$() vs )
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original sh.1 massaged, nroffed and htmlized.
SH(1) SH(1)
NAME
sh, rsh - shell, the standard/restricted command programming language
SYNOPSIS
sh [ -ceiknrstuvx ] [ args ]
rsh [ -ceiknrstuvx ] [ args ]
DESCRIPTION
Sh is a command programming language that executes commands read from a
terminal or a file. Rsh is a restricted version of the standard com-
mand interpreter sh; it is used to set up login names and execution
environments whose capabilities are more controlled than those of the
standard shell. See Invocation below 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 exec(2)). The value of a simple-command
is its exit status if it terminates normally, or (octal) 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 | (or,
for historical compatibility, by ^). The standard output of each com-
mand 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 &. Of these four symbols, ; and
& have equal precedence, which is lower than that of && and ||. The
symbols && and || also have equal precedence. A semicolon (;) causes
sequential execution of the preceding pipeline; an ampersand (&) causes
asynchronous execution of the preceding pipeline (i.e., the shell does
not wait for that pipeline to finish). The symbol && (||) causes the
list following it to be executed only if the preceding pipeline returns
a zero (non-zero) exit status. An arbitrary number of new-lines may
appear in a list, instead of semicolons, to delimit commands.
A command is either a simple-command or one of the following. Unless
otherwise stated, 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 taken from the in word list. If in word ... is omitted,
then the for command executes the do list once for each posi-
tional parameter that is set (see Parameter Substitution below).
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 (see File Name Generation
below).
if list then list [ elif list then list ] ... [ else list ] fi
The list following if is executed and, if it returns a zero exit
status, the list following the first then is executed. Other-
wise, the list following elif is executed and, if its value is
zero, the list following the next then is executed. Failing
that, the else list is executed. If no else list or then list
is executed, then the if command returns a zero exit status.
while list do list done
A while command repeatedly executes the while list and, if the
exit status of the last command in the list is zero, executes
the do list; otherwise the loop terminates. If no commands in
the do list are executed, then the while command returns a zero
exit status; until may be used in place of while to negate the
loop termination test.
(list)
Execute list in a sub-shell.
{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 esac for while until do done { }
Comments.
A word beginning with # causes that word and all the following charac-
ters up to a new-line to be ignored.
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 new-lines are
removed.
Parameter Substitution.
The character $ is used to introduce substitutable parameters. Posi-
tional parameters may be assigned values by set. Variables may be set
by writing:
name=value [ name=value ] ...
Pattern-matching is not performed on value.
${parameter}
A parameter is a sequence of letters, digits, or underscores (a
name), a digit, or any of the characters *, @, #, ?, -, $, and
!. 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. A name must begin with a letter or underscore. If
parameter is a digit then it is a positional parameter. If
parameter is * or @, then all the positional parameters, start-
ing with $1, are substituted (separated by spaces). Parameter
$0 is set from argument zero when the shell is invoked.
${parameter:-word}
If parameter is set and is non-null then substitute its value;
otherwise substitute word.
${parameter:=word}
If parameter is not set or is null then set it to word; the
value of the parameter is then substituted. Positional parame-
ters may not be assigned to in this way.
${parameter:?word}
If parameter is set and is non-null then substitute its value;
otherwise, print word and exit from the shell. If word is omit-
ted, then the message ``parameter null or not set'' is printed.
${parameter:+word}
If parameter is set and is non-null then substitute word; other-
wise substitute nothing.
In the above, word is not evaluated unless it is to be used as the sub-
stituted string, so that, in the following example, pwd is executed
only if d is not set or is null:
echo ${d:-`pwd`}
If the colon (:) is omitted from the above expressions, then the shell
only checks whether parameter is set or not.
The following parameters are automatically set by the shell:
# The number of positional parameters in decimal.
- Flags supplied to the shell on invocation or by the set
command.
? The decimal value returned by the last synchronously exe-
cuted command.
$ The process number of this shell.
! The process number of the last background command
invoked.
The following parameters are used by the shell:
HOME The default argument (home directory) for the cd command.
PATH The search path for commands (see Execution below). The
user may not change PATH if executing under rsh.
CDPATH The search path for the cd command.
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 new-
line.
The shell gives default values to PATH, PS1, PS2, and IFS, while HOME
and MAIL are not set at all by the shell (although HOME is set by
login(1)).
Blank Interpretation.
After parameter and command substitution, the 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 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 pattern, then the word is left unchanged. The character .
at the start of a file name or immediately following a /, as well as
the character / itself, must be matched explicitly.
* Matches any string, including the null string.
? Matches any single character.
[...] Matches any one of the enclosed characters. A pair of
characters separated by - matches any character lexically
between the pair, inclusive. If the first character fol-
lowing the opening ``[´´ is a ``!'' then any character
not enclosed is matched.
Quoting.
The following characters have a special meaning to the shell and cause
termination of a word unless quoted:
; & ( ) | ^ < > new-line space tab
A character may be quoted (i.e., made to stand for itself) by preceding
it with a \. The pair \new-line is ignored. All characters enclosed
between a pair of single quote marks (''), except a single quote, are
quoted. Inside double quote marks (""), parameter and command substi-
tution 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 new-line is typed and further
input is needed to complete a command, then the secondary prompt (i.e.,
the value of 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; otherwise, it
is truncated to zero length.
>>word Use file word as standard output. If the file exists
then output is appended to it (by first seeking to the
end-of-file); otherwise, the file is created.
<<[-]word The shell input is read up to a line that is the same as
word, or to an end-of-file. The resulting document
becomes the standard input. If any character of word is
quoted, then no interpretation is placed upon the charac-
ters of the document; otherwise, parameter and command
substitution occurs, (unescaped) \new-line is ignored,
and \ must be used to quote the characters \, $, `, and
the first character of word. If - is appended to <<,
then all leading tabs are stripped from word and from the
document.
<&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, 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 that is 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 invok-
ing shell as modified by input/output specifications.
Redirection of output is not allowed in the restricted shell.
Environment.
The environment (see environ(5)) is a list of name-value pairs that is
passed to an executed program in the same way as a normal argument
list. 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 com-
posed 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:
TERM=450 cmd args and
(export TERM; TERM=450; cmd args)
are equivalent (as far as the above execution of cmd is concerned).
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 first
prints a=b c and then 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, with the exception of signal 11 (but
see also the trap command below).
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 exec(2).
The shell parameter PATH defines the search path for the directory con-
taining the command. Alternative directory names are separated by a
colon (:). The default path is :/bin:/usr/bin (specifying the current
directory, /bin, and /usr/bin, in that order). Note that the current
directory is specified by a null path name, which can appear immedi-
ately after the equal sign or between the colon delimiters anywhere
else in the path list. If the command name contains a / then the
search path is not used; such commands will not be executed by the
restricted shell. 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 paren-
thesized command is also executed in a sub-shell.
Special Commands.
The following commands are executed in the shell process and, except as
specified, no input/output redirection is permitted for such commands:
: No effect; the command does nothing. A zero exit code is
returned.
. file Read and execute commands from file and return. The search path
specified by 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.
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. The shell parameter CDPATH defines the
search path for the directory containing arg. Alternative
directory names are separated by a colon (:). The default path
is <null> (specifying the current directory). Note that the
current directory is specified by a null path name, which can
appear immediately after the equal sign or between the colon
delimiters anywhere else in the path list. If arg begins with a
/ then the search path is not used. Otherwise, each directory
in the path is searched for arg. The cd command may not be exe-
cuted by rsh.
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 argu-
ments 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 then the exit status is that of the last command
executed (an end-of-file will also cause the shell to exit.)
export [ name ... ]
The given names are marked for automatic export to the environ-
ment of subsequently-executed commands. If no arguments are
given, then a list of all names that are exported in this shell
is printed.
newgrp [ arg ... ]
Equivalent to exec newgrp arg ....
read [ name ... ]
One line is read from the standard input and the first word is
assigned to the first name, the second word to the second name,
etc., with leftover words assigned to the last name. The return
code is 0 unless an 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 argu-
ments are given, then a list of all readonly names is printed.
set [ --ekntuvx [ arg ... ] ]
-e Exit immediately if a command exits with a non-zero exit
status.
-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.
-- Do not change any of the flags; useful in setting $1 to
-.
Using + rather than - causes these flags to be turned off.
These flags can also be used upon invocation of the shell. The
current set of flags may be found in $-. The remaining argu-
ments are positional parameters and are assigned, in order, to
$1, $2, .... If no arguments are given then the values of all
names are printed.
shift [ n ]
The positional parameters from $n+1 ... are renamed $1 .... If
n is not given, it is assumed to be 1.
test
Evaluate conditional expressions. See test(1) for usage and
description.
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 exe-
cuted in order of signal number. Any attempt to set a trap on a
signal that was ignored on entry to the current shell is inef-
fective. An attempt to trap on signal 11 (memory fault)
produces an error. 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 the commands it
invokes. If n is 0 then the command arg is executed on exit
from the shell. The trap command with no arguments prints a
list of commands associated with each signal number.
ulimit [ -fp ] [ n ]
imposes a size limit of n
-f imposes a size limit of n blocks on files written by
child processes (files of any size may be read). With no
argument, the current limit is printed.
-p changes the pipe size to n (UNIX System/RT only).
If no option is given, -f is assumed.
umask [ nnn ]
The user file-creation mask is set to 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 sta-
tus. If n is not given then all currently active child pro-
cesses are waited for and the return code is zero.
Invocation.
If the shell is invoked through exec(2) and the first character of
argument zero is -, commands are initially read from /etc/profile and
then from $HOME/.profile, if such files exist. Thereafter, commands
are read as described below, which is also the case when the shell is
invoked as /bin/sh. The flags below are interpreted by the shell on
invocation only; Note that unless the -c or -s flag is specified, the
first argument is assumed to be the name of a file containing commands,
and the remaining arguments are passed as positional parameters to that
command file:
-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 com-
mands are read from the standard input. Any remaining argu-
ments specify the positional parameters. 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, then this shell is interactive.
In this case TERMINATE is ignored (so that kill 0 does not
kill an interactive shell) and INTERRUPT is caught and
ignored (so that wait is interruptible). In all cases, QUIT
is ignored by the shell.
-r If the -r flag is present the shell is a restricted shell.
The remaining flags and arguments are described under the set command
above.
Rsh Only.
Rsh is used to set up login names and execution environments whose
capabilities are more controlled than those of the standard shell. The
actions of rsh are identical to those of sh, except that the following
are disallowed:
changing directory (see cd(1)),
setting the value of $PATH,
specifying path or command names containing /,
redirecting output (> and >>).
The restrictions above are enforced after .profile is interpreted.
When a command to be executed is found to be a shell procedure, rsh
invokes sh to execute it. Thus, it is possible to provide to the end-
user shell procedures that have access to the full power of the stan-
dard shell, while imposing a limited menu of commands; this scheme
assumes that the end-user does not have write and execute permissions
in the same directory.
The net effect of these rules is that the writer of the .profile has
complete control over user actions, by performing guaranteed setup
actions and leaving the user in an appropriate directory (probably not
the login directory).
The system administrator often sets up a directory of commands (i.e.,
/usr/rbin) that can be safely invoked by rsh. Some systems also pro-
vide a restricted editor red.
EXIT STATUS
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-interac-
tively then execution of the shell file is abandoned. Otherwise, the
shell returns the exit status of the last command executed (see also
the exit command above).
FILES
/etc/profile
$HOME/.profile
/tmp/sh*
/dev/null
SEE ALSO
cd(1), env(1), login(1), newgrp(1), test(1), umask(1), dup(2), exec(2),
fork(2), pipe(2), signal(2), ulimit(2), umask(2), wait(2), a.out(4),
profile(4), environ(5).
BUGS
The command readonly (without arguments) produces the same output as
the command export.
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 that file by another name.