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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
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 argu-
ment 0 (see exec(2)). The value of a simple-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 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 semi-
colons, 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 omit-
ted 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 the
first pattern that matches word. The form of the pat-
terns is the same as that used for file name genera-
tion.
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
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 under-
scores (a name), a digit, or any of the characters * @
# ? - $ !. The value, if any, of the parameter is sub-
stituted. 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 parame-
ter 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 from argument zero when the shell is invoked.
${parameter-word}
If parameter is set then substitute its value; other-
wise substitute word.
${parameter=word}
If parameter is not set then 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 then substitute its value; other-
wise, 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 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
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 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
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 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 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 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 then it is created; other-
wise 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); other-
wise 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 substi-
tution 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 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 exec(2) and environ(5). 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 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 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.
. file
Read and execute commands from file and return. The
search path $PATH is used to find the directory con-
taining file.
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 [arg]
Change the current directory to arg. The shell parame-
ter $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 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 [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; 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 encoun-
tered.
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 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 [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 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 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 string If the -c flag is present then 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 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
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 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.