sh(1)sh(1)Namesh - command language interpreter
Syntaxsh [ -ceiknrstuvx ] [ arg... ]
Description
The command invokes 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 nonblank 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. For further information, see The value of a sim‐
ple command is its exit status if it terminates normally. If it termi‐
nates abnormally, 0200 is added to the status. For a list of status
values, see
A pipeline is a sequence of one or more commands separated by a verti‐
cal bar ( | ) . The standard output of each command but the last is
connected by a 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 &. Note that ; and & have
equal precedence and their precedence is lower than that of && and ||.
Also, && and || 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 fol‐
lowing to be executed only if the preceding pipeline returns a zero
value. [The symbol (||) causes the list following to be executed only
if the preceding pipeline returns a nonzero value.] Newlines can be
used 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 pat‐
tern 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. If
no else list or then list is executed, the if command returns a
zero (0) exit value.
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 termi‐
nates. The value returned by a while command is that of the
last executed command in the do list. Use until in place of
while to negate the loop termination test.
( list )
Execute list in a subshell.
{ list }
list is executed.
The following words are only recognized as the first word of a command
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 new lines are
removed.
Parameter substitution
The $ character is used to introduce substitutable parameters. Posi‐
tional parameters may be assigned values by the set command. Variables
may be set by using the following format:
name=value [ name=value ] ...
${parameter}
A parameter is a sequence of letters, digits or underscores (a
name), a digit, or any of the characters * @ # ? - or $ . 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 param‐
eter 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 noth‐
ing.
In the above word is not evaluated unless it is to be used as the sub‐
stituted string. (So that, for example, echo ${d-`pwd`} will only exe‐
cute 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 deci‐
mal.
$ 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 new‐
line.
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 charac‐
ters 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 $.
Prompting
When used interactively, the shell prompts with the value of PS1 before
reading a command. If at any time a newline is typed and further input
is needed to complete a command, the secondary prompt ($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
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 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 exe‐
cuted program in the same way as a normal argument list; see and The
shell interacts with the environment in several ways. On invocation,
the shell scans the environment and creates a parameter for each valid
name found (except IFS), giving it the corresponding value. (IFS can‐
not be set by the environment; it can only be set in the current shell
session.) Executed commands inherit the same environment. If the user
modifies the values of the following 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 the following 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 environ‐
ment, even if they occur after the command name. The following
sequence of commands 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 inher‐
ited 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 with an
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 direc‐
tory 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 (that is, a separate process) is
spawned to read it. A parenthesized command is also executed in a sub‐
shell.
Special commands
The following commands are executed in the shell process and, except
where specified, no input/output redirection is permitted for such com‐
mands.
: 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 nth enclosing loop.
cd [ arg ] Change the current directory to arg. The shell param‐
eter $HOME is the default arg.
echo [ arg ... ] See for usage and description.
eval [ arg ... ] The arguments are read as input to the shell and
the resulting command(s) are 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 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 argu‐
ments 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 read-only and the val‐
ues of these names may not be changed by subsequent assignment.
If no arguments are given, a list of all read-only names is
printed.
set [ -ekntuvx [ arg ... ] ]
-e 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, and so forth. 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 ] ... The arg is a command to be read and exe‐
cuted 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 val‐
ues. 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 sig‐
nal n as numbered in The 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. For
further information, see If nnn is omitted, the current value of
the mask is printed.
wait Wait for all child background processes to terminate. The
return code from this command is that returned upon termination
of the last process being 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 is ignored (so that 'kill 0' does not kill an inter‐
active shell) and the interrupt signal SIGINT is caught and
ignored (so that wait is interruptible). For further infor‐
mation, see In all cases SIGQUIT is ignored by the shell.
The remaining flags and arguments are described under the set command.
Restrictions
If << is used to provide standard input to an asynchronous process
invoked by &, the shell becomes confused about naming the input docu‐
ment. A garbage file is created, and the shell complains about not
being able to find the file by another name.
If you are using the Version 7 Bourne Shell, the command is not 8-bit
clean. Only with the System V Bourne Shell is the command 8-bit clean.
Define the PWD variable in your environment if you use or If you run
PWD is defined for you. You can redefine as long as it exists in the
environment.
Command substitution in the command fails to produce output when is
closed. The following sequence demonstrates the problem:
% /bin/sh
$ exec >&-
$ echo `echo x` >&2
The command should print but nothing is printed. This example works
correctly with
VAX Only Restriction
If is run from another program (by the system or exec system calls)
whose maximum descriptor in use is number 10, the prompt string is not
printed.
Diagnostics
Errors detected by the shell, such as syntax errors, cause the shell to
return a nonzero exit status. If the shell is being used noninterac‐
tively, then execution of the shell file is abandoned. Otherwise, the
shell returns the exit status of the last command executed (see also
exit).
FilesSee Alsocsh(1), sh5(1), test(1), execve(2), environ(7)sh(1)
