SH(1) SH(1) NAME sh, for, case, if, while, :, ., break, continue, cd, eval, exec, exit, export, login, read, readonly, set, shift, times, trap, umask, wait - command language SYNOPSIS sh [ -ceiknrstuvx ] [ arg ] ... DESCRIPTION _S_h is a command programming language that executes commands read from a terminal or a file. See invocation for the meaning of arguments to the shell. Commands. A _s_i_m_p_l_e_-_c_o_m_m_a_n_d is a sequence of non blank _w_o_r_d_s 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 _e_x_e_c_v_e(2)). The _v_a_l_u_e of a simple-command is its exit status if it terminates normally or 200+_s_t_a_t_u_s if it termi‐ nates abnormally (see _s_i_g_v_e_c(2) for a list of status values). A _p_i_p_e_l_i_n_e is a sequence of one or more _c_o_m_m_a_n_d_s separated by |. The standard output of each command but the last is connected by a _p_i_p_e(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. A _l_i_s_t is a sequence of one or more _p_i_p_e_l_i_n_e_s separated by ;, &, && or || and optionally terminated by ; or &. ; and & have equal precedence which is lower than that of && and ||, && and || also have equal prece‐ dence. A semicolon causes sequential execution; an ampersand causes the preceding _p_i_p_e_l_i_n_e to be executed without waiting for it to finish. The symbol && (||) causes the _l_i_s_t following to be executed only if the preceding _p_i_p_e_l_i_n_e returns a zero (non zero) value. Newlines may appear in a _l_i_s_t_, instead of semicolons, to delimit commands. A _c_o_m_m_a_n_d is either a simple-command or one of the following. The value returned by a command is that of the last simple-command executed in the command. for _n_a_m_e [in _w_o_r_d ...] do _l_i_s_t done Each time a for command is executed _n_a_m_e is set to the next word in the for word list. If in _w_o_r_d ... is omitted, in "$@" is assumed. Execution ends when there are no more words in the list. case _w_o_r_d in [_p_a_t_t_e_r_n [ | _p_a_t_t_e_r_n ] ... ) _l_i_s_t ;;] ... esac A case command executes the _l_i_s_t associated with the first pat‐ tern that matches _w_o_r_d_. The form of the patterns is the same as that used for file name generation. if _l_i_s_t then _l_i_s_t [elif _l_i_s_t then _l_i_s_t] ... [else _l_i_s_t] fi The _l_i_s_t following if is executed and if it returns zero the _l_i_s_t following then is executed. Otherwise, the _l_i_s_t following elif is executed and if its value is zero the _l_i_s_t following then is executed. Failing that the else _l_i_s_t is executed. while _l_i_s_t [do _l_i_s_t] done A while command repeatedly executes the while _l_i_s_t and if its value is zero executes the do _l_i_s_t_; otherwise the loop termi‐ nates. The value returned by a while command is that of the last executed command in the do _l_i_s_t_. until may be used in place of while to negate the loop termination test. ( _l_i_s_t ) Execute _l_i_s_t in a subshell. { _l_i_s_t } _l_i_s_t is simply executed. The following words are only recognized as the first word of a command and when not quoted. if then else elif fi case in esac for while until do done { } Command substitution. The standard output from a command enclosed in a pair of back quotes (``) may be used as part or all of a word; trailing newlines are removed. Parameter substitution. The character $ is used to introduce substitutable parameters. Posi‐ tional parameters may be assigned values by set. Variables may be set by writing _n_a_m_e=_v_a_l_u_e [ _n_a_m_e=_v_a_l_u_e ] ... ${_p_a_r_a_m_e_t_e_r} A _p_a_r_a_m_e_t_e_r is a sequence of letters, digits or underscores (a _n_a_m_e), a digit, or any of the characters * @ # ? - $ !. The value, if any, of the parameter is substituted. The braces are required only when _p_a_r_a_m_e_t_e_r is followed by a letter, digit, or underscore that is not to be interpreted as part of its name. If _p_a_r_a_m_e_t_e_r is a digit, it is a positional parameter. If _p_a_r_a_m_e_t_e_r is * or @ then all the positional parameters, starting with $1, are substituted separated by spaces. $0 is set from argument zero when the shell is invoked. ${_p_a_r_a_m_e_t_e_r_-_w_o_r_d} If _p_a_r_a_m_e_t_e_r is set, substitute its value; otherwise substitute _w_o_r_d_. ${_p_a_r_a_m_e_t_e_r_=_w_o_r_d} If _p_a_r_a_m_e_t_e_r is not set, set it to _w_o_r_d_; the value of the param‐ eter is then substituted. Positional parameters may not be assigned to in this way. ${_p_a_r_a_m_e_t_e_r_?_w_o_r_d} If _p_a_r_a_m_e_t_e_r is set, substitute its value; otherwise, print _w_o_r_d and exit from the shell. If _w_o_r_d is omitted, a standard message is printed. ${_p_a_r_a_m_e_t_e_r_+_w_o_r_d} If _p_a_r_a_m_e_t_e_r is set, substitute _w_o_r_d_; otherwise substitute noth‐ ing. In the above _w_o_r_d is not evaluated unless it is to be used as the sub‐ stituted string. (So that, for example, echo ${d-´pwd´} will only exe‐ cute _p_w_d if _d is unset.) The following _p_a_r_a_m_e_t_e_r_s are automatically set by the shell. # The number of positional parameters in decimal. - Options supplied to the shell on invocation or by set. ? The value returned by the last executed command in deci‐ mal. $ The process number of this shell. ! The process number of the last background command invoked. The following _p_a_r_a_m_e_t_e_r_s are used but not set by the shell. HOME The default argument (home directory) for the cd command. PATH The search path for commands (see execution). MAIL If this variable is set to the name of a mail file, the shell informs the user of the arrival of mail in the specified file. PS1 Primary prompt string, by default ’$ ’. PS2 Secondary prompt string, by default ’> ’. IFS Internal field separators, normally space, tab, and new‐‐ line. IFS is ignored if _s_h is running as root or if the effective user id differs from the real user id. Blank interpretation. After parameter and command substitution, any 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 _p_a_r_a_m_e_t_e_r_s that have no values) are removed. File name 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 file names that match the pattern. If no file name is found that matches the pattern, the word is left unchanged. The character . at the start of a file name or immediately following a /, and the character /, must be matched explicitly. * Matches any string, including the null string. ? Matches any single character. [...] Matches any one of the characters enclosed. A pair of charac‐ ters separated by - matches any character lexically between the pair. 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 _q_u_o_t_e_d by preceding it with a \. \newline is ignored. All characters enclosed between a pair of quote marks (´´´´), except a single quote, are quoted. Inside double quotes ("") parameter and command substitution occurs and \ quotes the characters \ ´´ " and $. "$*" is equivalent to "$1 $2 ..." whereas "$@" is equivalent to "$1" "$2" ... . Prompting. When used interactively, the shell prompts with the value of PS1 before reading a command. If at any time a newline is typed and further input is needed to complete a command, the secondary prompt ($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 _c_o_m_m_a_n_d and are not passed on to the invoked command. Substitution occurs before _w_o_r_d or _d_i_g_i_t is used. <_w_o_r_d Use file _w_o_r_d as standard input (file descriptor 0). >_w_o_r_d Use file _w_o_r_d as standard output (file descriptor 1). If the file does not exist, it is created; otherwise it is truncated to zero length. >>_w_o_r_d Use file _w_o_r_d as standard output. If the file exists, output is appended (by seeking to the end); otherwise the file is created. <<_w_o_r_d The shell input is read up to a line the same as _w_o_r_d, or end of file. The resulting document becomes the standard input. If any character of _w_o_r_d is quoted, no interpretation is placed upon the characters of the document; otherwise, parameter and command substitution occurs, \newline is ignored, and \ is used to quote the characters \ $ ´´ and the first character of _w_o_r_d_. <&_d_i_g_i_t The standard input is duplicated from file descriptor _d_i_g_i_t_; see _d_u_p(2). 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 to be a duplicate of file descriptor 1. If a command is followed by & then 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 invok‐ ing shell as modified by input output specifications. Environment. The environment is a list of name-value pairs that is passed to an exe‐ cuted program in the same way as a normal argument list; see _e_x_e_c_v_e(2) and _e_n_v_i_r_o_n(7). The shell interacts with the environment in several ways. On invocation, the shell scans the environment and creates a _p_a_r_a_m_e_t_e_r for each name found, giving it the corresponding value. Exe‐ cuted commands inherit the same environment. If the user modifies the values of these _p_a_r_a_m_e_t_e_r_s or creates new ones, none of these affects the environment unless the export command is used to bind the shell’s _p_a_r_a_m_e_t_e_r to the environment. The environment seen by any executed command is thus composed of any unmodified name-value pairs originally inherited by the shell, plus any modifications or additions, all of which must be noted in export commands. The environment for any _s_i_m_p_l_e_-_c_o_m_m_a_n_d may be augmented by prefixing it with one or more assignments to _p_a_r_a_m_e_t_e_r_s_. Thus these two lines are equivalent TERM=450 cmd args (export TERM; TERM=450; cmd args) If the -k flag is set, _a_l_l keyword arguments are placed in the environ‐ ment, even if the occur after the command name. The following prints ’a=b c’ and ’c’: echo a=b c set -k echo a=b c Signals. The INTERRUPT and QUIT signals for an invoked command are ignored if the command is followed by &; otherwise signals have the values inher‐ ited by the shell from its parent. (But see also trap.) Execution. Each time a command is executed the above substitutions are carried out. Except for the ’special commands’ listed below a new process is created and an attempt is made to execute the command via an _e_x_e_c_v_e(2). The shell parameter $PATH defines the search path for the directory containing the command. Each alternative directory name is separated by a colon (:). The default path is :/bin:/usr/bin. If the command name contains a /, the search path is not used. Otherwise, each direc‐ tory in the path is searched for an executable file. If the file has execute permission but is not an _a_._o_u_t file, it is assumed to be a file containing shell commands. A subshell (i.e., a separate process) is spawned to read it. A parenthesized command is also executed in a sub‐ shell. Special commands. The following commands are executed in the shell process and except where specified no input output redirection is permitted for such com‐ mands. # For non-interactive shells, everything following the # is treated as a comment, i.e. the rest of the line is ignored. For interactive shells, the # has no special effect. : No effect; the command does nothing. . _f_i_l_e Read and execute commands from _f_i_l_e and return. The search path $PATH is used to find the directory containing _f_i_l_e. break [_n] Exit from the enclosing for or while loop, if any. If _n is specified, break _n levels. continue [_n] Resume the next iteration of the enclosing for or while loop. If _n is specified, resume at the _n-th enclosing loop. cd [_a_r_g] Change the current directory to _a_r_g_. The shell parameter $HOME is the default _a_r_g. eval [_a_r_g ...] The arguments are read as input to the shell and the resulting command(s) executed. exec [_a_r_g ...] The command specified by the arguments is executed in place of this shell without creating a new process. Input output argu‐ ments may appear and if no other arguments are given cause the shell input output to be modified. exit [_n] Causes a non interactive 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 exit from the shell.) export [_n_a_m_e ...] The given names are marked for automatic export to the _e_n_v_i_r_o_n_‐ _m_e_n_t of subsequently-executed commands. If no arguments are given, a list of exportable names is printed. login [_a_r_g ...] Equivalent to ’exec login arg ...’. read _n_a_m_e _._._. One line is read from the standard input; successive words of the input are assigned to the variables _n_a_m_e in order, with leftover words to the last variable. The return code is 0 unless the end-of-file is encountered. readonly [_n_a_m_e ...] The given names are marked readonly and the values of the these names may not be changed by subsequent assignment. If no argu‐ ments are given, a list of all readonly names is printed. set [-eknptuvx [_a_r_g ...]] -e If non interactive, exit immediately if a command fails. -k All keyword arguments are placed in the environment for a command, not just those that precede the command name. -n Read commands but do not execute them. -t Exit after reading and executing one command. -u Treat unset variables as an error when substituting. -v Print shell input lines as they are read. -x Print commands and their arguments as they are executed. - Turn off the -x and -v options. These flags can also be used upon invocation of the shell. The current set of flags may be found in $-. Remaining arguments are positional parameters and are assigned, in order, to $1, $2, etc. If no arguments are given, the values of all names are printed. shift The positional parameters from $2... are renamed $1... times Print the accumulated user and system times for processes run from the shell. trap [_a_r_g] [_n] ... _A_r_g is a command to be read and executed when the shell receives signal(s) _n_. (Note that _a_r_g is scanned once when the trap is set and once when the trap is taken.) Trap commands are exe‐ cuted in order of signal number. If _a_r_g is absent, all trap(s) _n are reset to their original values. If _a_r_g is the null string, this signal is ignored by the shell and by invoked com‐ mands. If _n is 0, the command _a_r_g is executed on exit from the shell, otherwise upon receipt of signal _n as numbered in _s_i_g_v_e_c(2). _T_r_a_p with no arguments prints a list of commands associated with each signal number. umask [ _n_n_n ] The user file creation mask is set to the octal value _n_n_n (see _u_m_a_s_k(2)). If _n_n_n is omitted, the current value of the mask is printed. wait [_n] Wait for the specified process and report its termination sta‐ tus. If _n is not given, all currently active child processes are waited for. The return code from this command is that of the process waited for. Invocation. If the first character of argument zero is -, commands are read from $HOME/.profile, if such a file exists. Commands are then read as described below. The following flags are interpreted by the shell when it is invoked. -c _s_t_r_i_n_g If the -c flag is present, commands are read from _s_t_r_i_n_g_. -s If the -s flag is present or if no arguments remain then commands are read from the standard input. Shell output is written to file descriptor 2. -i If the -i flag is present or if the shell input and output are attached to a terminal (as told by _g_t_t_y) then this shell is _i_n_t_e_r_a_c_t_i_v_e_. In this case the terminate signal SIGTERM (see _s_i_g_v_e_c(2)) is ignored (so that ’kill 0’ does not kill an interactive shell) and the interrupt signal SIGINT is caught and ignored (so that wait is interruptible). In all cases SIGQUIT is ignored by the shell. The remaining flags and arguments are described under the set command. FILES $HOME/.profile /tmp/sh* /dev/null SEE ALSO csh(1), test(1), execve(2), environ(7) DIAGNOSTICS Errors detected by the shell, such as syntax errors cause the shell to return a non zero exit status. If the shell is being used non interac‐ tively then execution of the shell file is abandoned. Otherwise, the shell returns the exit status of the last command executed (see also exit). BUGS 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 the file by another name. 7th Edition May 5, 1986 SH(1)