sh(1) sh(1)NAME sh, rsh - runs the Bourne shell SYNOPSIS sh [-c string] [-i] [-r] [-s] [-a] [-e] [-f] [-h] [-k] [-n] [-t] [-u] [-v] [-x] [args]...] rsh [-c string] [-i] [-r] [-s] [-a] [-e] [-f] [-h] [-k] [-n] [-t] [-u] [-v] [-x] [args]...] ARGUMENTS -a Marks variables which are modified or created for export. -c string Reads commands from the string. -e Exits immediately if a command exits with a nonzero exit status. -f Disables filename generation. -h Locates and remembers function commands as functions are defined (function commands are normally located when the function is executed). -i Causes the shell to be interactive if this option is present or if the shell input and output are attached to a terminal. In this case, Terminate is ignored (so that kill 0 does not kill an interactive shell) and Interrupt is caught and ignored (so that wait is interruptible). In all cases, Quit is ignored by the shell. -k Places all keyword arguments in the environment for a command, not just those that precede the command name. -n Reads commands, but does not execute them. -r Invokes a restricted shell. -s Reads commands from the standard input if this option is present or if no arguments remain. Any remaining arguments specify the positional parameters. Shell output, except for special commands (see ``Special Commands''), is written to file descriptor 2. -t Exits after reading and executing one command. -u Treats unset variables as an error when substituting. -v Prints shell input lines as they are read. January 1992 1
sh(1) sh(1)-x Prints commands and their arguments as they are executed. DESCRIPTION sh is a command programming language that executes commands read from a terminal or a file. rsh is a restricted version of the standard command interpreter sh; it is used to set up login names and execution environments whose capabilities are more controlled than those of the standard shell. Definitions A blank is a tab or a space. A name is a sequence of letters, digits, or underscores beginning with a letter or underscore. A parameter is a name, a digit, or any of the characters: *, @, #, ?, -, $, and !. Commands A simple-command is a sequence of nonblank words separated by blanks. The first word specifies the name of the command to be executed. Except as specified below, the remaining words are passed as arguments to the invoked command. The command name is passed as argument 0 (see exec(2)). The value of a simple-command is its exit status if it terminates normally, or (octal) 200+status if it terminates abnormally (see signal(3) for a list of status values). A pipeline is a sequence of one or more commands separated by ``|'' (or, for historical compatibility, by ^). The standard output of each command but the last is connected by a pipe(2) to the standard input of the next command. Each command is run as a separate process; the shell waits for the last command to terminate. The exit status of a pipeline is the exit status of the last command. A list is a sequence of one or more pipelines separated by a ;, &, &&, or ||, and optionally terminated by a ; or &. Of these four symbols, ; and & have equal precedence, which is lower than that of && and ||. The symbols && and || also have equal precedence. A semicolon (;) causes sequential execution of the preceding pipeline; an ampersand (&) causes asynchronous execution of the preceding pipeline (i.e., the shell does not wait for that pipeline to finish). The symbol && (||) causes the list following it to be executed only if the preceding pipeline returns a zero (nonzero) exit status. An arbitrary number of newlines may appear in a list, instead of semicolons, to delimit commands. A command is either a simple-command or one of the following. Unless otherwise stated, the value returned by a command is that of the last simple-command executed in the command. 2 January 1992
sh(1) sh(1)for name [in word ...] do list done Sets name to the next word taken from the in word list, each time a command is executed. If in word... is omitted, then the for command executes the do list once for each positional parameter that is set (see ``Parameter Substitution,'' below). Execution ends when there are no more words in the list. case word in [pattern[| pattern] list ;;]... esac Executes the list associated with the first pattern that matches word. The form of the patterns is the same as that used for file-name generation (see ``Filename Generation'') except that a slash, a leading dot, or a dot immediately following a slash need not be matched explicitly. if list then list [elif list then list]...[else list] fi Executes the list following if. If it returns a zero exit status, the list following the first then is executed. Otherwise, the list following elif is executed and, if its value is zero, the list following the next then is executed. Failing that, the else list is executed. If no else list or then list is executed, then the if command returns a zero exit status. while list do list done Repeatedly executes the while list and, if the exit status of the last command in the list is zero, executes the do list; otherwise the loop terminates. If no commands in the do list are executed, then the while command returns a zero exit status; until may be used in place of while to negate the loop termination test. (list) Executes list in a subshell. {list} Executes list. name () {list} Defines a function which is referenced by name. The body of the function is the list of commands between { and }. Execution of functions is described below (see ``Execution,'' below). The following words are recognized only as the first word of a command and when not quoted: if then else elif fi case esac for while until do done { } January 1992 3
sh(1) sh(1)Comments A word beginning with # causes that word and all the following characters up to a newline to be ignored. Command Substitution The standard output from a command enclosed in a pair of grave accents (``) may be used as part or all of a word; trailing newlines are removed. Parameter Substitution The character $ is used to introduce substitutable parameters. There are two types of parameters, positional and keyword. If parameter is a digit, it is a positional parameter. Positional parameters may be assigned values by set. Keyword parameters (also known as variables) may be assigned values by writing: name=value [name=value] Pattern-matching is not performed on value. There cannot be a function and a variable with the same name. ${parameter} The value, if any, of the parameter is substituted. The braces are required only when parameter is followed by a letter, digit, or underscore that is not to be interpreted as part of its name. If parameter is * or @, all the positional parameters, starting with $1, are substituted (separated by spaces). Parameter $0 is set from argument zero when the shell is invoked. ${parameter :-word} If parameter is set and is non-null, substitute its value; otherwise substitute word. ${parameter :=word} If parameter is not set or is null, set it to word; the value of the parameter is substituted. Positional parameters may not be assigned in this way. ${parameter :?word} If parameter is set and is non-null, substitute its value; otherwise, print word and exit from the shell. If word is omitted, the message ``parameter null or not set'' is printed. ${parameter :+word} If parameter is set and is non-null, substitute word; otherwise substitute nothing. In the above command, word is not evaluated unless it is to be used as the substituted string, so that, in the following 4 January 1992
sh(1) sh(1)example, pwd is executed only if d is not set or is null: echo ${d:- `pwd`} If the colon (:) is omitted from the above expressions, the shell checks only whether parameter is set or not. The following parameters are set automatically by the shell: # The number of positional parameters in decimal. - Flags supplied to the shell on invocation or by the set command. ? The decimal value returned by the last synchronously-executed command. $ The process number of this shell. ! The process number of the last background command invoked. The following parameters are used by the shell: HOME The default argument (home directory) for the cd command. PATH The search path for commands (see ``Execution,'' below). You may not change PATH if executing under rsh. CDPATH The search path for the cd command. MAIL If you have set this parameter to the name of a mail file and you have not set the MAILPATH parameter, the shell informs you of the arrival of mail in the specified file. MAILCHECK This parameter specifies how often (in seconds) the shell will check for the arrival of mail in the files specified by the MAILPATH or MAIL parameters. The default value is 600 seconds (10 minutes). If this parameter is set to 0, the shell will check before each prompt. MAILPATH A colon-separated (:) list of filenames. If this parameter is set, the shell informs the user of the arrival of mail in any of the specified files. Each filename may be followed by % and a message January 1992 5
sh(1) sh(1)that will be printed when the modification time changes. The default message is ``You have mail.'' PS1 Primary prompt string, by default ``$''. PS2 Secondary prompt string, by default ``>''. IFS Internal field separators, normally space, tab, and newline. SHACCT If this parameter is set to the name of a file writable by the user, the shell will write an accounting record in the file for each shell procedure executed. Accounting routines such as acctcom(1) and acctcms(1M) can be used to analyze the data collected. SHELL When the shell is invoked, it scans the environment (see ``Environment,'' below) for this name. If it is found and there is an r in the filename part of its value, the shell becomes a restricted shell. The shell gives default values to PATH, PS1, PS2, MAILCHECK, and IFS. HOME and MAIL are set by login(1)). Blank Interpretation After parameter and command substitution, the results of substitution are scanned for internal field separator characters (those found in IFS) and split into distinct arguments where such characters are found. Explicit null arguments (" or '') are retained. Implicit null arguments (those resulting from parameters that have no values) are removed. Filename Generation Following substitution, each command word is scanned for the characters *, ?, and [. If one of these characters appears, the word is regarded as a pattern. The word is replaced with alphabetically-sorted filenames that match the pattern. If no filename is found that matches the pattern, the word is left unchanged. The character . at the start of a filename or immediately following a /, as well as the character / itself, must be matched explicitly. * Matches any string, including the null string. ? Matches any single character. 6 January 1992
sh(1) sh(1)[...] Matches any one of the enclosed characters. A pair of characters separated by - matches any character lexically between the pair, inclusive. If the first character following the opening ``['' is a ``!'', any character not enclosed is matched. Quoting The following characters have a special meaning to the shell and cause termination of a word unless quoted: ; & ( ) | ^ < > newline space tab A character may be quoted (i.e., made to stand for itself) by preceding it with a \. The pair \newline is ignored. All characters enclosed between a pair of single quote marks (''), except a single quote, are quoted. Inside double quote marks (""), parameter and command substitution occurs and \ quotes the characters \, `, , and $. The string, $*, is equivalent to $1 $2 ..., whereas $@ is equivalent to $1 2 .... Prompting When used interactively, the shell prompts with the value of PS1 before reading a command. If, at any time, a newline is typed and further input is needed to complete a command, the secondary prompt (i.e., the value of PS2) is issued. Input/Output Before a command is executed, its input and output may be redirected using a special notation interpreted by the shell. The following may appear anywhere in a simple- command or may precede or follow a command and are not passed on to the invoked command; substitution occurs before word or digit is used: <word Uses file word as standard input (file descriptor 0). >word Uses file word as standard output (file descriptor 1). If the file does not exist, it is created; otherwise, it is truncated to zero length. >>word Uses file word as standard output. If the file exists, output is appended to it (by first seeking to the end- of-file); otherwise, the file is created. <<[-] word The shell input is read up to a line that is the January 1992 7
sh(1) sh(1)same as word, or to an end-of-file. The resulting document becomes the standard input. If any character of word is quoted, no interpretation is placed upon the characters of the document; otherwise, parameter and command substitution occurs, (unescaped) \newline is ignored, and \ must be used to quote the characters \, $, `, and the first character of word. If - is appended to <<, all leading tabs are stripped from word and from the document. <&digit Use the file associated with file descriptor digit as standard input. Similarly for the standard output using >&digit. <&- The standard input is closed. Similarly for the standard output using >&-. If any of the above is preceded by a digit, the file descriptor which will be associated with the file is that specified by the digit (instead of the default 0 or 1). For example: ... 2>&1 associates file descriptor 2 with the file currently associated with file descriptor 1. The order in which redirections are specified is significant. The shell evaluates redirections left-to- right. For example: ...1>xxx2>&1 first associates file descriptor 1 with file xxx. It associates file descriptor 2 with the file associated with file descriptor 1 (i.e., xxx). If the order of redirections were reversed, file descriptor 2 would be associated with the terminal (assuming file descriptor 1 had been) and file descriptor 1 would be associated with file xxx. If a command is followed by &, the default standard input for the command is the empty file /dev/null. Otherwise, the environment for the execution of a command contains the file descriptors of the invoking shell, as modified by input/output specifications. Redirection of output is not allowed in the restricted shell. 8 January 1992
sh(1) sh(1)Environment The environment (see environ(5)) is a list of name-value pairs that is passed to an executed program in the same way as a normal argument list. The shell interacts with the environment in several ways. On invocation, the shell scans the environment and creates a parameter for each name found, giving it the corresponding value. If the user modifies the values of any of these parameters or creates new parameters, none of these affects the environment unless the export command is used to bind the shell's parameter to the environment (see also set -a). A parameter may be removed from the environment with the unset command. The environment seen by any executed command is thus composed of any unmodified name-value pairs originally inherited by the shell, minus any pairs removed by unset, plus any modifications or additions, all of which must be noted in export commands. The environment for any simple-command may be augmented by prefixing it with one or more assignments to parameters. Thus: TERM=450 cmd and (export TERM; TERM=450; cmd) are equivalent (as far as the execution of cmd is concerned). If the -k option is set, all keyword arguments are placed in the environment, even if they occur after the command name. The following command first prints a=b c and then, after the -k option is set, prints only c: echo a=b c #first time prints a=b c set -k #puts all keyword args in env echo a=b c #nowprintsonly" c; a=b goes to env Signals The interrupt and quit signals for an invoked command are ignored if the command is followed by &; otherwise signals have the values inherited by the shell from its parent, with the exception of signal 11 (but see also the trap command below). Execution Each time a command is executed, the above substitutions are carried out. If a command name matches one of the special commands listed below (see ``Special Commands''), it is January 1992 9
sh(1) sh(1)executed in the shell process. If the command name does not match a special command, but matches the name of a defined function, the function is executed in the shell process (note that this differs from the execution of shell procedures, which takes place in subshells). The positional parameters $1, $2, ... are set to the arguments of the function. If the command name matches neither a special command nor the name of a defined function, a new process is created and an attempt is made to execute the command via exec(2). The shell parameter PATH defines the search path for the directory containing the command. Alternative directory names are separated by a colon (:). The default path is :/bin:/usr/bin (specifying the current directory, /bin, and /usr/bin, in that order). Note that the current directory is specified by a null pathname, which can appear immediately after the equals sign or between the colon delimiters anywhere else in the path list. If the command name contains a / the search path is not used; such commands will not be executed by the restricted shell. Otherwise, each directory in the path is searched for an executable file. If the file has execute permission but is not an a.out file, it is assumed to be a file containing shell commands. A subshell is spawned to read it. A parenthesized command is also executed in a subshell. The location in the search path where a command was found is remembered by the shell (to help avoid unnecessary execs later). If the command was found in a relative directory, its location must be redetermined whenever the current directory changes. The shell forgets all remembered locations whenever the PATH variable is changed or the hash -r command is executed (see below). Special Commands Input/output redirection is now permitted for these commands. File descriptor 1 is the default output location. : No effect; the command does nothing. A zero exit code is returned. . file Read and execute commands from file and return. The search path specified by PATH is used to find the directory containing file. break [n] Exit from the enclosing for or while loop, if any. If n is specified, break n levels. cd [arg] 10 January 1992
sh(1) sh(1)Change the current directory to arg. The shell parameter HOME is the default arg. The shell parameter CDPATH defines the search path for the directory containing arg. Alternative directory names are separated by a colon (:). The default path is null (i.e., the empty string, specifying the current directory). Note that the current directory is specified by a null pathname, which can appear immediately after the equals sign or between the colon delimiters anywhere else in the path list. If arg begins with a /, the search path is not used. Otherwise, each directory in the path is searched for arg. The cd command may not be executed by rsh. continue [n] Resume the next iteration of the enclosing for or while loop. If n is specified, resume at the n-th enclosing loop. echo [arg...] Echo arguments. Arguments are written separated by blanks and terminated by a newline on the standard output. It understands C-like escape conventions. eval [arg...] The arguments are read as input to the shell and the resulting command(s) executed. exec [arg...] The command specified by the arguments is executed in place of this shell without creating a new process. Input/output arguments may appear and, if no other arguments are given, cause the shell input/output to be modified. exit [n] Causes a shell to exit with the exit status specified by n. If n is omitted, the exit status is that of the last command executed (an end-of-file will also cause the shell to exit). export [name...] The given names are marked for automatic export to the environment of subsequently-executed commands. If no arguments are given, a list of all names that are exported in this shell is printed. Function names may not be exported. hash [-r][name...] For each name, the location in the search path of the command specified by name is determined and remembered by the shell. The -r option causes the shell to forget January 1992 11
sh(1) sh(1)all remembered locations. If no arguments are given, information (hits and cost) about remembered commands is presented. hits is the number of times a command has been invoked by the shell process. cost is a measure of the work required to locate a command in the search path. There are certain situations which require that the stored location of a command be recalculated. Commands for which this will be done are indicated by an asterisk (*) adjacent to the hits information. cost will be incremented when the recalculation is done. newgrp [arg...] Equivalent to exec newgrp arg... Changes a user's group identification. The user remains logged in, and the current directory is unchanged, but calculations of access permissions to files are performed with respect to the new real and effective group IDs. The user is always given a new shell, replacing the current shell, by newgrp, regardless of whether it terminated successfully or due to an error condition (i.e., unknown group). With no arguments, newgrp changes the group identification back to the group specified in the user's password file entry. If the first argument to newgrp is a -, the environment is changed to what would be expected if the user actually logged in again. This built-in version executes faster than the A/UX command newgrp(1) but is otherwise identical. pwd Print the current working directory. This built-in version executes faster than the A/UX command pwd(1) but is otherwise identical. read [name...] One line is read from the standard input and the first word is assigned to the first name, the second word to the second name, etc., with leftover words assigned to the last name. The return code is 0 unless an end-of- file is encountered. readonly[name...] The given names are marked readonly and the values of the these names may not be changed by subsequent assignment. If no arguments are given, a list of all readonly names is printed. return [n] 12 January 1992
sh(1) sh(1)Causes a function to exit with the return value specified by n. If n is omitted, the return status is that of the last command executed. set [aefhkntuvx--[arg]...] See the ``Arguments'' section at the beginning of this manual page for the argument descriptions to the set command. Using + rather than - causes these options to be turned off. These options can also be used upon invocation of the shell. The current set of options may be found in $-. The remaining arguments are positional parameters and are assigned, in order, to $1, $2, ... If no arguments are given, the values of all names are printed. shift [n] The positional parameters from $n+1... are renamed $1 ... . If n is not given, it is assumed to be 1. test Evaluates the expression expr and, if its value is true, returns a zero (true) exit status; otherwise, a nonzero (false) exit status is returned; test also returns a nonzero exit status if there are no arguments. The superuser is always granted execute permission even though (1) execute permission is meaningful only for directories and regular files, and (2) exec requires that at least one execute mode bit be set for a regular file to be executable. The following primitives are used to construct expr: -r file Returns true if file exists and is readable. -w file Returns true if file exists and is writable. -x file Returns true if file exists and is executable. -f file Returns true if file exists and is a regular file. -d file Returns true if file exists and is a directory. -c file Returns true if file exists and is a character special file. -b file Returns true if file exists and is a block special file. January 1992 13
sh(1) sh(1)-p file Returns true if file exists and is a named pipe (FIFO). -u file Returns true if file exists and its set user ID bit is set. -g file Returns true if file exists and its set group ID bit is set. -k file Returns true if file exists and its sticky bit is set. -s file Returns true if file exists and has a size greater than zero. -t [fildes] Returns true if the open file whose file descriptor number is fildes (1 by default) is associated with a terminal device. -z s1 Returns true if the length of string s1 is zero. -n s1 Returns true if the length of the string s1 is nonzero. s1 = s2 Returns true if strings s1 and s2 are identical. s1 != s2 Returns true if strings s1 and s2 are not identical. s1 Returns true if s1 is not the null string. n1 -eq n2 Returns true if the integers n1 and n2 are algebraically equal. Any of the comparisons -ne, -gt, -ge, -lt, and -le may be used in place of -eq. These primaries may be combined with the following operators: ! unary negation operator. 14 January 1992
sh(1) sh(1)-a binary AND operator. -o binary OR operator (-a has higher precedence than -o). (expr) parentheses for grouping. Notice that all the operators and options are separate arguments to test. Notice also that parentheses are meaningful to the shell and, therefore, must be escaped. test is typically used in shell scripts as in the following example, which prints the message ``foo is a directory'' if it is found to be one when test is run. For example, if test -d foo then echo "foo is a dir" fi times Print the accumulated user and system times for processes run from the shell. trap [arg][n]... The command arg is to be read and executed when the shell receives signal(s) n. (Note that arg is scanned once when the trap is set and once when the trap is taken.) Trap commands are executed in order of signal number. Any attempt to set a trap on a signal that was ignored on entry to the current shell is ineffective. An attempt to trap on signal 11 (memory fault) produces an error. If arg is absent, all trap(s) n are reset to their original values. If arg is the null string, this signal is ignored by the shell and by the commands it invokes. If n is 0, the command arg is executed on exit from the shell. The trap command with no arguments prints a list of commands associated with each signal number. type [name...] For each name, indicate how it would be interpreted if used as a command name. ulimit [f][n] Imposes a size limit of n. The -f option imposes a size limit of n blocks on files written by child January 1992 15
sh(1) sh(1)processes (files of any size may be read). With no argument, the current limit is printed. If no options option is given, -f is assumed. umask[nnn] The user file-creation mask is set to nnn (see umask(2)). If nnn is omitted, the current value of the mask is printed. unset[name...] For each name, remove the corresponding variable or function. The variables PATH, PS1, PS2, MAILCHECK, and IFS cannot be unset. wait [n] Waits for the specified process and reports its termination status. If n is not given, all currently active child processes are waited for and the return code is zero. Invocation If the shell is invoked through exec(2) and the first character of argument zero is -, commands are read initially from /etc/profile and from $HOME/.profile, if such files exist. Thereafter, commands are read as described below, which is also the case when the shell is invoked as /bin/sh. The -c, -i, -r, and -s options are interpreted by the shell on invocation only. Note that, unless the -c or -s option is specified, the first argument is assumed to be the name of a file containing commands, and the remaining arguments are passed as positional parameters to that command file. rsh Only rsh is used to set up login names and execution environments whose capabilities are more controlled than those of the standard shell. The actions of rsh are identical to those of sh, except that the following are disallowed: changing directory setting the value of $PATH specifying path or command names containing / redirecting output ( > and >> ) The restrictions above are enforced after .profile is interpreted. When a command to be executed is found to be a shell procedure, rsh invokes sh to execute it. Thus, it is possible to provide to the end-user shell procedures that have access to the full power of the standard shell, while imposing a limited menu of commands; this scheme assumes that the end-user does not have write and execute 16 January 1992
sh(1) sh(1)permissions in the same directory. The net effect of these rules is that the writer of the .profile has complete control over user actions, by performing guaranteed setup actions and leaving the user in an appropriate directory (probably not the login directory). The system administrator often sets up a directory of commands (i.e., /usr/rbin) that can be safely invoked by rsh. Some systems also provide a restricted editor red. EXAMPLES Enter the command: sh -x script1 to execute each command in script1, echoing the command just before executing it. STATUS MESSAGES AND VALUES Errors detected by the shell, such as syntax errors, cause the shell to return a nonzero exit status. If the shell is being used noninteractively, execution of the shell file is abandoned. Otherwise, the shell returns the exit status of the last command executed (see also the exit command above). WARNINGS If a command is executed, and a command with the same name is installed in a directory in the search path before the directory where the original command was found, the shell will continue to exec the original command. Use the hash command to correct this situation. If you move the current directory or one above it, pwd may not give the correct response. Use the cd command with a full pathname to correct this situation. LIMITATIONS Filename pattern matching is not done on redirected I/O filenames. FILES /bin/sh Executable file /etc/profile Profile file $HOME/.profile Profile file for the home directory /tmp/sh* Temporary file /dev/null Temporary file January 1992 17
sh(1) sh(1)SEE ALSO csh(1), chsh(1), echo(1), env(1), ksh(1), login(1), newgrp(1), pwd(1) acctcms(1M), acctcom(1M) in A/UX System Administrator's Reference dup(2), exec(2), fork(2), pipe(2), ulimit(2), umask(2), wait(2), signal(3), a.out(4), profile(4), environ(5) in A/UX Programmer's Reference ``Bourne Shell Reference'' in A/UX Shells and Shell Programming 18 January 1992