SH(1) 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
Sh is a command programming language that executes com-
mands 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 sepa-
rated 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(2)). The value of a sim-
ple-command is its exit status if it terminates normally
or 200+status if it terminates abnormally (see signal(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 pre-
ceding 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 follow-
ing. 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 then 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
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SH(1) SH(1)
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. Other-
wise, the list following elif is executed and if
its value is zero the list following then is exe-
cuted. 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; oth-
erwise the loop terminates. The value returned by
a while command is that of the last executed com-
mand 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
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
name=value [ name=value ] ...
${parameter}
A parameter is a sequence of letters, digits or
underscores (a name), a digit, or any of the char-
acters * @ # ? - $ !. 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 then 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
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SH(1) SH(1)
from argument zero when the shell is invoked.
${parameter-word}
If parameter is set then substitute its value; oth-
erwise substitute word.
${parameter=word}
If parameter is not set then set it to word; the
value of the parameter is then substituted. Posi-
tional parameters may not be assigned to in this
way.
${parameter?word}
If parameter is set then substitute its value; oth-
erwise, print word and exit from the shell. If
word is omitted then a standard message is printed.
${parameter+word}
If parameter is set then 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 deci-
mal.
- Options supplied to the shell on invocation
or by set.
? The value returned by the last executed com-
mand 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 execu-
tion).
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.
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SH(1) SH(1)
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 argu-
ments (those resulting from parameters that have no val-
ues) 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 pattern then the word is left unchanged. The charac-
ter . at the start of a file name or immediately follow-
ing 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 \. \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 substitu-
tion 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-com-
mand 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
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0).
>word Use file word as standard output (file descriptor
1). If the file does not exist then it is created;
otherwise it is truncated to zero length.
>>word Use file word as standard output. If the file
exists then 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 then 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 descrip-
tor 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 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
descriptor 1.
If a command is followed by & then the default standard
input for the command is the empty file (/dev/null). Oth-
erwise, 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 exec(2) and environ(5). The shell
interacts with the environment in several ways. On invo-
cation, 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
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SH(1) SH(1)
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 exec(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.out file, it is
assumed to be a file containing shell commands. A sub-
shell (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.
: 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 then break n levels.
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SH(1) SH(1)
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 [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 pro-
cess. 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 then
the exit status is that of the last command exe-
cuted. (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 then a list of exportable
names is printed.
login [arg ...]
Equivalent to `exec login arg ...'.
newgrp [arg ...]
Equivalent to `exec newgrp arg ...'.
read name ...
One line is read from the standard input; succes-
sive words of the input are assigned to the vari-
ables 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 then a list
of all readonly names is printed.
set [-eknptuvx [arg ...]]
-e If non interactive then exit immediately if a
command fails.
-k All keyword arguments are placed in the environ-
ment 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 substi-
tuting.
-v Print shell input lines as they are read.
-x Print commands and their arguments as they are
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SH(1) SH(1)
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 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 [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 then all
trap(s) n are reset to their original values. If
arg is the null string then this signal is ignored
by the shell and by invoked commands. If n is 0
then the command arg is executed on exit from the
shell, otherwise upon receipt of signal n as num-
bered in signal(2). Trap with no arguments prints
a list of commands associated with each signal num-
ber.
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 ter-
mination status. If n is not given then all cur-
rently active child processes are waited for. The
return code from this command is that of the pro-
cess 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 then commands are
read from string.
-s If the -s flag is present or if no arguments
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SH(1) SH(1)
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
signal(2)) is ignored (so that `kill 0' does
not kill an interactive shell) and the inter-
rupt signal SIGINT is caught and ignored (so
that wait is interruptable). 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.
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