and a value:
# same as map assignment above
%map = ('red',0x00f,'blue',0x0f0,'green',0xf00);
Array assignment in a scalar context returns the number of elements
produced by the expression on the right side of the assignment:
$x = (($foo,$bar) = (3,2,1)); # set $x to 3, not 2
There are several other pseudo-literals that you should know about.
If a string is enclosed by backticks (grave accents), it first
undergoes variable substitution just like a double quoted string. It
is then interpreted as a command, and the output of that command is
the value of the pseudo-literal, like in a shell. In a scalar
context, a single string consisting of all the output is returned.
In an array context, an array of values is returned, one for each
line of output. (You can set $/ to use a different line terminator.)
The command is executed each time the pseudo-literal is evaluated.
The status value of the command is returned in $? (see Predefined
Names for the interpretation of $?). Unlike in csh, no translation
is done on the return data--newlines remain newlines. Unlike in any
of the shells, single quotes do not hide variable names in the
command from interpretation. To pass a $ through to the shell you
need to hide it with a backslash.
Evaluating a filehandle in angle brackets yields the next line from
that file (newline included, so it's never false until EOF, at which
time an undefined value is returned). Ordinarily you must assign
that value to a variable, but there is one situation where an
automatic assignment happens. If (and only if) the input symbol is
the only thing inside the conditional of a while loop, the value is
automatically assigned to the variable "$_". (This may seem like an
odd thing to you, but you'll use the construct in almost every perl
script you write.) Anyway, the following lines are equivalent to
each other:
while ($_ = <STDIN>) { print; }
while (<STDIN>) { print; }
for (;<STDIN>;) { print; }
print while $_ = <STDIN>;
print while <STDIN>;
The filehandles STDIN, STDOUT and STDERR are predefined. (The
filehandles stdin, stdout and stderr will also work except in
packages, where they would be interpreted as local identifiers rather
than global.) Additional filehandles may be created with the open
function.
If a <FILEHANDLE> is used in a context that is looking for an array,
an array consisting of all the input lines is returned, one line per
array element. It's easy to make a LARGE data space this way, so use
with care.
The null filehandle <> is special and can be used to emulate the
behavior of sed and awk. Input from <> comes either from standard
input, or from each file listed on the command line. Here's how it
works: the first time <> is evaluated, the ARGV array is checked, and
if it is null, $ARGV[0] is set to '-', which when opened gives you
standard input. The ARGV array is then processed as a list of
filenames. The loop
while (<>) {
... # code for each line
}
is equivalent to the following Perl-like pseudo code:
unshift(@ARGV, '-') if $#ARGV < $[;
while ($ARGV = shift) {
open(ARGV, $ARGV);
while (<ARGV>) {
... # code for each line
}
}
except that it isn't as cumbersome to say, and will actually work.
It really does shift array ARGV and put the current filename into
variable ARGV. It also uses filehandle ARGV internally--<> is just a
synonym for <ARGV>, which is magical. (The pseudo code above doesn't
work because it treats <ARGV> as non-magical.)
You can modify @ARGV before the first <> as long as the array ends up
containing the list of filenames you really want. Line numbers ($.)
continue as if the input was one big happy file. (But see example
under eof for how to reset line numbers on each file.)
If you want to set @ARGV to your own list of files, go right ahead.
If you want to pass switches into your script, you can put a loop on
the front like this:
while ($_ = $ARGV[0], /^-/) {
shift;
last if /^--$/;
/^-D(.*)/ && ($debug = $1);
/^-v/ && $verbose++;
... # other switches
}
while (<>) {
... # code for each line
}
The <> symbol will return FALSE only once. If you call it again
after this it will assume you are processing another @ARGV list, and
if you haven't set @ARGV, will input from STDIN.
If the string inside the angle brackets is a reference to a scalar
variable (e.g. <$foo>), then that variable contains the name of the
filehandle to input from.
If the string inside angle brackets is not a filehandle, it is
interpreted as a filename pattern to be globbed, and either an array
of filenames or the next filename in the list is returned, depending
on context. One level of $ interpretation is done first, but you
can't say <$foo> because that's an indirect filehandle as explained
in the previous paragraph. You could insert curly brackets to force
interpretation as a filename glob: <${foo}>. Example:
while (<*.c>) {
chmod 0644, $_;
}
is equivalent to
open(foo, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
while (<foo>) {
chop;
chmod 0644, $_;
}
In fact, it's currently implemented that way. (Which means it will
not work on filenames with spaces in them unless you have /bin/csh on
your machine.) Of course, the shortest way to do the above is:
chmod 0644, <*.c>;
Syntax
A perl script consists of a sequence of declarations and commands.
The only things that need to be declared in perl are report formats
and subroutines. See the sections below for more information on
those declarations. All uninitialized user-created objects are
assumed to start with a null or 0 value until they are defined by
some explicit operation such as assignment. The sequence of commands
is executed just once, unlike in sed and awk scripts, where the
sequence of commands is executed for each input line. While this
means that you must explicitly loop over the lines of your input file
(or files), it also means you have much more control over which files
and which lines you look at. (Actually, I'm lying--it is possible to
do an implicit loop with either the -n or -p switch.)
A declaration can be put anywhere a command can, but has no effect on
the execution of the primary sequence of commands--declarations all
take effect at compile time. Typically all the declarations are put
at the beginning or the end of the script.
Perl is, for the most part, a free-form language. (The only
exception to this is format declarations, for fairly obvious
reasons.) Comments are indicated by the # character, and extend to
the end of the line. If you attempt to use /* */ C comments, it will
be interpreted either as division or pattern matching, depending on
the context. So don't do that.
Compound statements
In perl, a sequence of commands may be treated as one command by
enclosing it in curly brackets. We will call this a BLOCK.
The following compound commands may be used to control flow:
if (EXPR) BLOCK
if (EXPR) BLOCK else BLOCK
if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
LABEL while (EXPR) BLOCK
LABEL while (EXPR) BLOCK continue BLOCK
LABEL for (EXPR; EXPR; EXPR) BLOCK
LABEL foreach VAR (ARRAY) BLOCK
LABEL BLOCK continue BLOCK
Note that, unlike C and Pascal, these are defined in terms of BLOCKs,
not statements. This means that the curly brackets are required--no
dangling statements allowed. If you want to write conditionals
without curly brackets there are several other ways to do it. The
following all do the same thing:
if (!open(foo)) { die "Can't open $foo: $!"; }
die "Can't open $foo: $!" unless open(foo);
open(foo) || die "Can't open $foo: $!"; # foo or bust!
open(foo) ? 'hi mom' : die "Can't open $foo: $!";
# a bit exotic, that last one
The if statement is straightforward. Since BLOCKs are always bounded
by curly brackets, there is never any ambiguity about which if an
else goes with. If you use unless in place of if, the sense of the
test is reversed.
The while statement executes the block as long as the expression is
true (does not evaluate to the null string or 0). The LABEL is
optional, and if present, consists of an identifier followed by a
colon. The LABEL identifies the loop for the loop control statements
next, last, and redo (see below). If there is a continue BLOCK, it
is always executed just before the conditional is about to be
evaluated again, similarly to the third part of a for loop in C.
Thus it can be used to increment a loop variable, even when the loop
has been continued via the next statement (similar to the C
"continue" statement).
If the word while is replaced by the word until, the sense of the
test is reversed, but the conditional is still tested before the
first iteration.
In either the if or the while statement, you may replace "(EXPR)"
with a BLOCK, and the conditional is true if the value of the last
command in that block is true.
The for loop works exactly like the corresponding while loop:
for ($i = 1; $i < 10; $i++) {
...
}
is the same as
$i = 1;
while ($i < 10) {
...
} continue {
$i++;
}
The foreach loop iterates over a normal array value and sets the
variable VAR to be each element of the array in turn. The variable
is implicitly local to the loop, and regains its former value upon
exiting the loop. The "foreach" keyword is actually identical to the
"for" keyword, so you can use "foreach" for readability or "for" for
brevity. If VAR is omitted, $_ is set to each value. If ARRAY is an
actual array (as opposed to an expression returning an array value),
you can modify each element of the array by modifying VAR inside the
loop. Examples:
for (@ary) { s/foo/bar/; }
foreach $elem (@elements) {
$elem *= 2;
}
for ((10,9,8,7,6,5,4,3,2,1,'BOOM')) {
print $_, "\n"; sleep(1);
}
for (1..15) { print "Merry Christmas\n"; }
foreach $item (split(/:[\\\n:]*/, $ENV{'TERMCAP'})) {
print "Item: $item\n";
}
The BLOCK by itself (labeled or not) is equivalent to a loop that
executes once. Thus you can use any of the loop control statements
in it to leave or restart the block. The continue block is optional.
This construct is particularly nice for doing case structures.
foo: {
if (/^abc/) { $abc = 1; last foo; }
if (/^def/) { $def = 1; last foo; }
if (/^xyz/) { $xyz = 1; last foo; }
$nothing = 1;
}
There is no official switch statement in perl, because there are
already several ways to write the equivalent. In addition to the
above, you could write
foo: {
$abc = 1, last foo if /^abc/;
$def = 1, last foo if /^def/;
$xyz = 1, last foo if /^xyz/;
$nothing = 1;
}
or
foo: {
/^abc/ && do { $abc = 1; last foo; };
/^def/ && do { $def = 1; last foo; };
/^xyz/ && do { $xyz = 1; last foo; };
$nothing = 1;
}
or
foo: {
/^abc/ && ($abc = 1, last foo);
/^def/ && ($def = 1, last foo);
/^xyz/ && ($xyz = 1, last foo);
$nothing = 1;
}
or even
if (/^abc/)
{ $abc = 1; }
elsif (/^def/)
{ $def = 1; }
elsif (/^xyz/)
{ $xyz = 1; }
else
{$nothing = 1;}
As it happens, these are all optimized internally to a switch
structure, so perl jumps directly to the desired statement, and you
needn't worry about perl executing a lot of unnecessary statements
when you have a string of 50 elsifs, as long as you are testing the
same simple scalar variable using ==, eq, or pattern matching as
above. (If you're curious as to whether the optimizer has done this
for a particular case statement, you can use the -D1024 switch to
list the syntax tree before execution.)
Simple statements
The only kind of simple statement is an expression evaluated for its
side effects. Every simple statement must be terminated with a
semicolon, unless it is the final statement in a block, in which case
the semicolon is optional. (Semicolon is still encouraged there if
the block takes up more than one line).
Any simple statement may optionally be followed by a single modifier,
just before the terminating semicolon. The possible modifiers are:
if EXPR
unless EXPR
while EXPR
until EXPR
The if and unless modifiers have the expected semantics. The while
and until modifiers also have the expected semantics (conditional
evaluated first), except when applied to a do-BLOCK or a do-
SUBROUTINE command, in which case the block executes once before the
conditional is evaluated. This is so that you can write loops like:
do {
$_ = <STDIN>;
...
} until $_ eq ".\n";
(See the do operator below. Note also that the loop control commands
described later will NOT work in this construct, since modifiers
don't take loop labels. Sorry.)
Expressions
Since perl expressions work almost exactly like C expressions, only
the differences will be mentioned here.
Here's what perl has that C doesn't:
** The exponentiation operator.
**= The exponentiation assignment operator.
() The null list, used to initialize an array to null.
. Concatenation of two strings.
.= The concatenation assignment operator.
eq String equality (== is numeric equality). For a mnemonic
just think of "eq" as a string. (If you are used to the awk
behavior of using == for either string or numeric equality
based on the current form of the comparands, beware! You
must be explicit here.)
ne String inequality (!= is numeric inequality).
lt String less than.
gt String greater than.
le String less than or equal.
ge String greater than or equal.
cmp String comparison, returning -1, 0, or 1.
<=> Numeric comparison, returning -1, 0, or 1.
=~ Certain operations search or modify the string "$_" by
default. This operator makes that kind of operation work on
some other string. The right argument is a search pattern,
substitution, or translation. The left argument is what is
supposed to be searched, substituted, or translated instead
of the default "$_". The return value indicates the success
of the operation. (If the right argument is an expression
other than a search pattern, substitution, or translation, it
is interpreted as a search pattern at run time. This is less
efficient than an explicit search, since the pattern must be
compiled every time the expression is evaluated.) The
precedence of this operator is lower than unary minus and
autoincrement/decrement, but higher than everything else.
!~ Just like =~ except the return value is negated.
x The repetition operator. Returns a string consisting of the
left operand repeated the number of times specified by the
right operand. In an array context, if the left operand is a
list in parens, it repeats the list.
print '-' x 80; # print row of dashes
print '-' x80; # illegal, x80 is identifier
print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
@ones = (1) x 80; # an array of 80 1's
@ones = (5) x @ones; # set all elements to 5
x= The repetition assignment operator. Only works on scalars.
.. The range operator, which is really two different operators
depending on the context. In an array context, returns an
array of values counting (by ones) from the left value to the
right value. This is useful for writing "for (1..10)" loops
and for doing slice operations on arrays.
In a scalar context, .. returns a boolean value. The
operator is bistable, like a flip-flop, and emulates the
line-range (comma) operator of sed, awk, and various editors.
Each .. operator maintains its own boolean state. It is
false as long as its left operand is false. Once the left
operand is true, the range operator stays true until the
right operand is true, AFTER which the range operator becomes
false again. (It doesn't become false till the next time the
range operator is evaluated. It can test the right operand
and become false on the same evaluation it became true (as in
awk), but it still returns true once. If you don't want it
to test the right operand till the next evaluation (as in
sed), use three dots (...) instead of two.) The right
operand is not evaluated while the operator is in the "false"
state, and the left operand is not evaluated while the
operator is in the "true" state. The precedence is a little
lower than || and &&. The value returned is either the null
string for false, or a sequence number (beginning with 1) for
true. The sequence number is reset for each range
encountered. The final sequence number in a range has the
string 'E0' appended to it, which doesn't affect its numeric
value, but gives you something to search for if you want to
exclude the endpoint. You can exclude the beginning point by
waiting for the sequence number to be greater than 1. If
either operand of scalar .. is static, that operand is
implicitly compared to the $. variable, the current line
number. Examples:
As a scalar operator:
if (101 .. 200) { print; } # print 2nd hundred lines
next line if (1 .. /^$/); # skip header lines
s/^/> / if (/^$/ .. eof()); # quote body
As an array operator:
for (101 .. 200) { print; } # print $_ 100 times
@foo = @foo[$[ .. $#foo]; # an expensive no-op
@foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
-x A file test. This unary operator takes one argument, either
a filename or a filehandle, and tests the associated file to
see if something is true about it. If the argument is
omitted, tests $_, except for -t, which tests STDIN. It
returns 1 for true and '' for false, or the undefined value
if the file doesn't exist. Precedence is higher than logical
and relational operators, but lower than arithmetic
operators. The operator may be any of:
-r File is readable by effective uid/gid.
-w File is writable by effective uid/gid.
-x File is executable by effective uid/gid.
-o File is owned by effective uid.
-R File is readable by real uid/gid.
-W File is writable by real uid/gid.
-X File is executable by real uid/gid.
-O File is owned by real uid.
-e File exists.
-z File has zero size.
-s File has non-zero size (returns size).
-f File is a plain file.
-d File is a directory.
-l File is a symbolic link.
-p File is a named pipe (FIFO).
-S File is a socket.
-b File is a block special file.
-c File is a character special file.
-u File has setuid bit set.
-g File has setgid bit set.
-k File has sticky bit set.
-t Filehandle is opened to a tty.
-T File is a text file.
-B File is a binary file (opposite of -T).
-M Age of file in days when script started.
-A Same for access time.
-C Same for inode change time.
The interpretation of the file permission operators -r, -R,
-w, -W, -x and -X is based solely on the mode of the file and
the uids and gids of the user. There may be other reasons
you can't actually read, write or execute the file. Also
note that, when running with appropriate privilege, -r, -R,
-w and -W always return 1, and -x and -X return 1 if any
execute bit is set in the mode. Scripts run with appropriate
privilege may thus need to do a stat() in order to determine
the actual mode of the file, or temporarily set the uid to
something else.
On a generic DG/UX system, appropriate privilege is granted by having
an effective UID of 0 (root). See the appropriate_privilege(5) man
page for more information.
On a system with DG/UX information security, appropriate privilege is
granted by having one or more specific capabilities enabled in the
effective capability set of the user. See cap_defaults(5) for the
default capabilities for this command.
Example:
while (<>) {
chop;
next unless -f $_; # ignore specials
...
}
Note that -s/a/b/ does not do a negated substitution. Saying
-exp($foo) still works as expected, however--only single letters
following a minus are interpreted as file tests.
The -T and -B switches work as follows. The first block or so of the
file is examined for odd characters such as strange control codes or
metacharacters. If too many odd characters (>10%) are found, it's a
-B file, otherwise it's a -T file. Also, any file containing null in
the first block is considered a binary file. If -T or -B is used on
a filehandle, the current stdio buffer is examined rather than the
first block. Both -T and -B return TRUE on a null file, or a file at
EOF when testing a filehandle.
If any of the file tests (or either stat operator) are given the
special filehandle consisting of a solitary underline, then the stat
structure of the previous file test (or stat operator) is used,
saving a system call. (This doesn't work with -t, and you need to
remember that lstat and -l will leave values in the stat structure
for the symbolic link, not the real file.) Example:
print "Can do.\n" if -r $a || -w _ || -x _;
stat($filename);
print "Readable\n" if -r _;
print "Writable\n" if -w _;
print "Executable\n" if -x _;
print "Setuid\n" if -u _;
print "Setgid\n" if -g _;
print "Sticky\n" if -k _;
print "Text\n" if -T _;
print "Binary\n" if -B _;
Here is what C has that perl doesn't:
unary & Address-of operator.
unary * Dereference-address operator.
(TYPE) Type casting operator.
Like C, perl does a certain amount of expression evaluation at
compile time, whenever it determines that all of the arguments to an
operator are static and have no side effects. In particular, string
concatenation happens at compile time between literals that don't do
variable substitution. Backslash interpretation also happens at
compile time. You can say
'Now is the time for all' . "\n" .
'good men to come to.'
and this all reduces to one string internally.
The autoincrement operator has a little extra built-in magic to it.
If you increment a variable that is numeric, or that has ever been
used in a numeric context, you get a normal increment. If, however,
the variable has only been used in string contexts since it was set,
and has a value that is not null and matches the pattern
/^[a-zA-Z]*[0-9]*$/, the increment is done as a string, preserving
each character within its range, with carry:
print ++($foo = '99'); # prints '100'
print ++($foo = 'a0'); # prints 'a1'
print ++($foo = 'Az'); # prints 'Ba'
print ++($foo = 'zz'); # prints 'aaa'
The autodecrement is not magical.
The range operator (in an array context) makes use of the magical
autoincrement algorithm if the minimum and maximum are strings. You
can say
@alphabet = ('A' .. 'Z');
to get all the letters of the alphabet, or
$hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
to get a hexadecimal digit, or
@z2 = ('01' .. '31'); print @z2[$mday];
to get dates with leading zeros. (If the final value specified is
not in the sequence that the magical increment would produce, the
sequence goes until the next value would be longer than the final
value specified.)
The || and && operators differ from C's in that, rather than
returning 0 or 1, they return the last value evaluated. Thus, a
portable way to find out the home directory might be:
$home = $ENV{'HOME'} || $ENV{'LOGDIR'} ||
(getpwuid($<))[7] || die "You're homeless!\n";
Along with the literals and variables mentioned earlier, the
operations in the following section can serve as terms in an
expression. Some of these operations take a LIST as an argument.
Such a list can consist of any combination of scalar arguments or
array values; the array values will be included in the list as if
each individual element were interpolated at that point in the list,
forming a longer single-dimensional array value. Elements of the
LIST should be separated by commas. If an operation is listed both
with and without parentheses around its arguments, it means you can
either use it as a unary operator or as a function call. To use it
as a function call, the next token on the same line must be a left
parenthesis. (There may be intervening white space.) Such a
function then has highest precedence, as you would expect from a
function. If any token other than a left parenthesis follows, then
it is a unary operator, with a precedence depending only on whether
it is a LIST operator or not. LIST operators have lowest precedence.
All other unary operators have a precedence greater than relational
operators but less than arithmetic operators. See the section on
Precedence.
For operators that can be used in either a scalar or array context,
failure is generally indicated in a scalar context by returning the
undefined value, and in an array context by returning the null list.
Remember though that THERE IS NO GENERAL RULE FOR CONVERTING A LIST
INTO A SCALAR. Each operator decides which sort of scalar it would
be most appropriate to return. Some operators return the length of
the list that would have been returned in an array context. Some
operators return the first value in the list. Some operators return
the last value in the list. Some operators return a count of
successful operations. In general, they do what you want, unless you
want consistency.
/PATTERN/
See m/PATTERN/.
?PATTERN?
This is just like the /pattern/ search, except that it
matches only once between calls to the reset operator. This
is a useful optimization when you only want to see the first
occurrence of something in each file of a set of files, for
instance. Only ?? patterns local to the current package are
reset.
accept(NEWSOCKET,GENERICSOCKET)
Does the same thing that the accept system call does.
Returns true if it succeeded, false otherwise. See example
in section on Interprocess Communication.
alarm(SECONDS)
alarm SECONDS
Arranges to have a SIGALRM delivered to this process after
the specified number of seconds (minus 1, actually) have
elapsed. Thus, alarm(15) will cause a SIGALRM at some point
more than 14 seconds in the future. Only one timer may be
counting at once. Each call disables the previous timer, and
an argument of 0 may be supplied to cancel the previous timer
without starting a new one. The returned value is the amount
of time remaining on the previous timer.
atan2(Y,X)
Returns the arctangent of Y/X in the range -PI to PI.
bind(SOCKET,NAME)
Does the same thing that the bind system call does. Returns
true if it succeeded, false otherwise. NAME should be a
packed address of the proper type for the socket. See
example in section on Interprocess Communication.
binmode(FILEHANDLE)
binmode FILEHANDLE
Arranges for the file to be read in "binary" mode in
operating systems that distinguish between binary and text
files. Files that are not read in binary mode have CR LF
sequences translated to LF on input and LF translated to CR
LF on output. Binmode has no effect under Unix. If
FILEHANDLE is an expression, the value is taken as the name
of the filehandle.
caller(EXPR)
caller Returns the context of the current subroutine call:
($package,$filename,$line) = caller;
With EXPR, returns some extra information that the debugger
uses to print a stack trace. The value of EXPR indicates how
many call frames to go back before the current one.
chdir(EXPR)
chdir EXPR
Changes the working directory to EXPR, if possible. If EXPR
is omitted, changes to home directory. Returns 1 upon
success, 0 otherwise. See example under die.
chmod(LIST)
chmod LIST
Changes the permissions of a list of files. The first
element of the list must be the numerical mode. Returns the
number of files successfully changed.
$cnt = chmod 0755, 'foo', 'bar';
chmod 0755, @executables;
chop(LIST)
chop(VARIABLE)
chop VARIABLE
chop Chops off the last character of a string and returns the
character chopped. It's used primarily to remove the newline
from the end of an input record, but is much more efficient
than s/\n// because it neither scans nor copies the string.
If VARIABLE is omitted, chops $_. Example:
while (<>) {
chop; # avoid \n on last field
@array = split(/:/);
...
}
You can actually chop anything that's an lvalue, including an
assignment:
chop($cwd = `pwd`);
chop($answer = <STDIN>);
If you chop a list, each element is chopped. Only the value
of the last chop is returned.
chown(LIST)
chown LIST
Changes the owner (and group) of a list of files. The first
two elements of the list must be the NUMERICAL uid and gid,
in that order. Returns the number of files successfully
changed.
$cnt = chown $uid, $gid, 'foo', 'bar';
chown $uid, $gid, @filenames;
Here's an example that looks up non-numeric uids in the
passwd file:
print "User: ";
$user = <STDIN>;
chop($user);
print "Files: "
$pattern = <STDIN>;
chop($pattern);
open(pass, '/etc/passwd')
|| die "Can't open passwd: $!\n";
while (<pass>) {
($login,$pass,$uid,$gid) = split(/:/);
$uid{$login} = $uid;
$gid{$login} = $gid;
}
@ary = <${pattern}>; # get filenames
if ($uid{$user} eq '') {
die "$user not in passwd file";
}
else {
chown $uid{$user}, $gid{$user}, @ary;
}
chroot(FILENAME)
chroot FILENAME
Does the same as the system call of that name. If you don't
know what it does, don't worry about it. If FILENAME is
omitted, does chroot to $_.
close(FILEHANDLE)
close FILEHANDLE
Closes the file or pipe associated with the file handle. You
don't have to close FILEHANDLE if you are immediately going
to do another open on it, since open will close it for you.
(See open.) However, an explicit close on an input file
resets the line counter ($.), while the implicit close done
by open does not. Also, closing a pipe will wait for the
process executing on the pipe to complete, in case you want
to look at the output of the pipe afterwards. Closing a pipe
explicitly also puts the status value of the command into $?.
Example:
open(OUTPUT, '|sort >foo'); # pipe to sort
... # print stuff to output
close OUTPUT; # wait for sort to finish
open(INPUT, 'foo'); # get sort's results
FILEHANDLE may be an expression whose value gives the real
filehandle name.
closedir(DIRHANDLE)
closedir DIRHANDLE
Closes a directory opened by opendir().
connect(SOCKET,NAME)
Does the same thing that the connect system call does.
Returns true if it succeeded, false otherwise. NAME should
be a package address of the proper type for the socket. See
example in section on Interprocess Communication.
cos(EXPR)
cos EXPR
Returns the cosine of EXPR (expressed in radians). If EXPR
is omitted takes cosine of $_.
crypt(PLAINTEXT,SALT)
Encrypts a string exactly like the crypt() function in the C
library. Useful for checking the password file for lousy
passwords. Only the guys wearing white hats should do this.
dbmclose(ASSOC_ARRAY)
dbmclose ASSOC_ARRAY
Breaks the binding between a dbm file and an associative
array. The values remaining in the associative array are
meaningless unless you happen to want to know what was in the
cache for the dbm file. This function is only useful if you
have ndbm.
dbmopen(ASSOC,DBNAME,MODE)
This binds a dbm or ndbm file to an associative array. ASSOC
is the name of the associative array. (Unlike normal open,
the first argument is NOT a filehandle, even though it looks
like one). DBNAME is the name of the database (without the
.dir or .pag extension). If the database does not exist, it
is created with protection specified by MODE (as modified by
the umask). If your system only supports the older dbm
functions, you may perform only one dbmopen in your program.
If your system has neither dbm nor ndbm, calling dbmopen
produces a fatal error.
Values assigned to the associative array prior to the dbmopen
are lost. A certain number of values from the dbm file are
cached in memory. By default this number is 64, but you can
increase it by preallocating that number of garbage entries
in the associative array before the dbmopen. You can flush
the cache if necessary with the reset command.
If you don't have write access to the dbm file, you can only
read associative array variables, not set them. If you want
to test whether you can write, either use file tests or try
setting a dummy array entry inside an eval, which will trap
the error.
Note that functions such as keys() and values() may return
huge array values when used on large dbm files. You may
prefer to use the each() function to iterate over large dbm
files. Example:
# print out history file offsets
dbmopen(HIST,'/usr/lib/news/history',0666);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "\n";
}
dbmclose(HIST);
defined(EXPR)
defined EXPR
Returns a boolean value saying whether the lvalue EXPR has a
real value or not. Many operations return the undefined
value under exceptional conditions, such as end of file,
uninitialized variable, system error and such. This function
allows you to distinguish between an undefined null string
and a defined null string with operations that might return a
real null string, in particular referencing elements of an
array. You may also check to see if arrays or subroutines
exist. Use on predefined variables is not guaranteed to
produce intuitive results. Examples:
print if defined $switch{'D'};
print "$val\n" while defined($val = pop(@ary));
die "Can't readlink $sym: $!"
unless defined($value = readlink $sym);
eval '@foo = ()' if defined(@foo);
die "No XYZ package defined" unless defined %_XYZ;
sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
See also undef.
delete $ASSOC{KEY}
Deletes the specified value from the specified associative
array. Returns the deleted value, or the undefined value if
nothing was deleted. Deleting from $ENV{} modifies the
environment. Deleting from an array bound to a dbm file
deletes the entry from the dbm file.
The following deletes all the values of an associative array:
foreach $key (keys %ARRAY) {
delete $ARRAY{$key};
}
(But it would be faster to use the reset command. Saying
undef %ARRAY is faster yet.)
die(LIST)
die LIST
Outside of an eval, prints the value of LIST to STDERR and
exits with the current value of $! (errno). If $! is 0,
exits with the value of ($? >> 8) (`command` status). If ($?
>> 8) is 0, exits with 255. Inside an eval, the error
message is stuffed into $@ and the eval is terminated with
the undefined value.
Equivalent examples:
die "Can't cd to spool: $!\n"
unless chdir '/usr/spool/news';
chdir '/usr/spool/news' || die "Can't cd to spool: $!\n"
If the value of EXPR does not end in a newline, the current
script line number and input line number (if any) are also
printed, and a newline is supplied. Hint: sometimes
appending ", stopped" to your message will cause it to make
better sense when the string "at foo line 123" is appended.
Suppose you are running script "canasta".
die "/etc/games is no good";
die "/etc/games is no good, stopped";
produce, respectively
/etc/games is no good at canasta line 123.
/etc/games is no good, stopped at canasta line 123.
See also exit.
do BLOCK
Returns the value of the last command in the sequence of
commands indicated by BLOCK. When modified by a loop
modifier, executes the BLOCK once before testing the loop
condition. (On other statements the loop modifiers test the
conditional first.)
do SUBROUTINE (LIST)
Executes a SUBROUTINE declared by a sub declaration, and
returns the value of the last expression evaluated in
SUBROUTINE. If there is no subroutine by that name, produces
a fatal error. (You may use the "defined" operator to
determine if a subroutine exists.) If you pass arrays as
part of LIST you may wish to pass the length of the array in
front of each array. (See the section on subroutines later
on.) The parentheses are required to avoid confusion with
the "do EXPR" form.
SUBROUTINE may also be a single scalar variable, in which
case the name of the subroutine to execute is taken from the
variable.
As an alternate (and preferred) form, you may call a
subroutine by prefixing the name with an ampersand:
&foo(@args). If you aren't passing any arguments, you don't
have to use parentheses. If you omit the parentheses, no @_
array is passed to the subroutine. The & form is also used
to specify subroutines to the defined and undef operators:
if (defined &$var) { &$var($parm); undef &$var; }
do EXPR Uses the value of EXPR as a filename and executes the
contents of the file as a perl script. Its primary use is to
include subroutines from a perl subroutine library.
do 'stat.pl';
is just like
eval `cat stat.pl`;
except that it's more efficient, more concise, keeps track of
the current filename for error messages, and searches all the
-I libraries if the file isn't in the current directory (see
also the @INC array in Predefined Names). It's the same,
however, in that it does reparse the file every time you call
it, so if you are going to use the file inside a loop you
might prefer to use -P and #include, at the expense of a
little more startup time. (The main problem with #include is
that cpp doesn't grok # comments--a workaround is to use ";#"
for standalone comments.) Note that the following are NOT
equivalent:
do $foo; # eval a file
do $foo(); # call a subroutine
Note that inclusion of library routines is better done with
the "require" operator.
dump LABEL
This causes an immediate core dump. Primarily this is so
that you can use the undump program to turn your core dump
into an executable binary after having initialized all your
variables at the beginning of the program. When the new
binary is executed it will begin by executing a "goto LABEL"
(with all the restrictions that goto suffers). Think of it
as a goto with an intervening core dump and reincarnation.
If LABEL is omitted, restarts the program from the top.
WARNING: any files opened at the time of the dump will NOT be
open any more when the program is reincarnated, with possible
resulting confusion on the part of perl. See also -u.
Example:
#!/usr/bin/perl
require 'getopt.pl';
require 'stat.pl';
%days = (
'Sun',1,
'Mon',2,
'Tue',3,
'Wed',4,
'Thu',5,
'Fri',6,
'Sat',7);
dump QUICKSTART if $ARGV[0] eq '-d';
QUICKSTART:
do Getopt('f');
each(ASSOC_ARRAY)
each ASSOC_ARRAY
Returns a 2 element array consisting of the key and value for
the next value of an associative array, so that you can
iterate over it. Entries are returned in an apparently
random order. When the array is entirely read, a null array
is returned (which when assigned produces a FALSE (0) value).
The next call to each() after that will start iterating
again. The iterator can be reset only by reading all the
elements from the array. You must not modify the array while
iterating over it. There is a single iterator for each
associative array, shared by all each(), keys() and values()
function calls in the program. The following prints out your
environment like the printenv program, only in a different
order:
while (($key,$value) = each %ENV) {
print "$key=$value\n";
}
See also keys() and values().
eof(FILEHANDLE)
eof()
eof Returns 1 if the next read on FILEHANDLE will return end of
file, or if FILEHANDLE is not open. FILEHANDLE may be an
expression whose value gives the real filehandle name. (Note
that this function actually reads a character and then
ungetc's it, so it is not very useful in an interactive
context.) An eof without an argument returns the eof status
for the last file read. Empty parentheses () may be used to
indicate the pseudo file formed of the files listed on the
command line, i.e. eof() is reasonable to use inside a while
(<>) loop to detect the end of only the last file. Use
eof(ARGV) or eof without the parentheses to test EACH file in
a while (<>) loop. Examples:
# insert dashes just before last line of last file
while (<>) {
if (eof()) {
print "--------------\n";
}
print;
}
# reset line numbering on each input file
while (<>) {
print "$.\t$_";
if (eof) { # Not eof().
close(ARGV);
}
}
eval(EXPR)
eval EXPR
eval BLOCK
EXPR is parsed and executed as if it were a little perl
program. It is executed in the context of the current perl
program, so that any variable settings, subroutine or format
definitions remain afterwards. The value returned is the
value of the last expression evaluated, just as with
subroutines. If there is a syntax error or runtime error, or
a die statement is executed, an undefined value is returned
by eval, and $@ is set to the error message. If there was no
error, $@ is guaranteed to be a null string. If EXPR is
omitted, evaluates $_. The final semicolon, if any, may be
omitted from the expression.
Note that, since eval traps otherwise-fatal errors, it is
useful for determining whether a particular feature (such as
dbmopen or symlink) is implemented. It is also Perl's
exception trapping mechanism, where the die operator is used
to raise exceptions.
If the code to be executed doesn't vary, you may use the
eval-BLOCK form to trap run-time errors without incurring the
penalty of recompiling each time. The error, if any, is
still returned in $@. Evaluating a single-quoted string (as
EXPR) has the same effect, except that the eval-EXPR form
reports syntax errors at run time via $@, whereas the eval-
BLOCK form reports syntax errors at compile time. The eval-
EXPR form is optimized to eval-BLOCK the first time it
succeeds. (Since the replacement side of a substitution is
considered a single-quoted string when you use the e
modifier, the same optimization occurs there.) Examples:
# make divide-by-zero non-fatal
eval { $answer = $a / $b; }; warn $@ if $@;
# optimized to same thing after first use
eval '$answer = $a / $b'; warn $@ if $@;
# a compile-time error
eval { $answer = };
# a run-time error
eval '$answer ='; # sets $@
exec(LIST)
exec LIST
If there is more than one argument in LIST, or if LIST is an
array with more than one value, calls execvp() with the
arguments in LIST. If there is only one scalar argument, the
argument is checked for shell metacharacters. If there are
any, the entire argument is passed to "/bin/sh -c" for
parsing. If there are none, the argument is split into words
and passed directly to execvp(), which is more efficient.
Note: exec (and system) do not flush your output buffer, so
you may need to set $| to avoid lost output. Examples:
exec '/bin/echo', 'Your arguments are: ', @ARGV;
exec "sort $outfile | uniq";
If you don't really want to execute the first argument, but
want to lie to the program you are executing about its own
name, you can specify the program you actually want to run by
assigning that to a variable and putting the name of the
variable in front of the LIST without a comma. (This always
forces interpretation of the LIST as a multi-valued list,
even if there is only a single scalar in the list.) Example:
$shell = '/bin/csh';
exec $shell '-sh'; # pretend it's a login shell
exit(EXPR)
exit EXPR
Evaluates EXPR and exits immediately with that value.
Example:
$ans = <STDIN>;
exit 0 if $ans =~ /^[Xx]/;
See also die. If EXPR is omitted, exits with 0 status.
exp(EXPR)
exp EXPR
Returns e to the power of EXPR. If EXPR is omitted, gives
exp($_).
fcntl(FILEHANDLE,FUNCTION,SCALAR)
Implements the fcntl(2) function. You'll probably have to
say
require "fcntl.ph"; # probably /usr/local/lib/perl/fcntl.ph
first to get the correct function definitions. If fcntl.ph
doesn't exist or doesn't have the correct definitions you'll
have to roll your own, based on your C header files such as
<sys/fcntl.h>. (There is a perl script called h2ph that
comes with the perl kit which may help you in this.)
Argument processing and value return works just like ioctl
below. Note that fcntl will produce a fatal error if used on
a machine that doesn't implement fcntl(2).
fileno(FILEHANDLE)
fileno FILEHANDLE
Returns the file descriptor for a filehandle. Useful for
constructing bitmaps for select(). If FILEHANDLE is an
expression, the value is taken as the name of the filehandle.
flock(FILEHANDLE,OPERATION)
Calls flock(2) on FILEHANDLE. See manual page for flock(2)
for definition of OPERATION. Returns true for success, false
on failure. Will produce a fatal error if used on a machine
that doesn't implement flock(2). Here's a mailbox appender
for BSD systems.
$LOCK_SH = 1;
$LOCK_EX = 2;
$LOCK_NB = 4;
$LOCK_UN = 8;
sub lock {
flock(MBOX,$LOCK_EX);
# and, in case someone appended
# while we were waiting...
seek(MBOX, 0, 2);
}
sub unlock {
flock(MBOX,$LOCK_UN);
}
open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
|| die "Can't open mailbox: $!";
do lock();
print MBOX $msg,"\n\n";
do unlock();
fork Does a fork() call. Returns the child pid to the parent
process and 0 to the child process. Note: unflushed buffers
remain unflushed in both processes, which means you may need
to set $| to avoid duplicate output.
getc(FILEHANDLE)
getc FILEHANDLE
getc Returns the next character from the input file attached to
FILEHANDLE, or a null string at EOF. If FILEHANDLE is
omitted, reads from STDIN.
getlogin
Returns the current login from /etc/utmp, if any. If null,
use getpwuid.
$login = getlogin || (getpwuid($<))[0] || "Somebody";
getpeername(SOCKET)
Returns the packed sockaddr address of other end of the
SOCKET connection.
# An internet sockaddr
$sockaddr = 'S n a4 x8';
$hersockaddr = getpeername(S);
($family, $port, $heraddr) =
unpack($sockaddr,$hersockaddr);
getpgrp(PID)
getpgrp PID
Returns the current process group for the specified PID, 0
for the current process. Will produce a fatal error if used
on a machine that doesn't implement getpgrp(2). If EXPR is
omitted, returns process group of current process.
getppid Returns the process id of the parent process.
getpriority(WHICH,WHO)
Returns the current priority for a process, a process group,
or a user. (See getpriority(2).) Will produce a fatal error
if used on a machine that doesn't implement getpriority(2).
getpwnam(NAME)
getgrnam(NAME)
gethostbyname(NAME)
getnetbyname(NAME)
getprotobyname(NAME)
getpwuid(UID)
getgrgid(GID)
getservbyname(NAME,PROTO)
gethostbyaddr(ADDR,ADDRTYPE)
getnetbyaddr(ADDR,ADDRTYPE)
getprotobynumber(NUMBER)
getservbyport(PORT,PROTO)
getpwent
getgrent
gethostent
getnetent
getprotoent
getservent
setpwent
setgrent
sethostent(STAYOPEN)
setnetent(STAYOPEN)
setprotoent(STAYOPEN)
setservent(STAYOPEN)
endpwent
endgrent
endhostent
endnetent
endprotoent
endservent
These routines perform the same functions as their
counterparts in the system library. Within an array context,
the return values from the various get routines are as
follows:
($name,$passwd,$uid,$gid,
$quota,$comment,$gcos,$dir,$shell) = getpw...
($name,$passwd,$gid,$members) = getgr...
($name,$aliases,$addrtype,$length,@addrs) = gethost...
($name,$aliases,$addrtype,$net) = getnet...
($name,$aliases,$proto) = getproto...
($name,$aliases,$port,$proto) = getserv...
(If the entry doesn't exist you get a null list.)
Within a scalar context, you get the name, unless the
function was a lookup by name, in which case you get the
other thing, whatever it is. (If the entry doesn't exist you
get the undefined value.) For example:
$uid = getpwnam
$name = getpwuid
$name = getpwent
$gid = getgrnam
$name = getgrgid
$name = getgrent
etc.
The $members value returned by getgr... is a space separated
list of the login names of the members of the group.
For the gethost... functions, if the h_errno variable is
supported in C, it will be returned to you via $? if the
function call fails. The @addrs value returned by a
successful call is a list of the raw addresses returned by
the corresponding system library call. In the Internet
domain, each address is four bytes long and you can unpack it
by saying something like:
($a,$b,$c,$d) = unpack('C4',$addr[0]);
getsockname(SOCKET)
Returns the packed sockaddr address of this end of the SOCKET
connection.
# An internet sockaddr
$sockaddr = 'S n a4 x8';
$mysockaddr = getsockname(S);
($family, $port, $myaddr) =
unpack($sockaddr,$mysockaddr);
getsockopt(SOCKET,LEVEL,OPTNAME)
Returns the socket option requested, or undefined if there is
an error.
gmtime(EXPR)
gmtime EXPR
Converts a time as returned by the time function to a
9-element array with the time analyzed for the Greenwich
timezone. Typically used as follows:
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
gmtime(time);
All array elements are numeric, and come straight out of a
struct tm. In particular this means that $mon has the range
0..11 and $wday has the range 0..6. If EXPR is omitted, does
gmtime(time).
goto LABEL
Finds the statement labeled with LABEL and resumes execution
there. Currently you may only go to statements in the main
body of the program that are not nested inside a do {}
construct. This statement is not implemented very
efficiently, and is here only to make the sed-to-perl
translator easier. I may change its semantics at any time,
consistent with support for translated sed scripts. Use it
at your own risk. Better yet, don't use it at all.
grep(EXPR,LIST)
Evaluates EXPR for each element of LIST (locally setting $_
to each element) and returns the array value consisting of
those elements for which the expression evaluated to true.
In a scalar context, returns the number of times the
expression was true.
@foo = grep(!/^#/, @bar); # weed out comments
Note that, since $_ is a reference into the array value, it
can be used to modify the elements of the array. While this
is useful and supported, it can cause bizarre results if the
LIST is not a named array.
hex(EXPR)
hex EXPR
Returns the decimal value of EXPR interpreted as an hex
string. (To interpret strings that might start with 0 or 0x
see oct().) If EXPR is omitted, uses $_.
index(STR,SUBSTR,POSITION)
index(STR,SUBSTR)
Returns the position of the first occurrence of SUBSTR in STR
at or after POSITION. If POSITION is omitted, starts
searching from the beginning of the string. The return value
is based at 0, or whatever you've set the $[ variable to. If
the substring is not found, returns one less than the base,
ordinarily -1.
int(EXPR)
int EXPR
Returns the integer portion of EXPR. If EXPR is omitted,
uses $_.
ioctl(FILEHANDLE,FUNCTION,SCALAR)
Implements the ioctl(2) function. You'll probably have to
say
require "ioctl.ph"; # probably /usr/local/lib/perl/ioctl.ph
first to get the correct function definitions. If ioctl.ph
doesn't exist or doesn't have the correct definitions you'll
have to roll your own, based on your C header files such as
<sys/ioctl.h>. (There is a perl script called h2ph that
comes with the perl kit which may help you in this.) SCALAR
will be read and/or written depending on the FUNCTION--a
pointer to the string value of SCALAR will be passed as the
third argument of the actual ioctl call. (If SCALAR has no
string value but does have a numeric value, that value will
be passed rather than a pointer to the string value. To
guarantee this to be true, add a 0 to the scalar before using
it.) The pack() and unpack() functions are useful for
manipulating the values of structures used by ioctl(). The
following example sets the erase character to DEL.
require 'ioctl.ph';
$sgttyb_t = "ccccs"; # 4 chars and a short
if (ioctl(STDIN,$TIOCGETP,$sgttyb)) {
@ary = unpack($sgttyb_t,$sgttyb);
$ary[2] = 127;
$sgttyb = pack($sgttyb_t,@ary);
ioctl(STDIN,$TIOCSETP,$sgttyb)
|| die "Can't ioctl: $!";
}
The return value of ioctl (and fcntl) is as follows:
if OS returns: perl returns:
-1 undefined value
0 string "0 but true"
anything else that number
Thus perl returns true on success and false on failure, yet
you can still easily determine the actual value returned by
the operating system:
($retval = ioctl(...)) || ($retval = -1);
printf "System returned %d\n", $retval;
join(EXPR,LIST)
join(EXPR,ARRAY)
Joins the separate strings of LIST or ARRAY into a single
string with fields separated by the value of EXPR, and
returns the string. Example:
$_ = join(':',
$login,$passwd,$uid,$gid,$gcos,$home,$shell);
See split.
keys(ASSOC_ARRAY)
keys ASSOC_ARRAY
Returns a normal array consisting of all the keys of the
named associative array. The keys are returned in an
apparently random order, but it is the same order as either
the values() or each() function produces (given that the
associative array has not been modified). Here is yet
another way to print your environment:
@keys = keys %ENV;
@values = values %ENV;
while ($#keys >= 0) {
print pop(@keys), '=', pop(@values), "\n";
}
or how about sorted by key:
foreach $key (sort(keys %ENV)) {
print $key, '=', $ENV{$key}, "\n";
}
kill(LIST)
kill LIST
Sends a signal to a list of processes. The first element of
the list must be the signal to send. Returns the number of
processes successfully signaled.
$cnt = kill 1, $child1, $child2;
kill 9, @goners;
If the signal is negative, kills process groups instead of
processes. (On System V, a negative process number will also
kill process groups, but that's not portable.) You may use a
signal name in quotes.
last LABEL
last The last command is like the break statement in C (as used in
loops); it immediately exits the loop in question. If the
LABEL is omitted, the command refers to the innermost
enclosing loop. The continue block, if any, is not executed:
line: while (<STDIN>) {
last line if /^$/; # exit when done with header
...
}
length(EXPR)
length EXPR
Returns the length in characters of the value of EXPR. If
EXPR is omitted, returns length of $_.
link(OLDFILE,NEWFILE)
Creates a new filename linked to the old filename. Returns 1
for success, 0 otherwise.
listen(SOCKET,QUEUESIZE)
Does the same thing that the listen system call does.
Returns true if it succeeded, false otherwise. See example
in section on Interprocess Communication.
local(LIST)
Declares the listed variables to be local to the enclosing
block, subroutine, eval or "do". All the listed elements
must be legal lvalues. This operator works by saving the
current values of those variables in LIST on a hidden stack
and restoring them upon exiting the block, subroutine or
eval. This means that called subroutines can also reference
the local variable, but not the global one. The LIST may be
assigned to if desired, which allows you to initialize your
local variables. (If no initializer is given for a
particular variable, it is created with an undefined value.)
Commonly this is used to name the parameters to a subroutine.
Examples:
sub RANGEVAL {
local($min, $max, $thunk) = @_;
local($result) = '';
local($i);
# Presumably $thunk makes reference to $i
for ($i = $min; $i < $max; $i++) {
$result .= eval $thunk;
}
$result;
}
if ($sw eq '-v') {
# init local array with global array
local(@ARGV) = @ARGV;
unshift(@ARGV,'echo');
system @ARGV;
}
# @ARGV restored
# temporarily add to digits associative array
if ($base12) {
# (NOTE: not claiming this is efficient!)
local(%digits) = (%digits,'t',10,'e',11);
do parse_num();
}
Note that local() is a run-time command, and so gets executed
every time through a loop, using up more stack storage each
time until it's all released at once when the loop is exited.
localtime(EXPR)
localtime EXPR
Converts a time as returned by the time function to a
9-element array with the time analyzed for the local
timezone. Typically used as follows:
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
localtime(time);
All array elements are numeric, and come straight out of a
struct tm. In particular this means that $mon has the range
0..11 and $wday has the range 0..6. If EXPR is omitted, does
localtime(time).
log(EXPR)
log EXPR
Returns logarithm (base e) of EXPR. If EXPR is omitted,
returns log of $_.
lstat(FILEHANDLE)
lstat FILEHANDLE
lstat(EXPR)
lstat SCALARVARIABLE
Does the same thing as the stat() function, but stats a
symbolic link instead of the file the symbolic link points
to. If symbolic links are unimplemented on your system, a
normal stat is done.
m/PATTERN/gio
/PATTERN/gio
Searches a string for a pattern match, and returns true (1)
or false (''). If no string is specified via the =~ or !~
operator, the $_ string is searched. (The string specified
with =~ need not be an lvalue--it may be the result of an
expression evaluation, but remember the =~ binds rather
tightly.) See also the section on regular expressions.
If / is the delimiter then the initial 'm' is optional. With
the 'm' you can use any pair of non-alphanumeric characters
as delimiters. This is particularly useful for matching Unix
path names that contain '/'. If the final delimiter is
followed by the optional letter 'i', the matching is done in
a case-insensitive manner. PATTERN may contain references to
scalar variables, which will be interpolated (and the pattern
recompiled) every time the pattern search is evaluated.
(Note that $) and $| may not be interpolated because they
look like end-of-string tests.) If you want such a pattern
to be compiled only once, add an "o" after the trailing
delimiter. This avoids expensive run-time recompilations,
and is useful when the value you are interpolating won't
change over the life of the script. If the PATTERN evaluates
to a null string, the most recent successful regular
expression is used instead.
If used in a context that requires an array value, a pattern
match returns an array consisting of the subexpressions
matched by the parentheses in the pattern, i.e. ($1, $2,
$3...). It does NOT actually set $1, $2, etc. in this case,
nor does it set $+, $`, $& or $'. If the match fails, a null
array is returned. If the match succeeds, but there were no
parentheses, an array value of (1) is returned.
Examples:
open(tty, '/dev/tty');
<tty> =~ /^y/i && do foo(); # do foo if desired
if (/Version: *([0-9.]*)/) { $version = $1; }
next if m#^/usr/spool/uucp#;
# poor man's grep
$arg = shift;
while (<>) {
print if /$arg/o; # compile only once
}
if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
This last example splits $foo into the first two words and
the remainder of the line, and assigns those three fields to
$F1, $F2 and $Etc. The conditional is true if any variables
were assigned, i.e. if the pattern matched.
The "g" modifier specifies global pattern matching--that is,
matching as many times as possible within the string. How it
behaves depends on the context. In an array context, it
returns a list of all the substrings matched by all the
parentheses in the regular expression. If there are no
parentheses, it returns a list of all the matched strings, as
if there were parentheses around the whole pattern. In a
scalar context, it iterates through the string, returning
TRUE each time it matches, and FALSE when it eventually runs
out of matches. (In other words, it remembers where it left
off last time and restarts the search at that point.) It
presumes that you have not modified the string since the last
match. Modifying the string between matches may result in
undefined behavior. (You can actually get away with in-place
modifications via substr() that do not change the length of
the entire string. In general, however, you should be using
s///g for such modifications.) Examples:
# array context
($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
# scalar context
$/ = ""; $* = 1;
while ($paragraph = <>) {
while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
$sentences++;
}
}
print "$sentences\n";
mkdir(FILENAME,MODE)
Creates the directory specified by FILENAME, with permissions
specified by MODE (as modified by umask). If it succeeds it
returns 1, otherwise it returns 0 and sets $! (errno).
msgctl(ID,CMD,ARG)
Calls the System V IPC function msgctl. If CMD is &IPC_STAT,
then ARG must be a variable which will hold the returned
msqid_ds structure. Returns like ioctl: the undefined value
for error, "0 but true" for zero, or the actual return value
otherwise.
msgget(KEY,FLAGS)
Calls the System V IPC function msgget. Returns the message
queue id, or the undefined value if there is an error.
msgsnd(ID,MSG,FLAGS)
Calls the System V IPC function msgsnd to send the message
MSG to the message queue ID. MSG must begin with the long
integer message type, which may be created with pack("L",
$type). Returns true if successful, or false if there is an
error.
msgrcv(ID,VAR,SIZE,TYPE,FLAGS)
Calls the System V IPC function msgrcv to receive a message
from message queue ID into variable VAR with a maximum
message size of SIZE. Note that if a message is received,
the message type will be the first thing in VAR, and the
maximum length of VAR is SIZE plus the size of the message
type. Returns true if successful, or false if there is an
error.
next LABEL
next The next command is like the continue statement in C; it
starts the next iteration of the loop:
line: while (<STDIN>) {
next line if /^#/; # discard comments
...
}
Note that if there were a continue block on the above, it
would get executed even on discarded lines. If the LABEL is
omitted, the command refers to the innermost enclosing loop.
oct(EXPR)
oct EXPR
Returns the decimal value of EXPR interpreted as an octal
string. (If EXPR happens to start off with 0x, interprets it
as a hex string instead.) The following will handle decimal,
octal and hex in the standard notation:
$val = oct($val) if $val =~ /^0/;
If EXPR is omitted, uses $_.
open(FILEHANDLE,EXPR)
open(FILEHANDLE)
open FILEHANDLE
Opens the file whose filename is given by EXPR, and
associates it with FILEHANDLE. If FILEHANDLE is an
expression, its value is used as the name of the real
filehandle wanted. If EXPR is omitted, the scalar variable
of the same name as the FILEHANDLE contains the filename. If
the filename begins with "<" or nothing, the file is opened
for input. If the filename begins with ">", the file is
opened for output. If the filename begins with ">>", the
file is opened for appending. (You can put a '+' in front of
the '>' or '<' to indicate that you want both read and write
access to the file.) If the filename begins with "|", the
filename is interpreted as a command to which output is to be
piped, and if the filename ends with a "|", the filename is
interpreted as command which pipes input to us. (You may not
have a command that pipes both in and out.) Opening '-'
opens STDIN and opening '>-' opens STDOUT. Open returns non-
zero upon success, the undefined value otherwise. If the
open involved a pipe, the return value happens to be the pid
of the subprocess. Examples:
$article = 100;
open article || die "Can't find article $article: $!\n";
while (<article>) {...
open(LOG, '>>/usr/spool/news/twitlog');
# (log is reserved)
open(article, "caesar <$article |");
# decrypt article
open(extract, "|sort >/tmp/Tmp$$");
# $$ is our process#
# process argument list of files along with any includes
foreach $file (@ARGV) {
do process($file, 'fh00'); # no pun intended
}
sub process {
local($filename, $input) = @_;
$input++; # this is a string increment
unless (open($input, $filename)) {
print STDERR "Can't open $filename: $!\n";
return;
}
while (<$input>) { # note use of indirection
if (/^#include "(.*)"/) {
do process($1, $input);
next;
}
... # whatever
}
}
You may also, in the Bourne shell tradition, specify an EXPR
beginning with ">&", in which case the rest of the string is
interpreted as the name of a filehandle (or file descriptor,
if numeric) which is to be duped and opened. You may use &
after >, >>, <, +>, +>> and +<. The mode you specify should
match the mode of the original filehandle. Here is a script
that saves, redirects, and restores STDOUT and STDERR:
#!/usr/bin/perl
open(SAVEOUT, ">&STDOUT");
open(SAVEERR, ">&STDERR");
open(STDOUT, ">foo.out") || die "Can't redirect stdout";
open(STDERR, ">&STDOUT") || die "Can't dup stdout";
select(STDERR); $| = 1; # make unbuffered
select(STDOUT); $| = 1; # make unbuffered
print STDOUT "stdout 1\n"; # this works for
print STDERR "stderr 1\n"; # subprocesses too
close(STDOUT);
close(STDERR);
open(STDOUT, ">&SAVEOUT");
open(STDERR, ">&SAVEERR");
print STDOUT "stdout 2\n";
print STDERR "stderr 2\n";
If you open a pipe on the command "-", i.e. either "|-" or
"-|", then there is an implicit fork done, and the return
value of open is the pid of the child within the parent
process, and 0 within the child process. (Use defined($pid)
to determine if the open was successful.) The filehandle
behaves normally for the parent, but i/o to that filehandle
is piped from/to the STDOUT/STDIN of the child process. In
the child process the filehandle isn't opened--i/o happens
from/to the new STDOUT or STDIN. Typically this is used like
the normal piped open when you want to exercise more control
over just how the pipe command gets executed, such as when
you are running setuid, and don't want to have to scan shell
commands for metacharacters. The following pairs are more or
less equivalent:
open(FOO, "|tr '[a-z]' '[A-Z]'");
open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
open(FOO, "cat -n '$file'|");
open(FOO, "-|") || exec 'cat', '-n', $file;
Explicitly closing any piped filehandle causes the parent
process to wait for the child to finish, and returns the
status value in $?. Note: on any operation which may do a
fork, unflushed buffers remain unflushed in both processes,
which means you may need to set $| to avoid duplicate output.
The filename that is passed to open will have leading and
trailing whitespace deleted. In order to open a file with
arbitrary weird characters in it, it's necessary to protect
any leading and trailing whitespace thusly:
$file =~ s#^(\s)#./$1#;
open(FOO, "< $file\0");
opendir(DIRHANDLE,EXPR)
Opens a directory named EXPR for processing by readdir(),
telldir(), seekdir(), rewinddir() and closedir(). Returns
true if successful. DIRHANDLEs have their own namespace
separate from FILEHANDLEs.
ord(EXPR)
ord EXPR
Returns the numeric ascii value of the first character of
EXPR. If EXPR is omitted, uses $_.
pack(TEMPLATE,LIST)
Takes an array or list of values and packs it into a binary
structure, returning the string containing the structure.
The TEMPLATE is a sequence of characters that give the order
and type of values, as follows:
A An ascii string, will be space padded.
a An ascii string, will be null padded.
c A signed char value.
C An unsigned char value.
s A signed short value.
S An unsigned short value.
i A signed integer value.
I An unsigned integer value.
l A signed long value.
L An unsigned long value.
n A short in "network" order.
N A long in "network" order.
f A single-precision float in the native format.
d A double-precision float in the native format.
p A pointer to a string.
v A short in "VAX" (little-endian) order.
V A long in "VAX" (little-endian) order.
x A null byte.
X Back up a byte.
@ Null fill to absolute position.
u A uuencoded string.
b A bit string (ascending bit order, like vec()).
B A bit string (descending bit order).
h A hex string (low nybble first).
H A hex string (high nybble first).
Each letter may optionally be followed by a number which
gives a repeat count. With all types except "a", "A", "b",
"B", "h" and "H", the pack function will gobble up that many
values from the LIST. A * for the repeat count means to use
however many items are left. The "a" and "A" types gobble
just one value, but pack it as a string of length count,
padding with nulls or spaces as necessary. (When unpacking,
"A" strips trailing spaces and nulls, but "a" does not.)
Likewise, the "b" and "B" fields pack a string that many bits
long. The "h" and "H" fields pack a string that many nybbles
long. Real numbers (floats and doubles) are in the native
machine format only; due to the multiplicity of floating
formats around, and the lack of a standard "network"
representation, no facility for interchange has been made.
This means that packed floating point data written on one
machine may not be readable on another - even if both use
IEEE floating point arithmetic (as the endian-ness of the
memory representation is not part of the IEEE spec). Note
that perl uses doubles internally for all numeric
calculation, and converting from double -> float -> double
will lose precision (i.e. unpack("f", pack("f", $foo)) will
not in general equal $foo).
Examples:
$foo = pack("cccc",65,66,67,68);
# foo eq "ABCD"
$foo = pack("c4",65,66,67,68);
# same thing
$foo = pack("ccxxcc",65,66,67,68);
# foo eq "AB\0\0CD"
$foo = pack("s2",1,2);
# "\1\0\2\0" on little-endian
# "\0\1\0\2" on big-endian
$foo = pack("a4","abcd","x","y","z");
# "abcd"
$foo = pack("aaaa","abcd","x","y","z");
# "axyz"
$foo = pack("a14","abcdefg");
# "abcdefg\0\0\0\0\0\0\0"
$foo = pack("i9pl", gmtime);
# a real struct tm (on my system anyway)
sub bintodec {
unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
}
The same template may generally also be used in the unpack
function.
pipe(READHANDLE,WRITEHANDLE)
Opens a pair of connected pipes like the corresponding system
call. Note that if you set up a loop of piped processes,
deadlock can occur unless you are very careful. In addition,
note that perl's pipes use stdio buffering, so you may need
to set $| to flush your WRITEHANDLE after each command,
depending on the application. [Requires version 3.0
patchlevel 9.]
pop(ARRAY)
pop ARRAY
Pops and returns the last value of the array, shortening the
array by 1. Has the same effect as
$tmp = $ARRAY[$#ARRAY--];
If there are no elements in the array, returns the undefined
value.
print(FILEHANDLE LIST)
print(LIST)
print FILEHANDLE LIST
print LIST
print Prints a string or a comma-separated list of strings.
Returns non-zero if successful. FILEHANDLE may be a scalar
variable name, in which case the variable contains the name
of the filehandle, thus introducing one level of indirection.
(NOTE: If FILEHANDLE is a variable and the next token is a
term, it may be misinterpreted as an operator unless you
interpose a + or put parens around the arguments.) If
FILEHANDLE is omitted, prints by default to standard output
(or to the last selected output channel--see select()). If
LIST is also omitted, prints $_ to STDOUT. To set the
default output channel to something other than STDOUT use the
select operation. Note that, because print takes a LIST,
anything in the LIST is evaluated in an array context, and
any subroutine that you call will have one or more of its
expressions evaluated in an array context. Also be careful
not to follow the print keyword with a left parenthesis
unless you want the corresponding right parenthesis to
terminate the arguments to the print--interpose a + or put
parens around all the arguments.
printf(FILEHANDLE LIST)
printf(LIST)
printf FILEHANDLE LIST
printf LIST
Equivalent to a "print FILEHANDLE sprintf(LIST)".
push(ARRAY,LIST)
Treats ARRAY (@ is optional) as a stack, and pushes the
values of LIST onto the end of ARRAY. The length of ARRAY
increases by the length of LIST. Has the same effect as
for $value (LIST) {
$ARRAY[++$#ARRAY] = $value;
}
but is more efficient.
q/STRING/
qq/STRING/
qx/STRING/
These are not really functions, but simply syntactic sugar to
let you avoid putting too many backslashes into quoted
strings. The q operator is a generalized single quote, and
the qq operator a generalized double quote. The qx operator
is a generalized backquote. Any non-alphanumeric delimiter
can be used in place of /, including newline. If the
delimiter is an opening bracket or parenthesis, the final
delimiter will be the corresponding closing bracket or
parenthesis. (Embedded occurrences of the closing bracket
need to be backslashed as usual.) Examples:
$foo = q!I said, "You said, 'She said it.'"!;
$bar = q('This is it.');
$today = qx{ date };
$_ .= qq
*** The previous line contains the naughty word "$&".\n
if /(ibm|apple|awk)/; # :-)
rand(EXPR)
rand EXPR
rand Returns a random fractional number between 0 and the value of
EXPR. (EXPR should be positive.) If EXPR is omitted,
returns a value between 0 and 1. See also srand().
read(FILEHANDLE,SCALAR,LENGTH,OFFSET)
read(FILEHANDLE,SCALAR,LENGTH)
Attempts to read LENGTH bytes of data into variable SCALAR
from the specified FILEHANDLE. Returns the number of bytes
actually read, or undef if there was an error. SCALAR will
be grown or shrunk to the length actually read. An OFFSET
may be specified to place the read data at some other place
than the beginning of the string. This call is actually
implemented in terms of stdio's fread call. To get a true
read system call, see sysread.
readdir(DIRHANDLE)
readdir DIRHANDLE
Returns the next directory entry for a directory opened by
opendir(). If used in an array context, returns all the rest
of the entries in the directory. If there are no more
entries, returns an undefined value in a scalar context or a
null list in an array context.
readlink(EXPR)
readlink EXPR
Returns the value of a symbolic link, if symbolic links are
implemented. If not, gives a fatal error. If there is some
system error, returns the undefined value and sets $!
(errno). If EXPR is omitted, uses $_.
recv(SOCKET,SCALAR,LEN,FLAGS)
Receives a message on a socket. Attempts to receive LENGTH
bytes of data into variable SCALAR from the specified SOCKET
filehandle. Returns the address of the sender, or the
undefined value if there's an error. SCALAR will be grown or
shrunk to the length actually read. Takes the same flags as
the system call of the same name.
redo LABEL
redo The redo command restarts the loop block without evaluating
the conditional again. The continue block, if any, is not
executed. If the LABEL is omitted, the command refers to the
innermost enclosing loop. This command is normally used by
programs that want to lie to themselves about what was just
input:
# a simpleminded Pascal comment stripper
# (warning: assumes no { or } in strings)
line: while (<STDIN>) {
while (s|({.*}.*){.*}|$1 |) {}
s|{.*}| |;
if (s|{.*| |) {
$front = $_;
while (<STDIN>) {
if (/}/) { # end of comment?
s|^|$front{|;
redo line;
}
}
}
print;
}
rename(OLDNAME,NEWNAME)
Changes the name of a file. Returns 1 for success, 0
otherwise. Will not work across filesystem boundaries.
require(EXPR)
require EXPR
require Includes the library file specified by EXPR, or by $_ if EXPR
is not supplied. Has semantics similar to the following
subroutine:
sub require {
local($filename) = @_;
return 1 if $INC{$filename};
local($realfilename,$result);
ITER: {
foreach $prefix (@INC) {
$realfilename = "$prefix/$filename";
if (-f $realfilename) {
$result = do $realfilename;
last ITER;
}
}
die "Can't find $filename in \@INC";
}
die $@ if $@;
die "$filename did not return true value" unless $result;
$INC{$filename} = $realfilename;
$result;
}
Note that the file will not be included twice under the same
specified name. The file must return true as the last
statement to indicate successful execution of any
initialization code, so it's customary to end such a file
with "1;" unless you're sure it'll return true otherwise.
reset(EXPR)
reset EXPR
reset Generally used in a continue block at the end of a loop to
clear variables and reset ?? searches so that they work
again. The expression is interpreted as a list of single
characters (hyphens allowed for ranges). All variables and
arrays beginning with one of those letters are reset to their
pristine state. If the expression is omitted, one-match
searches (?pattern?) are reset to match again. Only resets
variables or searches in the current package. Always returns
1. Examples:
reset 'X'; # reset all X variables
reset 'a-z'; # reset lower case variables
reset; # just reset ?? searches
Note: resetting "A-Z" is not recommended since you'll wipe
out your ARGV and ENV arrays.
The use of reset on dbm associative arrays does not change
the dbm file. (It does, however, flush any entries cached by
perl, which may be useful if you are sharing the dbm file.
Then again, maybe not.)
return LIST
Returns from a subroutine with the value specified. (Note
that a subroutine can automatically return the value of the
last expression evaluated. That's the preferred method--use
of an explicit return is a bit slower.)
reverse(LIST)
reverse LIST
In an array context, returns an array value consisting of the
elements of LIST in the opposite order. In a scalar context,
returns a string value consisting of the bytes of the first
element of LIST in the opposite order.
rewinddir(DIRHANDLE)
rewinddir DIRHANDLE
Sets the current position to the beginning of the directory
for the readdir() routine on DIRHANDLE.
rindex(STR,SUBSTR,POSITION)
rindex(STR,SUBSTR)
Works just like index except that it returns the position of
the LAST occurrence of SUBSTR in STR. If POSITION is
specified, returns the last occurrence at or before that
position.
rmdir(FILENAME)
rmdir FILENAME
Deletes the directory specified by FILENAME if it is empty.
If it succeeds it returns 1, otherwise it returns 0 and sets
$! (errno). If FILENAME is omitted, uses $_.
s/PATTERN/REPLACEMENT/gieo
Searches a string for a pattern, and if found, replaces that
pattern with the replacement text and returns the number of
substitutions made. Otherwise it returns false (0). The "g"
is optional, and if present, indicates that all occurrences
of the pattern are to be replaced. The "i" is also optional,
and if present, indicates that matching is to be done in a
case-insensitive manner. The "e" is likewise optional, and
if present, indicates that the replacement string is to be
evaluated as an expression rather than just as a double-
quoted string. Any non-alphanumeric delimiter may replace
the slashes; if single quotes are used, no interpretation is
done on the replacement string (the e modifier overrides
this, however); if backquotes are used, the replacement
string is a command to execute whose output will be used as
the actual replacement text. If the PATTERN is delimited by
bracketing quotes, the REPLACEMENT has its own pair of
quotes, which may or may not be bracketing quotes, e.g.
s(foo)(bar) or s<foo>/bar/. If no string is specified via
the =~ or !~ operator, the $_ string is searched and
modified. (The string specified with =~ must be a scalar
variable, an array element, or an assignment to one of those,
i.e. an lvalue.) If the pattern contains a $ that looks like
a variable rather than an end-of-string test, the variable
will be interpolated into the pattern at run-time. If you
only want the pattern compiled once the first time the
variable is interpolated, add an "o" at the end. If the
PATTERN evaluates to a null string, the most recent
successful regular expression is used instead. See also the
section on regular expressions. Examples:
s/\bgreen\b/mauve/g; # don't change wintergreen
$path =~ s|/usr/bin|/usr/local/bin|;
s/Login: $foo/Login: $bar/; # run-time pattern
($foo = $bar) =~ s/bar/foo/;
$_ = 'abc123xyz';
s/\d+/$&*2/e; # yields 'abc246xyz'
s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
(Note the use of $ instead of \ in the last example. See
section on regular expressions.)
scalar(EXPR)
Forces EXPR to be interpreted in a scalar context and returns
the value of EXPR.
seek(FILEHANDLE,POSITION,WHENCE)
Randomly positions the file pointer for FILEHANDLE, just like
the fseek() call of stdio. FILEHANDLE may be an expression
whose value gives the name of the filehandle. Returns 1 upon
success, 0 otherwise.
seekdir(DIRHANDLE,POS)
Sets the current position for the readdir() routine on
DIRHANDLE. POS must be a value returned by telldir(). Has
the same caveats about possible directory compaction as the
corresponding system library routine.
select(FILEHANDLE)
select Returns the currently selected filehandle. Sets the current
default filehandle for output, if FILEHANDLE is supplied.
This has two effects: first, a write or a print without a
filehandle will default to this FILEHANDLE. Second,
references to variables related to output will refer to this
output channel. For example, if you have to set the top of
form format for more than one output channel, you might do
the following:
select(REPORT1);
$^ = 'report1_top';
select(REPORT2);
$^ = 'report2_top';
FILEHANDLE may be an expression whose value gives the name of
the actual filehandle. Thus:
$oldfh = select(STDERR); $| = 1; select($oldfh);
select(RBITS,WBITS,EBITS,TIMEOUT)
This calls the select system call with the bitmasks
specified, which can be constructed using fileno() and vec(),
along these lines:
$rin = $win = $ein = '';
vec($rin,fileno(STDIN),1) = 1;
vec($win,fileno(STDOUT),1) = 1;
$ein = $rin | $win;
If you want to select on many filehandles you might wish to
write a subroutine:
sub fhbits {
local(@fhlist) = split(' ',$_[0]);
local($bits);
for (@fhlist) {
vec($bits,fileno($_),1) = 1;
}
$bits;
}
$rin = &fhbits('STDIN TTY SOCK');
The usual idiom is:
($nfound,$timeleft) =
select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
or to block until something becomes ready:
$nfound = select($rout=$rin, $wout=$win,
$eout=$ein, undef);
Any of the bitmasks can also be undef. The timeout, if
specified, is in seconds, which may be fractional. NOTE: not
all implementations are capable of returning the $timeleft.
If not, they always return $timeleft equal to the supplied
$timeout.
semctl(ID,SEMNUM,CMD,ARG)
Calls the System V IPC function semctl. If CMD is &IPC_STAT
or &GETALL, then ARG must be a variable which will hold the
returned semid_ds structure or semaphore value array.
Returns like ioctl: the undefined value for error, "0 but
true" for zero, or the actual return value otherwise.
semget(KEY,NSEMS,SIZE,FLAGS)
Calls the System V IPC function semget. Returns the
semaphore id, or the undefined value if there is an error.
semop(KEY,OPSTRING)
Calls the System V IPC function semop to perform semaphore
operations such as signaling and waiting. OPSTRING must be a
packed array of semop structures. Each semop structure can
be generated with 'pack("sss", $semnum, $semop, $semflag)'.
The number of semaphore operations is implied by the length
of OPSTRING. Returns true if successful, or false if there
is an error. As an example, the following code waits on
semaphore $semnum of semaphore id $semid:
$semop = pack("sss", $semnum, -1, 0);
die "Semaphore trouble: $!\n" unless semop($semid, $semop);
To signal the semaphore, replace "-1" with "1".
send(SOCKET,MSG,FLAGS,TO)
send(SOCKET,MSG,FLAGS)
Sends a message on a socket. Takes the same flags as the
system call of the same name. On unconnected sockets you
must specify a destination to send TO. Returns the number of
characters sent, or the undefined value if there is an error.
setpgrp(PID,PGRP)
Sets the current process group for the specified PID, 0 for
the current process. Will produce a fatal error if used on a
machine that doesn't implement setpgrp(2).
setpriority(WHICH,WHO,PRIORITY)
Sets the current priority for a process, a process group, or
a user. (See setpriority(2).) Will produce a fatal error if
used on a machine that doesn't implement setpriority(2).
setsockopt(SOCKET,LEVEL,OPTNAME,OPTVAL)
Sets the socket option requested. Returns undefined if there
is an error. OPTVAL may be specified as undef if you don't
want to pass an argument.
shift(ARRAY)
shift ARRAY
shift Shifts the first value of the array off and returns it,
shortening the array by 1 and moving everything down. If
there are no elements in the array, returns the undefined
value. If ARRAY is omitted, shifts the @ARGV array in the
main program, and the @_ array in subroutines. (This is
determined lexically.) See also unshift(), push() and pop().
Shift() and unshift() do the same thing to the left end of an
array that push() and pop() do to the right end.
shmctl(ID,CMD,ARG)
Calls the System V IPC function shmctl. If CMD is &IPC_STAT,
then ARG must be a variable which will hold the returned
shmid_ds structure. Returns like ioctl: the undefined value
for error, "0 but true" for zero, or the actual return value
otherwise.
shmget(KEY,SIZE,FLAGS)
Calls the System V IPC function shmget. Returns the shared
memory segment id, or the undefined value if there is an
error.
shmread(ID,VAR,POS,SIZE)
shmwrite(ID,STRING,POS,SIZE)
Reads or writes the System V shared memory segment ID
starting at position POS for size SIZE by attaching to it,
copying in/out, and detaching from it. When reading, VAR
must be a variable which will hold the data read. When
writing, if STRING is too long, only SIZE bytes are used; if
STRING is too short, nulls are written to fill out SIZE
bytes. Return true if successful, or false if there is an
error.
shutdown(SOCKET,HOW)
Shuts down a socket connection in the manner indicated by
HOW, which has the same interpretation as in the system call
of the same name.
sin(EXPR)
sin EXPR
Returns the sine of EXPR (expressed in radians). If EXPR is
omitted, returns sine of $_.
sleep(EXPR)
sleep EXPR
sleep Causes the script to sleep for EXPR seconds, or forever if no
EXPR. May be interrupted by sending the process a SIGALRM.
Returns the number of seconds actually slept. You probably
cannot mix alarm() and sleep() calls, since sleep() is often
implemented using alarm().
socket(SOCKET,DOMAIN,TYPE,PROTOCOL)
Opens a socket of the specified kind and attaches it to
filehandle SOCKET. DOMAIN, TYPE and PROTOCOL are specified
the same as for the system call of the same name. You may
need to run h2ph on sys/socket.h to get the proper values
handy in a perl library file. Return true if successful.
See the example in the section on Interprocess Communication.
socketpair(SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL)
Creates an unnamed pair of sockets in the specified domain,
of the specified type. DOMAIN, TYPE and PROTOCOL are
specified the same as for the system call of the same name.
If unimplemented, yields a fatal error. Return true if
successful.
sort(SUBROUTINE LIST)
sort(LIST)
sort SUBROUTINE LIST
sort BLOCK LIST
sort LIST
Sorts the LIST and returns the sorted array value.
Nonexistent values of arrays are stripped out. If SUBROUTINE
or BLOCK is omitted, sorts in standard string comparison
order. If SUBROUTINE is specified, gives the name of a
subroutine that returns an integer less than, equal to, or
greater than 0, depending on how the elements of the array
are to be ordered. (The <=> and cmp operators are extremely
useful in such routines.) SUBROUTINE may be a scalar
variable name, in which case the value provides the name of
the subroutine to use. In place of a SUBROUTINE name, you
can provide a BLOCK as an anonymous, in-line sort subroutine.
In the interests of efficiency the normal calling code for
subroutines is bypassed, with the following effects: the
subroutine may not be a recursive subroutine, and the two
elements to be compared are passed into the subroutine not
via @_ but as $a and $b (see example below). They are passed
by reference so don't modify $a and $b.
Examples:
# sort lexically
@articles = sort @files;
# same thing, but with explicit sort routine
@articles = sort {$a cmp $b} @files;
# same thing in reversed order
@articles = sort {$b cmp $a} @files;
# sort numerically ascending
@articles = sort {$a <=> $b} @files;
# sort numerically descending
@articles = sort {$b <=> $a} @files;
# sort using explicit subroutine name
sub byage {
$age{$a} <=> $age{$b}; # presuming integers
}
@sortedclass = sort byage @class;
sub reverse { $b cmp $a; }
@harry = ('dog','cat','x','Cain','Abel');
@george = ('gone','chased','yz','Punished','Axed');
print sort @harry;
# prints AbelCaincatdogx
print sort reverse @harry;
# prints xdogcatCainAbel
print sort @george, 'to', @harry;
# prints AbelAxedCainPunishedcatchaseddoggonetoxyz
splice(ARRAY,OFFSET,LENGTH,LIST)
splice(ARRAY,OFFSET,LENGTH)
splice(ARRAY,OFFSET)
Removes the elements designated by OFFSET and LENGTH from an
array, and replaces them with the elements of LIST, if any.
Returns the elements removed from the array. The array grows
or shrinks as necessary. If LENGTH is omitted, removes
everything from OFFSET onward. The following equivalencies
hold (assuming $[ == 0):
push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y)
pop(@a) splice(@a,-1)
shift(@a) splice(@a,0,1)
unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
$a[$x] = $y splice(@a,$x,1,$y);
Example, assuming array lengths are passed before arrays:
sub aeq { # compare two array values
local(@a) = splice(@_,0,shift);
local(@b) = splice(@_,0,shift);
return 0 unless @a == @b; # same len?
while (@a) {
return 0 if pop(@a) ne pop(@b);
}
return 1;
}
if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
split(/PATTERN/,EXPR,LIMIT)
split(/PATTERN/,EXPR)
split(/PATTERN/)
split Splits a string into an array of strings, and returns it.
(If not in an array context, returns the number of fields
found and splits into the @_ array. (In an array context,
you can force the split into @_ by using ?? as the pattern
delimiters, but it still returns the array value.)) If EXPR
is omitted, splits the $_ string. If PATTERN is also
omitted, splits on whitespace (/[ \t\n]+/). Anything
matching PATTERN is taken to be a delimiter separating the
fields. (Note that the delimiter may be longer than one
character.) If LIMIT is specified, splits into no more than
that many fields (though it may split into fewer). If LIMIT
is unspecified, trailing null fields are stripped (which
potential users of pop() would do well to remember). A
pattern matching the null string (not to be confused with a
null pattern //, which is just one member of the set of
patterns matching a null string) will split the value of EXPR
into separate characters at each point it matches that way.
For example:
print join(':', split(/ */, 'hi there'));
produces the output 'h:i:t:h:e:r:e'.
The LIMIT parameter can be used to partially split a line
($login, $passwd, $remainder) = split(/:/, $_, 3);
(When assigning to a list, if LIMIT is omitted, perl supplies
a LIMIT one larger than the number of variables in the list,
to avoid unnecessary work. For the list above LIMIT would
have been 4 by default. In time critical applications it
behooves you not to split into more fields than you really
need.)
If the PATTERN contains parentheses, additional array
elements are created from each matching substring in the
delimiter.
split(/([,-])/,"1-10,20");
produces the array value
(1,'-',10,',',20)
The pattern /PATTERN/ may be replaced with an expression to
specify patterns that vary at runtime. (To do runtime
compilation only once, use /$variable/o.) As a special case,
specifying a space (' ') will split on white space just as
split with no arguments does, but leading white space does
NOT produce a null first field. Thus, split(' ') can be used
to emulate awk's default behavior, whereas split(/ /) will
give you as many null initial fields as there are leading
spaces.
Example:
open(passwd, '/etc/passwd');
while (<passwd>) {
($login, $passwd, $uid, $gid, $gcos, $home, $shell)
= split(/:/);
...
}
(Note that $shell above will still have a newline on it. See
chop().) See also join.
sprintf(FORMAT,LIST)
Returns a string formatted by the usual printf conventions.
The * character is not supported.
sqrt(EXPR)
sqrt EXPR
Return the square root of EXPR. If EXPR is omitted, returns
square root of $_.
srand(EXPR)
srand EXPR
Sets the random number seed for the rand operator. If EXPR
is omitted, does srand(time).
stat(FILEHANDLE)
stat FILEHANDLE
stat(EXPR)
stat SCALARVARIABLE
Returns a 13-element array giving the statistics for a file,
either the file opened via FILEHANDLE, or named by EXPR.
Returns a null list if the stat fails. Typically used as
follows:
($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
$atime,$mtime,$ctime,$blksize,$blocks)
= stat($filename);
If stat is passed the special filehandle consisting of an
underline, no stat is done, but the current contents of the
stat structure from the last stat or filetest are returned.
Example:
if (-x $file && (($d) = stat(_)) && $d < 0) {
print "$file is executable NFS file\n";
}
(This only works on machines for which the device number is
negative under NFS.)
study(SCALAR)
study SCALAR
study Takes extra time to study SCALAR ($_ if unspecified) in
anticipation of doing many pattern matches on the string
before it is next modified. This may or may not save time,
depending on the nature and number of patterns you are
searching on, and on the distribution of character
frequencies in the string to be searched--you probably want
to compare runtimes with and without it to see which runs
faster. Those loops which scan for many short constant
strings (including the constant parts of more complex
patterns) will benefit most. You may have only one study
active at a time--if you study a different scalar the first
is "unstudied". (The way study works is this: a linked list
of every character in the string to be searched is made, so
we know, for example, where all the 'k' characters are.
>From each search string, the rarest character is selected,
based on some static frequency tables constructed from some C
programs and English text. Only those places that contain
this "rarest" character are examined.)
For example, here is a loop which inserts index producing
entries before any line containing a certain pattern:
while (<>) {
study;
print ".IX foo\n" if /\bfoo\b/;
print ".IX bar\n" if /\bbar\b/;
print ".IX blurfl\n" if /\bblurfl\b/;
...
print;
}
In searching for /\bfoo\b/, only those locations in $_ that
contain 'f' will be looked at, because 'f' is rarer than 'o'.
In general, this is a big win except in pathological cases.
The only question is whether it saves you more time than it
took to build the linked list in the first place.
Note that if you have to look for strings that you don't know
till runtime, you can build an entire loop as a string and
eval that to avoid recompiling all your patterns all the
time. Together with undefining $/ to input entire files as
one record, this can be very fast, often faster than
specialized programs like fgrep. The following scans a list
of files (@files) for a list of words (@words), and prints
out the names of those files that contain a match:
$search = 'while (<>) { study;';
foreach $word (@words) {
$search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
}
$search .= "}";
@ARGV = @files;
undef $/;
eval $search; # this screams
$/ = "\n"; # put back to normal input delim
foreach $file (sort keys(%seen)) {
print $file, "\n";
}
substr(EXPR,OFFSET,LEN)
substr(EXPR,OFFSET)
Extracts a substring out of EXPR and returns it. First
character is at offset 0, or whatever you've set $[ to. If
OFFSET is negative, starts that far from the end of the
string. If LEN is omitted, returns everything to the end of
the string. You can use the substr() function as an lvalue,
in which case EXPR must be an lvalue. If you assign
something shorter than LEN, the string will shrink, and if
you assign something longer than LEN, the string will grow to
accommodate it. To keep the string the same length you may
need to pad or chop your value using sprintf().
symlink(OLDFILE,NEWFILE)
Creates a new filename symbolically linked to the old
filename. Returns 1 for success, 0 otherwise. On systems
that don't support symbolic links, produces a fatal error at
run time. To check for that, use eval:
$symlink_exists = (eval 'symlink("","");', $@ eq '');
syscall(LIST)
syscall LIST
Calls the system call specified as the first element of the
list, passing the remaining elements as arguments to the
system call. If unimplemented, produces a fatal error. The
arguments are interpreted as follows: if a given argument is
numeric, the argument is passed as an int. If not, the
pointer to the string value is passed. You are responsible
to make sure a string is pre-extended long enough to receive
any result that might be written into a string. If your
integer arguments are not literals and have never been
interpreted in a numeric context, you may need to add 0 to
them to force them to look like numbers.
require 'syscall.ph'; # may need to run h2ph
syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);
sysread(FILEHANDLE,SCALAR,LENGTH,OFFSET)
sysread(FILEHANDLE,SCALAR,LENGTH)
Attempts to read LENGTH bytes of data into variable SCALAR
from the specified FILEHANDLE, using the system call read(2).
It bypasses stdio, so mixing this with other kinds of reads
may cause confusion. Returns the number of bytes actually
read, or undef if there was an error. SCALAR will be grown
or shrunk to the length actually read. An OFFSET may be
specified to place the read data at some other place than the
beginning of the string.
system(LIST)
system LIST
Does exactly the same thing as "exec LIST" except that a fork
is done first, and the parent process waits for the child
process to complete. Note that argument processing varies
depending on the number of arguments. The return value is
the exit status of the program as returned by the wait()
call. To get the actual exit value divide by 256. See also
exec.
syswrite(FILEHANDLE,SCALAR,LENGTH,OFFSET)
syswrite(FILEHANDLE,SCALAR,LENGTH)
Attempts to write LENGTH bytes of data from variable SCALAR
to the specified FILEHANDLE, using the system call write(2).
It bypasses stdio, so mixing this with prints may cause
confusion. Returns the number of bytes actually written, or
undef if there was an error. An OFFSET may be specified to
place the read data at some other place than the beginning of
the string.
tell(FILEHANDLE)
tell FILEHANDLE
tell Returns the current file position for FILEHANDLE. FILEHANDLE
may be an expression whose value gives the name of the actual
filehandle. If FILEHANDLE is omitted, assumes the file last
read.
telldir(DIRHANDLE)
telldir DIRHANDLE
Returns the current position of the readdir() routines on
DIRHANDLE. Value may be given to seekdir() to access a
particular location in a directory. Has the same caveats
about possible directory compaction as the corresponding
system library routine.
time Returns the number of non-leap seconds since 00:00:00 UTC,
January 1, 1970. Suitable for feeding to gmtime() and
localtime().
times Returns a four-element array giving the user and system
times, in seconds, for this process and the children of this
process.
($user,$system,$cuser,$csystem) = times;
tr/SEARCHLIST/REPLACEMENTLIST/cds
y/SEARCHLIST/REPLACEMENTLIST/cds
Translates all occurrences of the characters found in the
search list with the corresponding character in the
replacement list. It returns the number of characters
replaced or deleted. If no string is specified via the =~ or
!~ operator, the $_ string is translated. (The string
specified with =~ must be a scalar variable, an array
element, or an assignment to one of those, i.e. an lvalue.)
For sed devotees, y is provided as a synonym for tr. If the
SEARCHLIST is delimited by bracketing quotes, the
REPLACEMENTLIST has its own pair of quotes, which may or may
not be bracketing quotes, e.g. tr[A-Z][a-z] or
tr(+-*/)/ABCD/.
If the c modifier is specified, the SEARCHLIST character set
is complemented. If the d modifier is specified, any
characters specified by SEARCHLIST that are not found in
REPLACEMENTLIST are deleted. (Note that this is slightly
more flexible than the behavior of some tr programs, which
delete anything they find in the SEARCHLIST, period.) If the
s modifier is specified, sequences of characters that were
translated to the same character are squashed down to 1
instance of the character.
If the d modifier was used, the REPLACEMENTLIST is always
interpreted exactly as specified. Otherwise, if the
REPLACEMENTLIST is shorter than the SEARCHLIST, the final
character is replicated till it is long enough. If the
REPLACEMENTLIST is null, the SEARCHLIST is replicated. This
latter is useful for counting characters in a class, or for
squashing character sequences in a class.
Examples:
$ARGV[1] =~ y/A-Z/a-z/; # canonicalize to lower case
$cnt = tr/*/*/; # count the stars in $_
$cnt = tr/0-9//; # count the digits in $_
tr/a-zA-Z//s; # bookkeeper -> bokeper
($HOST = $host) =~ tr/a-z/A-Z/;
y/a-zA-Z/ /cs; # change non-alphas to single space
tr/\200-\377/\0-\177/; # delete 8th bit
truncate(FILEHANDLE,LENGTH)
truncate(EXPR,LENGTH)
Truncates the file opened on FILEHANDLE, or named by EXPR, to
the specified length. Produces a fatal error if truncate
isn't implemented on your system.
umask(EXPR)
umask EXPR
umask Sets the umask for the process and returns the old one. If
EXPR is omitted, merely returns current umask.
undef(EXPR)
undef EXPR
undef Undefines the value of EXPR, which must be an lvalue. Use
only on a scalar value, an entire array, or a subroutine name
(using &). (Undef will probably not do what you expect on
most predefined variables or dbm array values.) Always
returns the undefined value. You can omit the EXPR, in which
case nothing is undefined, but you still get an undefined
value that you could, for instance, return from a subroutine.
Examples:
undef $foo;
undef $bar{'blurfl'};
undef @ary;
undef %assoc;
undef &mysub;
return (wantarray ? () : undef) if $they_blew_it;
unlink(LIST)
unlink LIST
Deletes a list of files. Returns the number of files
successfully deleted.
$cnt = unlink 'a', 'b', 'c';
unlink @goners;
unlink <*.bak>;
Note: unlink will not delete directories unless you have
appropriate privilege and the -U flag is supplied to perl.
On a generic DG/UX system, appropriate privilege is granted
by having an effective UID of 0 (root). See the
appropriate_privilege(5) man page for more information.
On a system with DG/UX information security, appropriate privilege is
granted by having one or more specific capabilities enabled in the
effective capability set of the user. See cap_defaults(5) for the
default capabilities for this command.
Even if these conditions are met, be warned that unlinking a
directory can inflict damage on your filesystem. Use rmdir instead.
unpack(TEMPLATE,EXPR)
Unpack does the reverse of pack: it takes a string
representing a structure and expands it out into an array
value, returning the array value. (In a scalar context, it
merely returns the first value produced.) The TEMPLATE has
the same format as in the pack function. Here's a subroutine
that does substring:
sub substr {
local($what,$where,$howmuch) = @_;
unpack("x$where a$howmuch", $what);
}
and then there's
sub ord { unpack("c",$_[0]); }
In addition, you may prefix a field with a %number to
indicate that you want a number-bit checksum of the items
instead of the items themselves. Default is a 16-bit
checksum. For example, the following computes the same
number as the System V sum program:
while (<>) {
$checksum += unpack("%16C*", $_);
}
$checksum %= 65536;
unshift(ARRAY,LIST)
Does the opposite of a shift. Or the opposite of a push,
depending on how you look at it. Prepends list to the front
of the array, and returns the number of elements in the new
array.
unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
utime(LIST)
utime LIST
Changes the access and modification times on each file of a
list of files. The first two elements of the list must be
the NUMERICAL access and modification times, in that order.
Returns the number of files successfully changed. The inode
modification time of each file is set to the current time.
Example of a "touch" command:
#!/usr/bin/perl
$now = time;
utime $now, $now, @ARGV;
values(ASSOC_ARRAY)
values ASSOC_ARRAY
Returns a normal array consisting of all the values of the
named associative array. The values are returned in an
apparently random order, but it is the same order as either
the keys() or each() function would produce on the same
array. See also keys() and each().
vec(EXPR,OFFSET,BITS)
Treats a string as a vector of unsigned integers, and returns
the value of the bitfield specified. May also be assigned
to. BITS must be a power of two from 1 to 32.
Vectors created with vec() can also be manipulated with the
logical operators |, & and ^, which will assume a bit vector
operation is desired when both operands are strings. This
interpretation is not enabled unless there is at least one
vec() in your program, to protect older programs.
To transform a bit vector into a string or array of 0's and
1's, use these:
$bits = unpack("b*", $vector);
@bits = split(//, unpack("b*", $vector));
If you know the exact length in bits, it can be used in place
of the *.
wait Waits for a child process to terminate and returns the pid of
the deceased process, or -1 if there are no child processes.
The status is returned in $?.
waitpid(PID,FLAGS)
Waits for a particular child process to terminate and returns
the pid of the deceased process, or -1 if there is no such
child process. The status is returned in $?. If you say
require "sys/wait.h";
...
waitpid(-1,&WNOHANG);
then you can do a non-blocking wait for any process. Non-
blocking wait is only available on machines supporting either
the waitpid(2) or wait4(2) system calls. However, waiting
for a particular pid with FLAGS of 0 is implemented
everywhere. (Perl emulates the system call by remembering
the status values of processes that have exited but have not
been harvested by the Perl script yet.)
wantarray
Returns true if the context of the currently executing
subroutine is looking for an array value. Returns false if
the context is looking for a scalar.
return wantarray ? () : undef;
warn(LIST)
warn LIST
Produces a message on STDERR just like "die", but doesn't
exit.
write(FILEHANDLE)
write(EXPR)
write Writes a formatted record (possibly multi-line) to the
specified file, using the format associated with that file.
By default the format for a file is the one having the same
name is the filehandle, but the format for the current output
channel (see select) may be set explicitly by assigning the
name of the format to the $~ variable.
Top of form processing is handled automatically: if there is
insufficient room on the current page for the formatted
record, the page is advanced by writing a form feed, a
special top-of-page format is used to format the new page
header, and then the record is written. By default the top-
of-page format is the name of the filehandle with "_TOP"
appended, but it may be dynamically set to the format of your
choice by assigning the name to the $^ variable while the
filehandle is selected. The number of lines remaining on the
current page is in variable $-, which can be set to 0 to
force a new page.
If FILEHANDLE is unspecified, output goes to the current
default output channel, which starts out as STDOUT but may be
changed by the select operator. If the FILEHANDLE is an
EXPR, then the expression is evaluated and the resulting
string is used to look up the name of the FILEHANDLE at run
time. For more on formats, see the section on formats later
on.
Note that write is NOT the opposite of read.
Precedence
Perl operators have the following associativity and precedence:
nonassoc print printf exec system sort reverse
chmod chown kill unlink utime die return
left ,
right = += -= *= etc.
right ?:
nonassoc ..
left ||
left &&
left | ^
left &
nonassoc == != <=> eq ne cmp
nonassoc < > <= >= lt gt le ge
nonassoc chdir exit eval reset sleep rand umask
nonassoc -r -w -x etc.
left << >>
left + - .
left * / % x
left =~ !~
right ! ~ and unary minus
right **
nonassoc ++ --
left '('
As mentioned earlier, if any list operator (print, etc.) or any unary
operator (chdir, etc.) is followed by a left parenthesis as the next
token on the same line, the operator and arguments within parentheses
are taken to be of highest precedence, just like a normal function
call. Examples:
chdir $foo || die; # (chdir $foo) || die
chdir($foo) || die; # (chdir $foo) || die
chdir ($foo) || die; # (chdir $foo) || die
chdir +($foo) || die; # (chdir $foo) || die
but, because * is higher precedence than ||:
chdir $foo * 20; # chdir ($foo * 20)
chdir($foo) * 20; # (chdir $foo) * 20
chdir ($foo) * 20; # (chdir $foo) * 20
chdir +($foo) * 20; # chdir ($foo * 20)
rand 10 * 20; # rand (10 * 20)
rand(10) * 20; # (rand 10) * 20
rand (10) * 20; # (rand 10) * 20
rand +(10) * 20; # rand (10 * 20)
In the absence of parentheses, the precedence of list operators such
as print, sort or chmod is either very high or very low depending on
whether you look at the left side of operator or the right side of
it. For example, in
@ary = (1, 3, sort 4, 2);
print @ary; # prints 1324
the commas on the right of the sort are evaluated before the sort,
but the commas on the left are evaluated after. In other words, list
operators tend to gobble up all the arguments that follow them, and
then act like a simple term with regard to the preceding expression.
Note that you have to be careful with parens:
# These evaluate exit before doing the print:
print($foo, exit); # Obviously not what you want.
print $foo, exit; # Nor is this.
# These do the print before evaluating exit:
(print $foo), exit; # This is what you want.
print($foo), exit; # Or this.
print ($foo), exit; # Or even this.
Also note that
print ($foo & 255) + 1, "\n";
probably doesn't do what you expect at first glance.
Subroutines
A subroutine may be declared as follows:
sub NAME BLOCK
Any arguments passed to the routine come in as array @_, that is
($_[0], $_[1], ...). The array @_ is a local array, but its values
are references to the actual scalar parameters. The return value of
the subroutine is the value of the last expression evaluated, and can
be either an array value or a scalar value. Alternately, a return
statement may be used to specify the returned value and exit the
subroutine. To create local variables see the local operator.
A subroutine is called using the do operator or the & operator.
Example:
sub MAX {
local($max) = pop(@_);
foreach $foo (@_) {
$max = $foo if $max < $foo;
}
$max;
}
...
$bestday = &MAX($mon,$tue,$wed,$thu,$fri);
Example:
# get a line, combining continuation lines
# that start with whitespace
sub get_line {
$thisline = $lookahead;
line: while ($lookahead = <STDIN>) {
if ($lookahead =~ /^[ \t]/) {
$thisline .= $lookahead;
}
else {
last line;
}
}
$thisline;
}
$lookahead = <STDIN>; # get first line
while ($_ = do get_line()) {
...
}
Use array assignment to a local list to name your formal arguments:
sub maybeset {
local($key, $value) = @_;
$foo{$key} = $value unless $foo{$key};
}
This also has the effect of turning call-by-reference into call-by-
value, since the assignment copies the values.
Subroutines may be called recursively. If a subroutine is called
using the & form, the argument list is optional. If omitted, no @_
array is set up for the subroutine; the @_ array at the time of the
call is visible to subroutine instead.
do foo(1,2,3); # pass three arguments
&foo(1,2,3); # the same
do foo(); # pass a null list
&foo(); # the same
&foo; # pass no arguments--more efficient
Passing By Reference
Sometimes you don't want to pass the value of an array to a
subroutine but rather the name of it, so that the subroutine can
modify the global copy of it rather than working with a local copy.
In perl you can refer to all the objects of a particular name by
prefixing the name with a star: *foo. When evaluated, it produces a
scalar value that represents all the objects of that name, including
any filehandle, format or subroutine. When assigned to within a
local() operation, it causes the name mentioned to refer to whatever
* value was assigned to it. Example:
sub doubleary {
local(*someary) = @_;
foreach $elem (@someary) {
$elem *= 2;
}
}
do doubleary(*foo);
do doubleary(*bar);
Assignment to *name is currently recommended only inside a local().
You can actually assign to *name anywhere, but the previous referent
of *name may be stranded forever. This may or may not bother you.
Note that scalars are already passed by reference, so you can modify
scalar arguments without using this mechanism by referring explicitly
to the $_[nnn] in question. You can modify all the elements of an
array by passing all the elements as scalars, but you have to use the
* mechanism to push, pop or change the size of an array. The *
mechanism will probably be more efficient in any case.
Since a *name value contains unprintable binary data, if it is used
as an argument in a print, or as a %s argument in a printf or
sprintf, it then has the value '*name', just so it prints out pretty.
Even if you don't want to modify an array, this mechanism is useful
for passing multiple arrays in a single LIST, since normally the LIST
mechanism will merge all the array values so that you can't extract
out the individual arrays.
Regular Expressions
The patterns used in pattern matching are regular expressions such as
those supplied in the Version 8 regexp routines. (In fact, the
routines are derived from Henry Spencer's freely redistributable
reimplementation of the V8 routines.) In addition, \w matches an
alphanumeric character (including "_") and \W a nonalphanumeric.
Word boundaries may be matched by \b, and non-boundaries by \B. A
whitespace character is matched by \s, non-whitespace by \S. A
numeric character is matched by \d, non-numeric by \D. You may use
\w, \s and \d within character classes. Also, \n, \r, \f, \t and
\NNN have their normal interpretations. Within character classes \b
represents backspace rather than a word boundary. Alternatives may
be separated by |. The bracketing construct ( ... ) may also be
used, in which case \<digit> matches the digit'th substring.
(Outside of the pattern, always use $ instead of \ in front of the
digit. The scope of $digit (and $`, $& and $') extends to the end of
the enclosing BLOCK or eval string, or to the next pattern match with
subexpressions. The \digit notation sometimes works outside the
current pattern, but should not be relied upon.) You may have as
many parentheses as you wish. If you have more than 9 substrings,
the variables $10, $11, ... refer to the corresponding substring.
Within the pattern, \10, \11, etc. refer back to substrings if there
have been at least that many left parens before the backreference.
Otherwise (for backward compatibility) \10 is the same as \010, a
backspace, and \11 the same as \011, a tab. And so on. (\1 through
\9 are always backreferences.)
$+ returns whatever the last bracket match matched. $& returns the
entire matched string. ($0 used to return the same thing, but not
any more.) $` returns everything before the matched string. $'
returns everything after the matched string. Examples:
s/^([^ ]*) *([^ ]*)/$2 $1/; # swap first two words
if (/Time: (..):(..):(..)/) {
$hours = $1;
$minutes = $2;
$seconds = $3;
}
By default, the ^ character is only guaranteed to match at the
beginning of the string, the $ character only at the end (or before
the newline at the end) and perl does certain optimizations with the
assumption that the string contains only one line. The behavior of ^
and $ on embedded newlines will be inconsistent. You may, however,
wish to treat a string as a multi-line buffer, such that the ^ will
match after any newline within the string, and $ will match before
any newline. At the cost of a little more overhead, you can do this
by setting the variable $* to 1. Setting it back to 0 makes perl
revert to its old behavior.
To facilitate multi-line substitutions, the . character never matches
a newline (even when $* is 0). In particular, the following leaves a
newline on the $_ string:
$_ = <STDIN>;
s/.*(some_string).*/$1/;
If the newline is unwanted, try one of
s/.*(some_string).*\n/$1/;
s/.*(some_string)[^\000]*/$1/;
s/.*(some_string)(.|\n)*/$1/;
chop; s/.*(some_string).*/$1/;
/(some_string)/ && ($_ = $1);
Any item of a regular expression may be followed with digits in curly
brackets of the form {n,m}, where n gives the minimum number of times
to match the item and m gives the maximum. The form {n} is
equivalent to {n,n} and matches exactly n times. The form {n,}
matches n or more times. (If a curly bracket occurs in any other
context, it is treated as a regular character.) The * modifier is
equivalent to {0,}, the + modifier to {1,} and the ? modifier to
{0,1}. There is no limit to the size of n or m, but large numbers
will chew up more memory.
You will note that all backslashed metacharacters in perl are
alphanumeric, such as \b, \w, \n. Unlike some other regular
expression languages, there are no backslashed symbols that aren't
alphanumeric. So anything that looks like \\, \(, \), \<, \>, \{, or
\} is always interpreted as a literal character, not a metacharacter.
This makes it simple to quote a string that you want to use for a
pattern but that you are afraid might contain metacharacters. Simply
quote all the non-alphanumeric characters:
$pattern =~ s/(\W)/\\$1/g;
Formats
Output record formats for use with the write operator may declared as
follows:
format NAME =
FORMLIST
.
If name is omitted, format "STDOUT" is defined. FORMLIST consists of
a sequence of lines, each of which may be of one of three types:
1. A comment.
2. A "picture" line giving the format for one output line.
3. An argument line supplying values to plug into a picture line.
Picture lines are printed exactly as they look, except for certain
fields that substitute values into the line. Each picture field
starts with either @ or ^. The @ field (not to be confused with the
array marker @) is the normal case; ^ fields are used to do
rudimentary multi-line text block filling. The length of the field
is supplied by padding out the field with multiple <, >, or |
characters to specify, respectively, left justification, right
justification, or centering. As an alternate form of right
justification, you may also use # characters (with an optional .) to
specify a numeric field. (Use of ^ instead of @ causes the field to
be blanked if undefined.) If any of the values supplied for these
fields contains a newline, only the text up to the newline is
printed. The special field @* can be used for printing multi-line
values. It should appear by itself on a line.
The values are specified on the following line, in the same order as
the picture fields. The values should be separated by commas.
Picture fields that begin with ^ rather than @ are treated specially.
The value supplied must be a scalar variable name which contains a
text string. Perl puts as much text as it can into the field, and
then chops off the front of the string so that the next time the
variable is referenced, more of the text can be printed. Normally
you would use a sequence of fields in a vertical stack to print out a
block of text. If you like, you can end the final field with ...,
which will appear in the output if the text was too long to appear in
its entirety. You can change which characters are legal to break on
by changing the variable $: to a list of the desired characters.
Since use of ^ fields can produce variable length records if the text
to be formatted is short, you can suppress blank lines by putting the
tilde (~) character anywhere in the line. (Normally you should put
it in the front if possible, for visibility.) The tilde will be
translated to a space upon output. If you put a second tilde
contiguous to the first, the line will be repeated until all the
fields on the line are exhausted. (If you use a field of the @
variety, the expression you supply had better not give the same value
every time forever!)
Examples:
# a report on the /etc/passwd file
format STDOUT_TOP =
Passwd File
Name Login Office Uid Gid Home
------------------------------------------------------------------
.
format STDOUT =
@<<<<<<<<<<<<<<<<<< @||||||| @<<<<<<@>>>> @>>>> @<<<<<<<<<<<<<<<<<
$name, $login, $office,$uid,$gid, $home
.
# a report from a bug report form
format STDOUT_TOP =
Bug Reports
@<<<<<<<<<<<<<<<<<<<<<<< @||| @>>>>>>>>>>>>>>>>>>>>>>>
$system, $%, $date
------------------------------------------------------------------
.
format STDOUT =
Subject: @<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
$subject
Index: @<<<<<<<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
$index, $description
Priority: @<<<<<<<<<< Date: @<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
$priority, $date, $description
From: @<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
$from, $description
Assigned to: @<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
$programmer, $description
~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
$description
~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
$description
~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
$description
~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
$description
~ ^<<<<<<<<<<<<<<<<<<<<<<<...
$description
.
It is possible to intermix prints with writes on the same output
channel, but you'll have to handle $- (lines left on the page)
yourself.
If you are printing lots of fields that are usually blank, you should
consider using the reset operator between records. Not only is it
more efficient, but it can prevent the bug of adding another field
and forgetting to zero it.
Interprocess Communication
The IPC facilities of perl are built on the Berkeley socket
mechanism. If you don't have sockets, you can ignore this section.
The calls have the same names as the corresponding system calls, but
the arguments tend to differ, for two reasons. First, perl file
handles work differently than C file descriptors. Second, perl
already knows the length of its strings, so you don't need to pass
that information. Here is a sample client (untested):
($them,$port) = @ARGV;
$port = 2345 unless $port;
$them = 'localhost' unless $them;
$SIG{'INT'} = 'dokill';
sub dokill { kill 9,$child if $child; }
require 'sys/socket.ph';
$sockaddr = 'S n a4 x8';
chop($hostname = `hostname`);
($name, $aliases, $proto) = getprotobyname('tcp');
($name, $aliases, $port) = getservbyname($port, 'tcp')
unless $port =~ /^\d+$/;
($name, $aliases, $type, $len, $thisaddr) =
gethostbyname($hostname);
($name, $aliases, $type, $len, $thataddr) = gethostbyname($them);
$this = pack($sockaddr, &AF_INET, 0, $thisaddr);
$that = pack($sockaddr, &AF_INET, $port, $thataddr);
socket(S, &PF_INET, &SOCK_STREAM, $proto) || die "socket: $!";
bind(S, $this) || die "bind: $!";
connect(S, $that) || die "connect: $!";
select(S); $| = 1; select(stdout);
if ($child = fork) {
while (<>) {
print S;
}
sleep 3;
do dokill();
}
else {
while (<S>) {
print;
}
}
And here's a server:
($port) = @ARGV;
$port = 2345 unless $port;
require 'sys/socket.ph';
$sockaddr = 'S n a4 x8';
($name, $aliases, $proto) = getprotobyname('tcp');
($name, $aliases, $port) = getservbyname($port, 'tcp')
unless $port =~ /^\d+$/;
$this = pack($sockaddr, &AF_INET, $port, "\0\0\0\0");
select(NS); $| = 1; select(stdout);
socket(S, &PF_INET, &SOCK_STREAM, $proto) || die "socket: $!";
bind(S, $this) || die "bind: $!";
listen(S, 5) || die "connect: $!";
select(S); $| = 1; select(stdout);
for (;;) {
print "Listening again\n";
($addr = accept(NS,S)) || die $!;
print "accept ok\n";
($af,$port,$inetaddr) = unpack($sockaddr,$addr);
@inetaddr = unpack('C4',$inetaddr);
print "$af $port @inetaddr\n";
while (<NS>) {
print;
print NS;
}
}
Predefined Names
The following names have special meaning to perl. I could have used
alphabetic symbols for some of these, but I didn't want to take the
chance that someone would say reset "a-zA-Z" and wipe them all out.
You'll just have to suffer along with these silly symbols. Most of
them have reasonable mnemonics, or analogues in one of the shells.
$_ The default input and pattern-searching space. The following
pairs are equivalent:
while (<>) {... # only equivalent in while!
while ($_ = <>) {...
/^Subject:/
$_ =~ /^Subject:/
y/a-z/A-Z/
$_ =~ y/a-z/A-Z/
chop
chop($_)
(Mnemonic: underline is understood in certain operations.)
$. The current input line number of the last filehandle that was
read. Readonly. Remember that only an explicit close on the
filehandle resets the line number. Since <> never does an
explicit close, line numbers increase across ARGV files (but
see examples under eof). (Mnemonic: many programs use . to
mean the current line number.)
$/ The input record separator, newline by default. Works like
awk's RS variable, including treating blank lines as
delimiters if set to the null string. You may set it to a
multicharacter string to match a multi-character delimiter.
Note that setting it to "\n\n" means something slightly
different than setting it to "", if the file contains
consecutive blank lines. Setting it to "" will treat two or
more consecutive blank lines as a single blank line. Setting
it to "\n\n" will blindly assume that the next input
character belongs to the next paragraph, even if it's a
newline. (Mnemonic: / is used to delimit line boundaries
when quoting poetry.)
$, The output field separator for the print operator.
Ordinarily the print operator simply prints out the comma
separated fields you specify. In order to get behavior more
like awk, set this variable as you would set awk's OFS
variable to specify what is printed between fields.
(Mnemonic: what is printed when there is a , in your print
statement.)
$"" This is like $, except that it applies to array values
interpolated into a double-quoted string (or similar
interpreted string). Default is a space. (Mnemonic:
obvious, I think.)
$\ The output record separator for the print operator.
Ordinarily the print operator simply prints out the comma
separated fields you specify, with no trailing newline or
record separator assumed. In order to get behavior more like
awk, set this variable as you would set awk's ORS variable to
specify what is printed at the end of the print. (Mnemonic:
you set $\ instead of adding \n at the end of the print.
Also, it's just like /, but it's what you get "back" from
perl.)
$# The output format for printed numbers. This variable is a
half-hearted attempt to emulate awk's OFMT variable. There
are times, however, when awk and perl have differing notions
of what is in fact numeric. Also, the initial value is %.20g
rather than %.6g, so you need to set $# explicitly to get
awk's value. (Mnemonic: # is the number sign.)
$% The current page number of the currently selected output
channel. (Mnemonic: % is page number in nroff.)
$= The current page length (printable lines) of the currently
selected output channel. Default is 60. (Mnemonic: = has
horizontal lines.)
$- The number of lines left on the page of the currently
selected output channel. (Mnemonic: lines_on_page -
lines_printed.)
$~ The name of the current report format for the currently
selected output channel. Default is name of the filehandle.
(Mnemonic: brother to $^.)
$^ The name of the current top-of-page format for the currently
selected output channel. Default is name of the filehandle
with "_TOP" appended. (Mnemonic: points to top of page.)
$| If set to nonzero, forces a flush after every write or print
on the currently selected output channel. Default is 0.
Note that STDOUT will typically be line buffered if output is
to the terminal and block buffered otherwise. Setting this
variable is useful primarily when you are outputting to a
pipe, such as when you are running a perl script under rsh
and want to see the output as it's happening. (Mnemonic:
when you want your pipes to be piping hot.)
$$ The process number of the perl running this script.
(Mnemonic: same as shells.)
$? The status returned by the last pipe close, backtick (``)
command or system operator. Note that this is the status
word returned by the wait() system call, so the exit value of
the subprocess is actually ($? >> 8). $? & 255 gives which
signal, if any, the process died from, and whether there was
a core dump. (Mnemonic: similar to sh and ksh.)
$& The string matched by the last successful pattern match (not
counting any matches hidden within a BLOCK or eval enclosed
by the current BLOCK). (Mnemonic: like & in some editors.)
$` The string preceding whatever was matched by the last
successful pattern match (not counting any matches hidden
within a BLOCK or eval enclosed by the current BLOCK).
(Mnemonic: ` often precedes a quoted string.)
$' The string following whatever was matched by the last
successful pattern match (not counting any matches hidden
within a BLOCK or eval enclosed by the current BLOCK).
(Mnemonic: ' often follows a quoted string.) Example:
$_ = 'abcdefghi';
/def/;
print "$`:$&:$'\n"; # prints abc:def:ghi
$+ The last bracket matched by the last search pattern. This is
useful if you don't know which of a set of alternative
patterns matched. For example:
/Version: (.*)|Revision: (.*)/ && ($rev = $+);
(Mnemonic: be positive and forward looking.)
$* Set to 1 to do multiline matching within a string, 0 to tell
perl that it can assume that strings contain a single line,
for the purpose of optimizing pattern matches. Pattern
matches on strings containing multiple newlines can produce
confusing results when $* is 0. Default is 0. (Mnemonic: *
matches multiple things.) Note that this variable only
influences the interpretation of ^ and $. A literal newline
can be searched for even when $* == 0.
$0 Contains the name of the file containing the perl script
being executed. Assigning to $0 modifies the argument area
that the ps(1) program sees. (Mnemonic: same as sh and ksh.)
$digit Contains the subpattern from the corresponding set of
parentheses in the last pattern matched, not counting
patterns matched in nested blocks that have been exited
already. (Mnemonic: like \digit.)
$[ The index of the first element in an array, and of the first
character in a substring. Default is 0, but you could set it
to 1 to make perl behave more like awk (or Fortran) when
subscripting and when evaluating the index() and substr()
functions. (Mnemonic: [ begins subscripts.)
$] The string printed out when you say "perl -v". It can be
used to determine at the beginning of a script whether the
perl interpreter executing the script is in the right range
of versions. If used in a numeric context, returns the
version + patchlevel / 1000. Example:
# see if getc is available
($version,$patchlevel) =
$] =~ /(\d+\.\d+).*\nPatch level: (\d+)/;
print STDERR "(No filename completion available.)\n"
if $version * 1000 + $patchlevel < 2016;
or, used numerically,
warn "No checksumming!\n" if $] < 3.019;
(Mnemonic: Is this version of perl in the right bracket?)
$; The subscript separator for multi-dimensional array
emulation. If you refer to an associative array element as
$foo{$a,$b,$c}
it really means
$foo{join($;, $a, $b, $c)}
But don't put
@foo{$a,$b,$c} # a slice--note the @
which means
($foo{$a},$foo{$b},$foo{$c})
Default is "\034", the same as SUBSEP in awk. Note that if
your keys contain binary data there might not be any safe
value for $;. (Mnemonic: comma (the syntactic subscript
separator) is a semi-semicolon. Yeah, I know, it's pretty
lame, but $, is already taken for something more important.)
$! If used in a numeric context, yields the current value of
errno, with all the usual caveats. (This means that you
shouldn't depend on the value of $! to be anything in
particular unless you've gotten a specific error return
indicating a system error.) If used in a string context,
yields the corresponding system error string. You can assign
to $! in order to set errno if, for instance, you want $! to
return the string for error n, or you want to set the exit
value for the die operator. (Mnemonic: What just went bang?)
$@ The perl syntax error message from the last eval command. If
null, the last eval parsed and executed correctly (although
the operations you invoked may have failed in the normal
fashion). (Mnemonic: Where was the syntax error "at"?)
$< The real uid of this process. (Mnemonic: it's the uid you
came FROM, if you're running setuid.)
$> The effective uid of this process. Example:
$< = $>; # set real uid to the effective uid
($<,$>) = ($>,$<); # swap real and effective uid
(Mnemonic: it's the uid you went TO, if you're running
setuid.) Note: $< and $> can only be swapped on machines
supporting setreuid().
$( The real gid of this process. If you are on a machine that
supports membership in multiple groups simultaneously, gives
a space separated list of groups you are in. The first
number is the one returned by getgid(), and the subsequent
ones by getgroups(), one of which may be the same as the
first number. (Mnemonic: parentheses are used to GROUP
things. The real gid is the group you LEFT, if you're
running setgid.)
$) The effective gid of this process. If you are on a machine
that supports membership in multiple groups simultaneously,
gives a space separated list of groups you are in. The first
number is the one returned by getegid(), and the subsequent
ones by getgroups(), one of which may be the same as the
first number. (Mnemonic: parentheses are used to GROUP
things. The effective gid is the group that's RIGHT for you,
if you're running setgid.)
Note: $<, $>, $( and $) can only be set on machines that
support the corresponding set[re][ug]id() routine. $( and $)
can only be swapped on machines supporting setregid().
$: The current set of characters after which a string may be
broken to fill continuation fields (starting with ^) in a
format. Default is " \n-", to break on whitespace or
hyphens. (Mnemonic: a "colon" in poetry is a part of a
line.)
$^D The current value of the debugging flags. (Mnemonic: value
of -D switch.)
$^F The maximum system file descriptor, ordinarily 2. System
file descriptors are passed to subprocesses, while higher
file descriptors are not. During an open, system file
descriptors are preserved even if the open fails. Ordinary
file descriptors are closed before the open is attempted.
$^I The current value of the inplace-edit extension. Use undef
to disable inplace editing. (Mnemonic: value of -i switch.)
$^L What formats output to perform a formfeed. Default is \f.
$^P The internal flag that the debugger clears so that it doesn't
debug itself. You could conceivable disable debugging
yourself by clearing it.
$^T The time at which the script began running, in seconds since
the epoch. The values returned by the -M, -A and -C
filetests are based on this value.
$^W The current value of the warning switch. (Mnemonic: related
to the -w switch.)
$^X The name that Perl itself was executed as, from argv[0].
$ARGV contains the name of the current file when reading from <>.
@ARGV The array ARGV contains the command line arguments intended
for the script. Note that $#ARGV is the generally number of
arguments minus one, since $ARGV[0] is the first argument,
NOT the command name. See $0 for the command name.
@INC The array INC contains the list of places to look for perl
scripts to be evaluated by the "do EXPR" command or the
"require" command. It initially consists of the arguments to
any -I command line switches, followed by the default perl
library, probably "/usr/local/lib/perl", followed by ".", to
represent the current directory.
%INC The associative array INC contains entries for each filename
that has been included via "do" or "require". The key is the
filename you specified, and the value is the location of the
file actually found. The "require" command uses this array
to determine whether a given file has already been included.
$ENV{expr}
The associative array ENV contains your current environment.
Setting a value in ENV changes the environment for child
processes.
$SIG{expr}
The associative array SIG is used to set signal handlers for
various signals. Example:
sub handler { # 1st argument is signal name
local($sig) = @_;
print "Caught a SIG$sig--shutting down\n";
close(LOG);
exit(0);
}
$SIG{'INT'} = 'handler';
$SIG{'QUIT'} = 'handler';
...
$SIG{'INT'} = 'DEFAULT'; # restore default action
$SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
The SIG array only contains values for the signals actually
set within the perl script.
Packages
Perl provides a mechanism for alternate namespaces to protect
packages from stomping on each others variables. By default, a perl
script starts compiling into the package known as "main". By use of
the package declaration, you can switch namespaces. The scope of the
package declaration is from the declaration itself to the end of the
enclosing block (the same scope as the local() operator). Typically
it would be the first declaration in a file to be included by the
"require" operator. You can switch into a package in more than one
place; it merely influences which symbol table is used by the
compiler for the rest of that block. You can refer to variables and
filehandles in other packages by prefixing the identifier with the
package name and a single quote. If the package name is null, the
"main" package as assumed.
Only identifiers starting with letters are stored in the packages
symbol table. All other symbols are kept in package "main". In
addition, the identifiers STDIN, STDOUT, STDERR, ARGV, ARGVOUT, ENV,
INC and SIG are forced to be in package "main", even when used for
other purposes than their built-in one. Note also that, if you have
a package called "m", "s" or "y", the you can't use the qualified
form of an identifier since it will be interpreted instead as a
pattern match, a substitution or a translation.
Eval'ed strings are compiled in the package in which the eval was
compiled in. (Assignments to $SIG{}, however, assume the signal
handler specified is in the main package. Qualify the signal handler
name if you wish to have a signal handler in a package.) For an
example, examine perldb.pl in the perl library. It initially
switches to the DB package so that the debugger doesn't interfere
with variables in the script you are trying to debug. At various
points, however, it temporarily switches back to the main package to
evaluate various expressions in the context of the main package.
The symbol table for a package happens to be stored in the
associative array of that name prepended with an underscore. The
value in each entry of the associative array is what you are
referring to when you use the *name notation. In fact, the following
have the same effect (in package main, anyway), though the first is
more efficient because it does the symbol table lookups at compile
time:
local(*foo) = *bar;
local($_main{'foo'}) = $_main{'bar'};
You can use this to print out all the variables in a package, for
instance. Here is dumpvar.pl from the perl library:
package dumpvar;
sub main'dumpvar {
($package) = @_;
local(*stab) = eval("*_$package");
while (($key,$val) = each(%stab)) {
{
local(*entry) = $val;
if (defined $entry) {
print "\$$key = '$entry'\n";
}
if (defined @entry) {
print "\@$key = (\n";
foreach $num ($[ .. $#entry) {
print " $num\t'",$entry[$num],"'\n";
}
print ")\n";
}
if ($key ne "_$package" && defined %entry) {
print "\%$key = (\n";
foreach $key (sort keys(%entry)) {
print " $key\t'",$entry{$key},"'\n";
}
print ")\n";
}
}
}
}
Note that, even though the subroutine is compiled in package dumpvar,
the name of the subroutine is qualified so that its name is inserted
into package "main".
Style
Each programmer will, of course, have his or her own preferences in
regards to formatting, but there are some general guidelines that
will make your programs easier to read.
1. Just because you CAN do something a particular way doesn't mean
that you SHOULD do it that way. Perl is designed to give you
several ways to do anything, so consider picking the most
readable one. For instance
open(FOO,$foo) || die "Can't open $foo: $!";
is better than
die "Can't open $foo: $!" unless open(FOO,$foo);
because the second way hides the main point of the statement in a
modifier. On the other hand
print "Starting analysis\n" if $verbose;
is better than
$verbose && print "Starting analysis\n";
since the main point isn't whether the user typed -v or not.
Similarly, just because an operator lets you assume default
arguments doesn't mean that you have to make use of the defaults.
The defaults are there for lazy systems programmers writing one-
shot programs. If you want your program to be readable, consider
supplying the argument.
Along the same lines, just because you can omit parentheses in
many places doesn't mean that you ought to:
return print reverse sort num values array;
return print(reverse(sort num (values(%array))));
When in doubt, parenthesize. At the very least it will let some
poor schmuck bounce on the % key in vi.
Even if you aren't in doubt, consider the mental welfare of the
person who has to maintain the code after you, and who will
probably put parens in the wrong place.
2. Don't go through silly contortions to exit a loop at the top or
the bottom, when perl provides the "last" operator so you can
exit in the middle. Just outdent it a little to make it more
visible:
line:
for (;;) {
statements;
last line if $foo;
next line if /^#/;
statements;
}
3. Don't be afraid to use loop labels--they're there to enhance
readability as well as to allow multi-level loop breaks. See
last example.
4. For portability, when using features that may not be implemented
on every machine, test the construct in an eval to see if it
fails. If you know what version or patchlevel a particular
feature was implemented, you can test $] to see if it will be
there.
5. Choose mnemonic identifiers.
6. Be consistent.
Debugging
If you invoke perl with a -d switch, your script will be run under a
debugging monitor. It will halt before the first executable
statement and ask you for a command, such as:
h Prints out a help message.
T Stack trace.
s Single step. Executes until it reaches the beginning of
another statement.
n Next. Executes over subroutine calls, until it reaches
the beginning of the next statement.
f Finish. Executes statements until it has finished the
current subroutine.
c Continue. Executes until the next breakpoint is reached.
c line Continue to the specified line. Inserts a one-time-only
breakpoint at the specified line.
<CR> Repeat last n or s.
l min+incr List incr+1 lines starting at min. If min is omitted,
starts where last listing left off. If incr is omitted,
previous value of incr is used.
l min-max List lines in the indicated range.
l line List just the indicated line.
l List next window.
- List previous window.
w line List window around line.
l subname List subroutine. If it's a long subroutine it just lists
the beginning. Use "l" to list more.
/pattern/ Regular expression search forward for pattern; the final
/ is optional.
?pattern? Regular expression search backward for pattern; the final
? is optional.
L List lines that have breakpoints or actions.
S Lists the names of all subroutines.
t Toggle trace mode on or off.
b line condition
Set a breakpoint. If line is omitted, sets a breakpoint
on the line that is about to be executed. If a condition
is specified, it is evaluated each time the statement is
reached and a breakpoint is taken only if the condition
is true. Breakpoints may only be set on lines that begin
an executable statement.
b subname condition
Set breakpoint at first executable line of subroutine.
d line Delete breakpoint. If line is omitted, deletes the
breakpoint on the line that is about to be executed.
D Delete all breakpoints.
a line command
Set an action for line. A multi-line command may be
entered by backslashing the newlines.
A Delete all line actions.
< command Set an action to happen before every debugger prompt. A
multi-line command may be entered by backslashing the
newlines.
> command Set an action to happen after the prompt when you've just
given a command to return to executing the script. A
multi-line command may be entered by backslashing the
newlines.
V package List all variables in package. Default is main package.
! number Redo a debugging command. If number is omitted, redoes
the previous command.
! -number Redo the command that was that many commands ago.
H -number Display last n commands. Only commands longer than one
character are listed. If number is omitted, lists them
all.
q or ^D Quit.
command Execute command as a perl statement. A missing semicolon
will be supplied.
p expr Same as "print DB'OUT expr". The DB'OUT filehandle is
opened to /dev/tty, regardless of where STDOUT may be
redirected to.
If you want to modify the debugger, copy perldb.pl from the perl
library to your current directory and modify it as necessary.
(You'll also have to put -I. on your command line.) You can do some
customization by setting up a .perldb file which contains
initialization code. For instance, you could make aliases like
these:
$DB'alias{'len'} = 's/^len(.*)/p length($1)/';
$DB'alias{'stop'} = 's/^stop (at|in)/b/';
$DB'alias{'.'} =
's/^\./p "\$DB\'sub(\$DB\'line):\t",\$DB\'line[\$DB\'line]/';
Setuid Scripts
Perl is designed to make it easy to write secure setuid and setgid
scripts. Unlike shells, which are based on multiple substitution
passes on each line of the script, perl uses a more conventional
evaluation scheme with fewer hidden "gotchas". Additionally, since
the language has more built-in functionality, it has to rely less
upon external (and possibly untrustworthy) programs to accomplish its
purposes.
In an unpatched 4.2 or 4.3bsd kernel, setuid scripts are
intrinsically insecure, but this kernel feature can be disabled. If
it is, perl can emulate the setuid and setgid mechanism when it
notices the otherwise useless setuid/gid bits on perl scripts. If
the kernel feature isn't disabled, perl will complain loudly that
your setuid script is insecure. You'll need to either disable the
kernel setuid script feature, or put a C wrapper around the script.
When perl is executing a setuid script, it takes special precautions
to prevent you from falling into any obvious traps. (In some ways, a
perl script is more secure than the corresponding C program.) Any
command line argument, environment variable, or input is marked as
"tainted", and may not be used, directly or indirectly, in any
command that invokes a subshell, or in any command that modifies
files, directories or processes. Any variable that is set within an
expression that has previously referenced a tainted value also
becomes tainted (even if it is logically impossible for the tainted
value to influence the variable). For example:
$foo = shift; # $foo is tainted
$bar = $foo,'bar'; # $bar is also tainted
$xxx = <>; # Tainted
$path = $ENV{'PATH'}; # Tainted, but see below
$abc = 'abc'; # Not tainted
system "echo $foo"; # Insecure
system "/bin/echo", $foo; # Secure (doesn't use sh)
system "echo $bar"; # Insecure
system "echo $abc"; # Insecure until PATH set
$ENV{'PATH'} = '/bin:/usr/bin';
$ENV{'IFS'} = '' if $ENV{'IFS'} ne '';
$path = $ENV{'PATH'}; # Not tainted
system "echo $abc"; # Is secure now!
open(FOO,"$foo"); # OK
open(FOO,">$foo"); # Not OK
open(FOO,"echo $foo|"); # Not OK, but...
open(FOO,"-|") || exec 'echo', $foo; # OK
$zzz = `echo $foo`; # Insecure, zzz tainted
unlink $abc,$foo; # Insecure
umask $foo; # Insecure
exec "echo $foo"; # Insecure
exec "echo", $foo; # Secure (doesn't use sh)
exec "sh", '-c', $foo; # Considered secure, alas
The taintedness is associated with each scalar value, so some
elements of an array can be tainted, and others not.
If you try to do something insecure, you will get a fatal error
saying something like "Insecure dependency" or "Insecure PATH". Note
that you can still write an insecure system call or exec, but only by
explicitly doing something like the last example above. You can also
bypass the tainting mechanism by referencing subpatterns--perl
presumes that if you reference a substring using $1, $2, etc, you
knew what you were doing when you wrote the pattern:
$ARGV[0] =~ /^-P(\w+)$/;
$printer = $1; # Not tainted
This is fairly secure since \w+ doesn't match shell metacharacters.
Use of .+ would have been insecure, but perl doesn't check for that,
so you must be careful with your patterns. This is the ONLY
mechanism for untainting user supplied filenames if you want to do
file operations on them (unless you make $> equal to $<).
It's also possible to get into trouble with other operations that
don't care whether they use tainted values. Make judicious use of
the file tests in dealing with any user-supplied filenames. When
possible, do opens and such after setting $> = $<. Perl doesn't
prevent you from opening tainted filenames for reading, so be careful
what you print out. The tainting mechanism is intended to prevent
stupid mistakes, not to remove the need for thought.
ENVIRONMENT
HOME Used if chdir has no argument.
LOGDIR Used if chdir has no argument and HOME is not set.
PATH Used in executing subprocesses, and in finding the script
if -S is used.
PERLLIB A colon-separated list of directories in which to look
for Perl library files before looking in the standard
library and the current directory.
PERLDB The command used to get the debugger code. If unset,
uses
require 'perldb.pl'
Apart from these, perl uses no other environment variables, except to
make them available to the script being executed, and to child
processes. However, scripts running setuid would do well to execute
the following lines before doing anything else, just to keep people
honest:
$ENV{'PATH'} = '/bin:/usr/bin'; # or whatever you need
$ENV{'SHELL'} = '/bin/sh' if $ENV{'SHELL'} ne '';
$ENV{'IFS'} = '' if $ENV{'IFS'} ne '';
AUTHOR
Larry Wall <lwall@netlabs.com>
MS-DOS port by Diomidis Spinellis <dds@cc.ic.ac.uk>
AVAILABILITY
The source code for perl is available without charge from many sites
on the Internet, including the host ftp.uu.net (192.48.96.9) in the
directory languages/perl.
FILES
/tmp/perl-eXXXXXX temporary file for -e commands.
SEE ALSO
a2p(1) awk to perl translator
s2p(1) sed to perl translator
DIAGNOSTICS
Compilation errors will tell you the line number of the error, with
an indication of the next token or token type that was to be
examined. (In the case of a script passed to perl via -e switches,
each -e is counted as one line.)
Setuid scripts have additional constraints that can produce error
messages such as "Insecure dependency". See the section on setuid
scripts.
TRAPS
Accustomed awk users should take special note of the following:
* Semicolons are required after all simple statements in perl
(except at the end of a block). Newline is not a statement
delimiter.
* Curly brackets are required on ifs and whiles.
* Variables begin with $ or @ in perl.
* Arrays index from 0 unless you set $[. Likewise string positions
in substr() and index().
* You have to decide whether your array has numeric or string
indices.
* Associative array values do not spring into existence upon mere
reference.
* You have to decide whether you want to use string or numeric
comparisons.
* Reading an input line does not split it for you. You get to
split it yourself to an array. And the split operator has
different arguments.
* The current input line is normally in $_, not $0. It generally
does not have the newline stripped. ($0 is the name of the
program executed.)
* $digit does not refer to fields--it refers to substrings matched
by the last match pattern.
* The print statement does not add field and record separators
unless you set $, and $\.
* You must open your files before you print to them.
* The range operator is "..", not comma. (The comma operator works
as in C.)
* The match operator is "=~", not "~". ("~" is the one's
complement operator, as in C.)
* The exponentiation operator is "**", not "^". ("^" is the XOR
operator, as in C.)
* The concatenation operator is ".", not the null string. (Using
the null string would render "/pat/ /pat/" unparsable, since the
third slash would be interpreted as a division operator--the
tokener is in fact slightly context sensitive for operators like
/, ?, and <. And in fact, . itself can be the beginning of a
number.)
* Next, exit and continue work differently.
* The following variables work differently
Awk Perl
ARGC $#ARGV
ARGV[0] $0
FILENAME $ARGV
FNR $. - something
FS (whatever you like)
NF $#Fld, or some such
NR $.
OFMT $#
OFS $,
ORS $\
RLENGTH length($&)
RS $/
RSTART length($`)
SUBSEP $;
* When in doubt, run the awk construct through a2p and see what it
gives you.
Cerebral C programmers should take note of the following:
* Curly brackets are required on ifs and whiles.
* You should use "elsif" rather than "else if"
* Break and continue become last and next, respectively.
* There's no switch statement.
* Variables begin with $ or @ in perl.
* Printf does not implement *.
* Comments begin with #, not /*.
* You can't take the address of anything.
* ARGV must be capitalized.
* The "system" calls link, unlink, rename, etc. return nonzero for
success, not 0.
* Signal handlers deal with signal names, not numbers.
Seasoned sed programmers should take note of the following:
* Backreferences in substitutions use $ rather than \.
* The pattern matching metacharacters (, ), and | do not have
backslashes in front.
* The range operator is .. rather than comma.
Sharp shell programmers should take note of the following:
* The backtick operator does variable interpretation without regard
to the presence of single quotes in the command.
* The backtick operator does no translation of the return value,
unlike csh.
* Shells (especially csh) do several levels of substitution on each
command line. Perl does substitution only in certain constructs
such as double quotes, backticks, angle brackets and search
patterns.
* Shells interpret scripts a little bit at a time. Perl compiles
the whole program before executing it.
* The arguments are available via @ARGV, not $1, $2, etc.
* The environment is not automatically made available as variables.
ERRATA AND ADDENDA
The Perl book, Programming Perl, has the following omissions and
goofs.
On page 5, the examples which read
eval "/usr/bin/perl
should read
eval "exec /usr/bin/perl
On page 195, the equivalent to the System V sum program only works
for very small files. To do larger files, use
undef $/;
$checksum = unpack("%32C*",<>) % 32767;
The descriptions of alarm and sleep refer to signal SIGALARM. These
should refer to SIGALRM.
The -0 switch to set the initial value of $/ was added to Perl after
the book went to press.
The -l switch now does automatic line ending processing.
The qx// construct is now a synonym for backticks.
$0 may now be assigned to set the argument displayed by ps(1).
The new @###.## format was omitted accidentally from the description
on formats.
It wasn't known at press time that s///ee caused multiple evaluations
of the replacement expression. This is to be construed as a feature.
(LIST) x $count now does array replication.
There is now no limit on the number of parentheses in a regular
expression.
In double-quote context, more escapes are supported: \e, \a, \x1b,
\c[, \l, \L, \u, \U, \E. The latter five control up/lower case
translation.
The $/ variable may now be set to a multi-character delimiter.
There is now a g modifier on ordinary pattern matching that causes it
to iterate through a string finding multiple matches.
All of the $^X variables are new except for $^T.
The default top-of-form format for FILEHANDLE is now FILEHANDLE_TOP
rather than top.
The eval {} and sort {} constructs were added in version 4.018.
The v and V (little-endian) template options for pack and unpack were
added in 4.019.
BUGS
Perl is at the mercy of your machine's definitions of various
operations such as type casting, atof() and sprintf().
If your stdio requires an seek or eof between reads and writes on a
particular stream, so does perl. (This doesn't apply to sysread()
and syswrite().)
While none of the built-in data types have any arbitrary size limits
(apart from memory size), there are still a few arbitrary limits: a
given identifier may not be longer than 255 characters, and no
component of your PATH may be longer than 255 if you use -S. A
regular expression may not compile to more than 32767 bytes
internally.
Perl actually stands for Pathologically Eclectic Rubbish Lister, but
don't tell anyone I said that.
Licensed material--property of copyright holder(s)