sh(C) 06 January 1993 sh(C) Name sh - invoke the shell command interpreter Syntax sh [ -aceiknrstuvx ] [ args ] Description The shell is the standard command programming language that executes com- mands read from a terminal or a file. See ``Invocation'' below for the meaning of arguments to the shell. Commands A simple-command is a sequence of nonblank words separated by blanks (a blank is a tab or a space). 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(S)). The value of a simple-command is its exit status if it terminates normally, or (octal) 1000+status if it ter- minates abnormally. See signal(S) for a list of status values. A pipeline is a sequence of one or more commands separated by a vertical bar (|). (The caret (^), is an obsolete synonym for the vertical bar and should not be used in a pipeline. Scripts that use ``^'' for pipelines are incompatible with the Korn shell.) The standard output of each com- mand but the last is connected by a pipe(S) 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. A list is a sequence of one or more pipelines separated by ;, &, &&, or ||, and optionally terminated by ; or &. Of these four symbols, ; and & have equal precedence, which is lower than that of && and ||. The sym- bols && and || also have equal precedence. A semicolon (;) causes sequential execution of the preceding pipeline; an ampersand (&) causes asynchronous execution of the preceding pipeline (that is, 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 commands. Unless otherwise stated, the value returned by a command is that of the last simple-command executed in the command: for name [ in word ... ] do list done Each time a for command is executed, name is set to the next word taken from the in word list. If in word is omitted, then the for command exe- cutes 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 A case command executes the list associated with the first pattern that matches word. The form of the patterns is the same as that used for filename generation (see ``Filename generation'' below). if list then list [ elif list then list ] ... [ else list ] fi The list following if is executed and, if it returns a zero exit status, the list following the first then is executed. Otherwise, the list fol- lowing 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 A while command 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 exe- cuted, then the while command returns a zero exit status; until may be used in place of while to negate the loop termination test. until list do list done until is similar to while, only until continues execution until the first list returns a zero exit status. In other words, until works until the test condition succeeds (it works the whole time the command is failing); while works until the test condition fails. until is useful when you are waiting for a particular event to occur. (list) Executes list in a subshell. {list;} list is simply executed. name () {list;} Define a function which is referenced by name. The body of functions is the list of commands between { and }. Execution of functions is described later (see ``Execution''.) 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 { } 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 between grave accents ( ` ` ) may be used as part or all of a word; trailing newlines are removed. No interpretation is done on the command string before the string is read, except to remove backslashes (\) used to escape other characters. Backslashes may be used to escape grave accents (`) or other backslashes and are removed before the command string is read. Escaping grave accents allows nested command substitution. If the command substitution lies within a pair of double quotes ( " ` ... ` " ), backslashes used to escape a double quote (\") will be removed; otherwise, they will be left intact. If a backslash is used to escape a newline character, both the backslash and the newline are removed (see the section on ``Quoting''). In addi- tion, backslashes used to escape dollar signs ( \$ ) are removed. Since no interpretation is done on the command string before it is read, inserting a backslash to escape a dollar sign has no effect. Backslashes that precede characters other than \, `, ", newline, and $ are left intact. 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} A parameter is a sequence of letters, digits, or underscores (a name), a digit, or any of the characters *, @, #, ?, -, $, and !. 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. A name must begin with a letter or underscore. If parameter is a digit then it is a positional parameter. If parameter is * or @, then all the positional parameters, starting with $1, are substi- tuted (separated by spaces). Parameter $0 is set from argument zero when the shell is invoked. ${parameter:-word} If parameter is set and is not a null argument, substitute its value; otherwise substitute word. ${parameter:=word} If parameter is not set or is null, then set it to word; the value of the parameter is then substituted. Positional parameters may not be assigned to in this way. ${parameter:?word} If parameter is set and is not a null argument, 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 not a null argument, substitute word; otherwise substitute nothing. In the above, word is not evaluated unless it is to be used as the sub- stituted string, so that in the following example, pwd is executed only if d is not set or is null: echo ${d:-(gapwd`} If the colon (:) is omitted from the above expressions, then the shell only checks whether parameter is set. The following parameters are automatically set 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 com- mand $ The process number of this shell ! The process number of the last background command invoked The following parameters are used by the shell: CDPATH Defines search path for the cd command. See the section ``cd'' under ``Special commands'' below. HOME The default argument (home directory) for the cd command PATH The search path for commands (see ``Execution'' below) MAIL If this variable is set to the name of a mail file, then the shell informs the user 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 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 can be fol- lowed by ``%'' and a message 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 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 file name part of its value, the shell becomes a restricted shell. The shell gives default values to PATH, PS1, PS2, and IFS, while HOME and MAIL are not set at all by the shell (although HOME is set by login(M)). 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 pat- tern, 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. These characters and their matching patterns are: * Matches any string, including the null string. ? Matches any single character. [...] 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 bracket ([) is an exclamation mark (!), then 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 (that is, made to stand for itself) by preced- ing it with a ``\''. The pair \newline is ignored. All characters enclosed between a pair of single quotation marks (' '), except a single quotation mark, are quoted. Inside double quotation marks (" "), parame- ter and command substitution occurs and ``\'' quotes the characters \, `, ", and $. "$*" 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 (that is, the value of PS2) is issued. Spelling checker When using cd(C) the shell checks spelling. For example, if you change to a different directory using cd and misspell the directory name, the shell responds with an alternative spelling of an existing directory. Enter ``y'' and press <Return> (or just press <Return>) to change to the offered directory. If the offered spelling is incorrect, enter ``n'', then retype the command line. In this example the sh(C) response is boldfaced: $ cd /usr/spol/uucp cd /usr/spool/uucp?y ok 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. They are not passed on to the invoked command; substitution occurs before word or digit is used: <word Use file word as standard input (file descriptor 0). >word Use 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 Use file word as standard output. If the file exists, out- put is appended to it (by first seeking the end-of-file); otherwise, the file is created. <<[-]word The shell input is read up to a line that is the 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 docu- ment; 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 The standard input is duplicated from file descriptor digit (see dup(S)). Similarly for the standard output using >. <&- The standard input is closed. Similarly for the standard output using >. If one of the above is preceded by a digit, the file descriptor created is that specified by the digit (instead of the default 0 or 1). For example: ... 2>&1 creates file descriptor 2 that is a duplicate of file descriptor 1. 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. Environment The environment (see environ(M)) 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. Executed commands inherit the same environment. If the user modifies the values of these parameters or creates new ones, none of these affect the environment unless the export command is used to bind the shell's parameter to the environment. The environment seen by any executed command is 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=wy60 cmd args and (export TERM; TERM=wy60; cmd args) are equivalent (as far as the above execution of cmd is concerned). If the -k flag is set, all keyword arguments are placed in the environ- ment, even if they occur after the command name. 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. See the trap command below. Execution Each time a command is executed, the above substitutions are carried out. 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 how this differs from the execution of shell procedures). The posi- tional 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 exe- cute the command via exec(S). The shell parameter PATH defines the search path for the directory con- taining 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 equal sign or between the colon delimiters anywhere else in the path list. If the command name contains a ``/'', then the search path is not used. Otherwise, each directory in the path is searched for an exe- cutable 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 (that is, a separate process) is spawned to read it. A parenthesized command is also executed in a subshell. Shell procedures are often used by users running the csh. However, if the first character of the procedure is a ``#'' (comment character), csh assumes the procedure is a csh script, and invokes /bin/csh to execute it. Always start sh procedures with some other character if csh users are to run the procedure at any time. This invokes the standard shell /bin/sh. 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 re-determined when- ever the current directory changes. The shell forgets all remembered locations whenever the PATH variable is changed or the hash -r command is executed (see hash in next section). Special commands Input/output redirection is permitted for these commands: : No effect; the command does nothing. A zero exit code is returned. . file Reads and executes commands from file and returns. The search path specified by PATH is used to find the directory containing file. break [ n ] Exits from the enclosing for, while, or until loop, if any. If n is specified, it breaks n levels. continue [ n ] Resumes the next iteration of the enclosing for, while, or until loop. If n is specified, it resumes at the n-th enclosing loop. cd [ arg ] Changes 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> (specifying the current directory). Note that the current directory is speci- fied by a null path name, which can appear immediately after the equal 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. If the shell is reading its commands from a terminal, and the speci- fied directory does not exist (or some component cannot be searched), spelling correction is applied to each component of directory, in a search for the ``correct'' name. The shell then asks whether or not to try and change directory to the corrected directory name; an answer of n means ``no'', and anything else is taken as ``yes''. echo [ arg ] Writes arguments separated by blanks and terminated by a newline on the standard output. Arguments may be enclosed in quotes. Quotes are required so that the shell correctly interprets these special escape sequences: \b Backspace \c Prints line without newline. \f Form feed \n Newline \r Carriage return \t Tab \v Vertical tab \\ Backslash \n The 8-bit character whose ASCII code is the 1, 2 or 3-digit octal number n. n must start with a zero. 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 the 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. getopts Used in shell scripts to support command syntax standards (see Intro(C)); it parses positional parameters and checks for legal options. See getopts(C) for usage and description. hash [ -r ] [ name ... ] For each name, the location in the search path of the command speci- fied by name is determined and remembered by the shell. The -r option causes the shell to forget all remembered locations. If no arguments are given, information 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 ... pwd Print the current working directory. See pwd(C) for usage and description. 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 these names may not be changed by subsequent assignment. If no arguments are given, a list of all readonly names is printed. return [ n ] 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 [ -aefhknuvx [ arg ... ] ] -a Mark variables which are modified or created for export. -e If the shell is noninteractive, 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). For example, if h is set, /bin/tty is added to the hash table when: showtty(){ tty } is declared. If h is unset, the function is not added to the hash table until showtty is called. -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. -u Treats unset variables as an error when substituting. -v Prints shell input lines as they are read. -x Prints commands and their arguments as they are executed. Although this flag is passed to subshells, it does not enable tracing in those subshells. -- Does not change any of the flags; useful in setting $1 to ``-''. Using ``+'' rather than ``-'' causes these flags to be turned off. These flags can also be used upon invocation of the shell. The current set of flags 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 $2 ... are renamed $1 ... If n is specified, shift the positional parameters by n places. shift is the only way to access positional parameters above $9. test Evaluates conditional expressions. See test(C) for usage and description. times Prints the accumulated user and system times for processes run from the shell. trap [ arg ] [ n ] ... arg is a command 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. The highest signal number allowed is 16. 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 [ n ] imposes a size limit of n blocks on files written by the shell and its child processes (files of any size may be read). Any user may decrease the file size limit, but only the super user (root) can increase the limit. With no argument, the current limit is printed. If no option is given and a number is specified, -f is assumed. unset [ name ... ] For each name, remove the corresponding variable or function. The variables PATH, PS1, PS2, MAILCHECK and IFS cannot be unset. umask [ ooo ] The user file-creation mask is set to the octal number ooo where o is an octal digit (see umask(C)). If ooo is omitted, the current value of the mask is printed. wait [ n ] Waits for the specified process to terminate, and reports the termi- nation status. If n is not given, all currently active child pro- cesses are waited for. The return code from this command is always 0. Invocation If the shell is invoked through exec(S) and the first character of argu- ment 0 is ``-'', commands are initially read from /etc/profile and then 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 flags below are interpreted by the shell on invocation only; note that unless the -c or -s flag is specified, the first argument is assumed to be the name of a file containing commands, and the remain- ing arguments are passed as positional parameters to that command file: -c string If the -c flag is present, commands are read from string. -s If the -s flag is present or if no arguments remain, commands are read from the standard input. Any remaining arguments specify the positional parameters. Shell output is written to file descriptor 2. -t If the -t flag is present, a single command is read and exe- cuted, and the shell exits. This flag is intended for use by C programs only and is not useful interactively. -i If the -i flag is present or if the shell input and output are attached to a terminal, this shell is interactive. 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. -r If the -r flag is present, the shell is a restricted shell (see rsh(C)). The remaining flags and arguments are described under the set command above. Exit status Errors detected by the shell, such as syntax errors, cause the shell to return a nonzero exit status. If the shell is being used noninterac- tively, execution of the shell file is abandoned. Otherwise, the shell returns the exit status of the last command executed. See the exit com- mand above. Files /etc/profile system default profile, read by login shells before $HOME/.profile $HOME/.profile read by login shell at login /tmp/sh* temporary file for << /dev/null source of empty file See also a.out(FP), cd(C), dup(S), env(C), environ(M), exec(S), fork(S), ksh(C), login(M), newgrp(C), pipe(S), profile(M), rsh(C), signal(S), test(C), umask(C), umask(S) and wait(S). Notes The command readonly (without arguments) produces the same type of output as the command export. If << is used to provide standard input to an asynchronous process invoked by &, the shell gets mixed up about naming the input document; a garbage file /tmp/sh* is created and the shell complains about not being able to find that file by another name. 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. When a sh user logs in, the system reads and executes commands in /etc/profile before executing commands in the user's $HOME/.profile. You can, therefore, modify the environment for all sh users on the system by editing /etc/profile. The shell doesn't treat the high (eighth) bit in the characters of a com- mand line argument specially, nor does it strip the eighth bit from the characters of error messages. Previous versions of the shell used the eighth bit as a quoting mechanism. Existing programs that set the eighth bit of characters in order to quote them as part of the shell command line should be changed to use of the standard shell quoting mechanisms (see the section on ``Quoting''). Words used to specify filenames in input/output redirection are not expanded for filename generation (see the section on ``Filename generation''). For example, cat file1 > a* will create a file named a*. Because commands in pipelines are run as separate processes, variables set in a pipeline have no effect on the parent shell. If you get the error message: fork failed - too many processes try using the wait(C) command to clean up your background processes. If this doesn't help, the system process table is probably full or you have too many active foreground processes (there is a limit to the number of processes that be can associated with your login, and the number the sys- tem can keep track of). Warnings Not all processes of a 3 or more stage pipeline are children of the shell, and thus cannot be waited for. For wait n, if n is not an active process id, all your shell's currently active background processes are waited for and the return code will be zero. Standards conformance sh is conformant with: AT&T SVID Issue 2; and X/Open Portability Guide, Issue 3, 1989.