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
Sh is a command programming language that executes commands read from a
terminal or a file; jsh is identical, but with csh-style job control
enabled (job control is described in the csh(1) man page). 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.
${parameter=word}
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 (:). 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 ALSOcsh(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.
7th Edition May 5, 1986 SH(1)
