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
Sh 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 simple-command is a sequence of non blank 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 execve(2)). The value of a simple-command is its exit status if it terminates normally or 200+status if it terminates abnormally (see sigvec(2) for a list of status values).

A pipeline is a sequence of one or more commands separated 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.

A list is a sequence of one or more pipelines separated by ;, &, && or || and optionally terminated by ; or &. ; and & have equal precedence which is lower than that of && and ||, && and || also have equal precedence. A semicolon causes sequential execution; an ampersand causes the preceding pipeline to be executed without waiting for it to finish. The symbol && (||) causes the list following to be executed only if the preceding pipeline returns a zero (non zero) value. Newlines may appear in a list, instead of semicolons, to delimit commands.

A command 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 name [in word ...] do list done
    Each time a for command is executed name is set to the next word in the for word list. If in word ... is omitted, in "$@" is assumed. 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 file name generation.

if list then list [elif list then list] ... [else list] fi
    The list following if is executed and if it returns zero the list following then is executed. Otherwise, the list following elif is executed and if its value is zero the list following then is executed. Failing that the else list is executed.

while list [do list] done
    A while command repeatedly executes the while list and if its value is zero executes the do list; otherwise the loop terminates. The value returned by a while command is that of the last executed command in the do list. until may be used in place of while to negate the loop termination test.

( list )
    Execute list in a subshell.

{ list }
    list 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. Positional parameters may be assigned values by set. Variables may be set by writing

    name=value [ name=value ] ...

${parameter}
    A parameter is a sequence of letters, digits or underscores (a name), a digit, or any of the characters * @ # ? - $ !. 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 a digit, it is a positional parameter. If parameter 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.

${parameter-word}
    If parameter is set, substitute its value; otherwise substitute word.

${parameterword}
    If parameter is not set, 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, substitute its value; otherwise, print word and exit from the shell. If word is omitted, a standard message is printed.

${parameter+word}
    If parameter is set, substitute word; otherwise substitute nothing.

In the above word is not evaluated unless it is to be used as the substituted string. (So that, for example, echo ${d-'pwd'} will only execute pwd if d is unset.)

The following parameters 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 decimal.
    $   The process number of this shell.
    !   The process number of the last background command invoked.

The following parameters 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 newline. IFS is ignored if sh 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 parameters 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 characters 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 quoted 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 command and 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, output is appended (by seeking to the end); otherwise the file is created.

<<word
    The shell input is read up to a line the same as word, or 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, \newline is ignored, and \ is used to quote the characters \ $ ' and the first character of word.

<&digit
    The standard input is duplicated from file descriptor digit; see dup(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 invoking shell as modified by input output specifications.

Environment.
The environment is a list of name-value pairs that is passed to an executed program in the same way as a normal argument list; see execve(2) and environ(7). 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 affects 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 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 simple-command may be augmented by prefixing it with one or more assignments to parameters. Thus these two lines are equivalent

    TERM=450 cmd args
    (export TERM; TERM=450; cmd args)

If the -k flag is set, all keyword arguments are placed in the environment, 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 inherited 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 execve(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 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 (i.e., a separate process) is spawned to read it. A parenthesized command is also executed in a subshell.

Special commands.
The following commands are executed in the shell process and except where specified no input output redirection is permitted for such commands.

#   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.
. file
    Read and execute commands from file and return. The search path $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.
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 [arg]
    Change the current directory to arg. The shell parameter $HOME is the default arg.
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 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 [name ...]
    The given names are marked for automatic export to the environment of subsequently-executed commands. If no arguments are given, a list of exportable names is printed.
login [arg ...]
    Equivalent to 'exec login arg ...'.
read name ...
    One line is read from the standard input; successive words of the input are assigned to the variables name in order, with leftover words to the last variable. The return code is 0 unless the 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.
set [-eknptuvx [arg ...]]
    -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 [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. 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 invoked commands. If n is 0, the command arg is executed on exit from the shell, otherwise upon receipt of signal n as numbered in sigvec(2). Trap with no arguments prints a list of commands associated with each signal number.

umask [ nnn ]
    The user file creation mask is set to the octal value nnn (see umask(2)). If nnn is omitted, the current value of the mask is printed.

wait [n]
    Wait for the specified process and report its termination status. 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 string
    If the -c flag is present, commands are read from string.
-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 gtty) then this shell is interactive. In this case the terminate signal SIGTERM (see sigvec(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 interactively 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.