32v 1m sh
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SH(1) UNIX Programmer's Manual SH(1)
NAME
sh, for, case, if, while, :, ., break, continue, cd, eval,
exec, exit, export, login, newgrp, 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 argu-
ment 0 (see _e_x_e_c(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
terminates abnormally (see _s_i_g_n_a_l(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 precedence. 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 semi-
colons, to delimit commands.
A _c_o_m_m_a_n_d is either a simple-command or one of the follow-
ing. 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 omit-
ted then 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 pattern that matches _w_o_r_d. The form of the pat-
terns is the same as that used for file name genera-
tion.
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 terminates. 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
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 parame-
ters. Positional 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 under-
scores (a _n_a_m_e), a digit, or any of the characters * @
# ? - $ !. The value, if any, of the parameter is sub-
stituted. 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 then 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 then substitute its value; other-
wise 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 then set it to _w_o_r_d; the value
of the parameter 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 then substitute its value; other-
wise, print _w_o_r_d and exit from the shell. If _w_o_r_d is
omitted then 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 then substitute _w_o_r_d; otherwise
substitute nothing.
In the above _w_o_r_d is not evaluated unless it is to be used
as the substituted string. (So that, for example, echo
${d-`pwd`} will only execute _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
decimal.
$ 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
then 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.
Blank interpretation.
After parameter and command substitution, any results of
substitution are scanned for internal field separator char-
acters (those found in $IFS) and split into distinct argu-
ments where such characters are found. Explicit null argu-
ments ("" 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
then 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 pat-
tern then 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 lexi-
cally 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 \. \new-
line 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" "$2" ... .
Prompting.
When used interactively, the shell prompts with the value of
PS1 before reading a command. If at any time a newline is
typed and further input is needed to complete a command then
the secondary prompt ($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 then it is created; other-
wise it is truncated to zero length.
>>_w_o_r_d
Use file _w_o_r_d as standard output. If the file exists
then output is appended (by seeking to the end); other-
wise 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
then no interpretation is placed upon the characters of
the document; otherwise, parameter and command substi-
tution 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 stan-
dard output using >.
If one of the above is preceded by a digit then 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 descrip-
tor 1.
If a command is followed by & then the default standard
input for the command is the empty file (/dev/null). Other-
wise, the environment for the execution of a command con-
tains 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 _e_x_e_c(2) and _e_n_v_i_r_o_n(5). 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. Executed 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 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 _e_x_e_c(2).
The shell parameter $PATH defines the search path for the
directory containing the command. Each alternative direc-
tory name is separated by a colon (:). The default path is
:/bin:/usr/bin. If the command name contains a / then 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._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 subshell.
Special commands.
The following commands are executed in the shell process and
except where specified no input output redirection is per-
mitted for such commands.
: 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 con-
taining _f_i_l_e.
break [_n]
Exit from the enclosing for or while loop, if any. If
_n is specified then break _n levels.
continue [_n]
Resume the next iteration of the enclosing for or while
loop. If _n is specified then resume at the _n-th
enclosing loop.
cd [_a_r_g]
Change the current directory to _a_r_g. The shell parame-
ter $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 arguments may appear and if no other argu-
ments 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 then 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 then a list of exportable names is
printed.
login [_a_r_g ...]
Equivalent to `exec login arg ...'.
newgrp [_a_r_g ...]
Equivalent to `exec newgrp 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 encoun-
tered.
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 arguments are given then a list of
all readonly names is printed.
set [-eknptuvx [_a_r_g ...]]
-e If non interactive then exit immediately if a com-
mand fails.
-k All keyword arguments are placed in the environment
for a command, not just those that precede the com-
mand 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 exe-
cuted.
- 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 then 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 executed in order of signal number.
If _a_r_g is absent then all trap(s) _n are reset to their
original values. If _a_r_g is the null string then this
signal is ignored by the shell and by invoked commands.
If _n is 0 then the command _a_r_g is executed on exit from
the shell, otherwise upon receipt of signal _n as num-
bered in _s_i_g_n_a_l(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 termina-
tion status. If _n is not given then 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 then 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 descrip-
tor 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_n_a_l(2))
is ignored (so that `kill 0' does not kill an
interactive shell) and the interrupt signal SIG-
INT is caught and ignored (so that wait is inter-
ruptable). 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
test(1), exec(2),
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.
