NAME

expr − evaluate arguments as an expression

SYNOPSIS

expr arg ...

DESCRIPTION

expr evaluates expressions as specified by its arguments.  After evaluation, the result is written on the standard output.  Each token of the expression is a separate argument, so terms of the expression must be separated by blanks.  Characters special to the shell must be escaped.  Note that 0 is returned to indicate a zero value, rather than the null string.  Strings containing blanks or other special characters should be quoted.  Internally, integers are treated as 32-bit, 2s complement numbers. 

The operators and keywords are listed below.  Characters that need to be escaped are preceded by \.  The list is in order of increasing precedence, with equal precedence operators grouped within {} symbols. 

expr \│ expr
returns the first expr if it is neither null nor 0, otherwise returns the second expr.

expr \& expr
returns the first expr if neither expr is null or 0, otherwise returns 0. 

expr { =, \>, \>=, \<, \<=, != } expr
returns the result of an integer comparison if both arguments are integers, otherwise returns the result of a lexical comparison.

expr { +, − } expr
addition or subtraction of integer-valued arguments.

expr { \∗, /, % } expr
multiplication, division, or remainder of the integer-valued arguments.

expr : expr

match string regular-expression
The two forms of the matching operator above are synonymous. The matching operators : and match compare the first argument with the second argument which must be a regular expression.  Regular expression syntax is the same as that of ed(1), except that all patterns are “anchored” (i.e., begin with ^) and, therefore, ^ is not a special character, in that context.  Normally, the matching operator returns the number of characters matched (0 on failure).  Alternatively, the \(...\) pattern symbols can be used to return a portion of the first argument. 

substr string integer-1 integer-2
extracts the subtring of string starting at position integer-1 and of length integer-2 characters.  If integer-1 has a value greater than the length of string, expr returns a null string.  If you try to extract more characters than there are in string, expr returns all the remaining characters from string. Beware of using negative values for either integer-1 or integer-2 as expr tends to run forever in these cases. 

index string character-list
reports the first position in string at which any one of the characters in character-list matches a character in string.

length string
returns the length (that is, the number of characters) of string.

SYSTEM V DESCRIPTION

Integer-valued arguments may be preceded by a unary minus sign. 

The operators substr, index, and length are not supported. 

EXAMPLES

1.  a=`expr $a + 1`

adds 1 to the shell variable a. 

2.  #  ´For $a equal to either "/usr/abc/file" or just "file"´
expr  $a  :  ´.∗/\(.∗\)´  \│  $a

returns the last segment of a path name (i.e., file).  Watch out for / alone as an argument: expr will take it as the division operator (see BUGS below). 

3.  #  A better representation of example 2. 
expr  //$a  :  ´.∗/\(.∗\)´

The addition of the // characters eliminates any ambiguity about the division operator and simplifies the whole expression. 

4.  expr  $VAR  :  ´.∗´

returns the number of characters in $VAR. 

SEE ALSO

ed(1), sh(1), test(1)

EXIT CODE

Expr returns the following exit codes:

0if the expression is neither null nor 0
1if the expression is null or 0
2for invalid expressions. 

DIAGNOSTICS

syntax error for operator/operand errors

non-numeric argument
if arithmetic is attempted on such a string

BUGS

After argument processing by the shell, expr cannot tell the difference between an operator and an operand except by the value.  If $a is an =, the command:

expr  $a  =  ´=´

looks like:

expr  =  =  =

as the arguments are passed to expr (and they will all be taken as the = operator).  The following works:

expr  X$a  =  X=

Note that the match, substr, length, and index operators cannot themselves be used as ordinary strings.  That is, the expression:

tutorial% expr index expurgatorious length
syntax error
tutorial%

generates the ‘syntax error’ message as shown instead of the value 1 as you might expect. 

Sun Release 3.2  —  Last change: 26 March 1984