sh(1) sh(1)
NAME
sh, rsh - shell, the standard/restricted command programming
language
SYNOPSIS
sh [-a] [-c] [-e] [-f] [-h] [-i] [-k] [-n] [-r] [-s] [-t]
[-u] [-v] [-x] [args]
rsh [-a] [-c] [-e] [-f] [-h] [-i] [-k] [-n] [-r] [-s] [-t]
[-u] [-v] [-x] [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 command 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.
Definitions
A blank is a tab or a space. A name is a sequence of
letters, digits, or underscores beginning with a letter or
underscore. A parameter is a name, a digit, or any of the
characters , @, #, ?, -, $, and !.
Commands
A simple-command is a sequence of non-blank words separated
by blanks. 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(3) 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 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. The exit status of a
pipeline is the exit status of the last command.
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
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only if the preceding pipeline returns a zero (non-zero)
exit status. An arbitrary number of newlines 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 positional 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 pattern that matches word. The form of the
patterns is the same as that used for file-name
generation (see ``Filename Generation'') except that a
slash, a leading dot, or a dot immediately following a
slash need not be matched explicitly.
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. Otherwise, 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.
name (){list}
Define a function which is referenced by name. The body
of the function is the list of commands between { and }.
Execution of functions is described below (see
``Execution,'' below).
The following words are recognized only as the first word of
a command and when not quoted:
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if then else elif fi case esac for while until do done { }
Comments
A word beginning with # causes that word and all the
following characters up to a newline 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 newlines are removed.
Parameter Substitution
The character $ is used to introduce substitutable
parameters. There are two types of parameters, positional
and keyword. If parameter is a digit, it is a positional
parameter. Positional parameters may be assigned values by
set. Keyword parameters (also known as variables) may be
assigned values by writing:
name=value [ name=value ] ...
Pattern-matching is not performed on value. There cannot be
a function and a variable with the same name.
${parameter}
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 * or @,
all the positional parameters, starting 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, substitute its
value; otherwise substitute word.
${parameter :=word}
If parameter is not set or is null, set it to word; the
value of the parameter is substituted. Positional
parameters may not be assigned to in this way.
${parameter :?word}
If parameter is set and is non-null, substitute its
value; otherwise, print word and exit from the shell. If
word is omitted, the message parameter null or not set is
printed.
${parameter :+word}
If parameter is set and is non-null, substitute word;
otherwise substitute nothing.
In the above, word is not evaluated unless it is to be used
as the substituted string, so that, in the following
example, pwd is executed only if d is not set or is null:
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echo ${d:-`pwd`}
If the colon (:) is omitted from the above expressions, the
shell checks only whether parameter is set or not.
The following parameters are set automatically 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-executed 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). You may not change
PATH if executing under rsh.
CDPATH The search path for the cd command.
MAIL If you have set this parameter to the name of
a mail file and you have not set the MAILPATH
parameter, the shell informs you of the
arrival of mail in the specified file.
MAILCHECK This parameter specifies how often (in
seconds) the shell will check for the arrival
of mail in the files specified by the MAILPATH
or MAIL parameters. The default value is 600
seconds (10 minutes). If this parameter is
set to 0, the shell will check before each
prompt.
MAILPATH A colon-separated (:) list of filenames. If
this parameter is set, the shell informs the
user of the arrival of mail in any of the
specified files. Each filename may be
followed by % and a message that will be
printed when the modification time changes.
The default message is you have mail.
PS1 Primary prompt string, by default "$".
PS2 Secondary prompt string, by default ">".
IFS Internal field separators, normally space,
tab, and newline.
SHACCT If this parameter is set to the name of a file
writable by the user, the shell will write an
accounting record in the file for each shell
procedure executed. Accounting routines such
as acctcom(1) and acctcms(1M) can be used to
analyze the data collected.
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SHELL When the shell is invoked, it scans the
environment (see ``Environment,'' below) for
this name. If it is found and there is an r
in the filename part of its value, the shell
becomes a restricted shell.
The shell gives default values to PATH, PS1, PS2, MAILCHECK,
and IFS. HOME and MAIL are 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 arguments
(those resulting from parameters that have no values) are
removed.
Filename 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 filenames that match the pattern.
If no filename is found that matches the pattern, the word
is left unchanged. The character . at the start of a
filename 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 following the opening "["
is a "!", 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:
; & ( ) | ^ < > newline space tab
A character may be quoted (i.e., made to stand for itself)
by preceding it with a \. The pair \newline 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 substitution occurs
and \ quotes the characters \, `, , and $. "$*" is
equivalent to "$1 $2 ...", whereas "$@" is equivalent to
"$1" "$2" ....
Prompting
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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 (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, it
is created; otherwise, it is truncated to zero
length.
>>word Use file word as standard output. If the file
exists, 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, no
interpretation is placed upon the characters of
the document; otherwise, parameter and command
substitution occurs, (unescaped) \newline is
ignored, and \ must be used to quote the
characters \, $, `, and the first character of
word. If - is appended to <<, all leading tabs
are stripped from word and from the document.
<&digit Use the file associated with file descriptor
digit as standard input. Similarly for the
standard output using >&digit.
<&- The standard input is closed. Similarly for the
standard output using >&-.
If any of the above is preceded by a digit, the file
descriptor which will be associated with the file is that
specified by the digit (instead of the default 0 or 1). For
example:
... 2>&1
associates file descriptor 2 with the file currently
associated with file descriptor 1.
The order in which redirections are specified is
significant. The shell evaluates redirections left-to-
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right. For example:
... 1>xxx 2>&1
first associates file descriptor 1 with file xxx. It
associates file descriptor 2 with the file associated with
file descriptor 1 (i.e., xxx). If the order of redirections
were reversed, file descriptor 2 would be associated with
the terminal (assuming file descriptor 1 had been) and file
descriptor 1 would be associated with file xxx.
If a command is followed by &, 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 invoking 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. If the user modifies the
values of any of these parameters or creates new parameters,
none of these affects the environment unless the export
command is used to bind the shell's parameter to the
environment (see also set -a). A parameter may be removed
from the environment with the unset command. The
environment seen by any executed command is thus composed of
any unmodified name-value pairs originally inherited by the
shell, minus any pairs removed by unset, 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
and
(export TERM; TERM=450; cmd)
are equivalent (as far as the execution of cmd is
concerned).
If the -k flag is set, all keyword arguments are placed in
the environment, even if they occur after the command name.
The following command first prints a=b c and then, after the
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-k flag is set, prints only c:
echo a=b c #first time prints a=b c
set -k #puts all keyword args in env
echo a=b c #now prints only c; a=b goes to env
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, 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. If a command name matches one of the special
commands listed below (see ``Special Commands''), it is
executed in the shell process. If the command name does not
match a special command, but matches the name of a defined
function, the function is executed in the shell process
(note that this differs from the execution of shell
procedures, which takes place in subshells). The positional
parameters $1, $2, .... are set to the arguments of the
function. If the command name matches neither a special
command nor the name of a defined function, 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 containing 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 pathname, which can appear
immediately after the equals sign or between the colon
delimiters anywhere else in the path list. If the command
name contains a / 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 is spawned to read it. A
parenthesized command is also executed in a sub-shell.
The location in the search path where a command was found is
remembered by the shell (to help avoid unnecessary execs
later). If the command was found in a relative directory,
its location must be re-determined whenever the current
directory changes. The shell forgets all remembered
locations whenever the PATH variable is changed or the hash
-r command is executed (see below).
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Special Commands
Input/output redirection is now permitted for these
commands. File descriptor 1 is the default output location.
: 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, break n levels.
continue [n]
Resume the next iteration of the enclosing for or while
loop. If n is specified, 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 (i.e., the empty string,
specifying the current directory). Note that the current
directory is specified by a null pathname, which can
appear immediately after the equals sign or between the
colon delimiters anywhere else in the path list. If arg
begins with a /, the search path is not used. Otherwise,
each directory in the path is searched for arg. The cd
command may not be executed by rsh.
echo [arg ...]
Echo arguments. See echo(1) for usage and description.
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
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 cause the
shell to exit.)
export [name ...]
The given names are marked for automatic export to the
environment of subsequently-executed commands. If no
arguments are given, a list of all names that are
exported in this shell is printed. Function names may
not be exported.
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hash[-r] [name ...]
For each name, the location in the search path of the
command specified by name is determined and remembered by
the shell. The -r flag option causes the shell to forget
all remembered locations. If no arguments are given,
information (hits and cost) about remembered commands is
presented. hits is the number of times a command has
been invoked by the shell process. cost is a measure of
the work required to locate a command in the search path.
There are certain situations which require that the
stored location of a command be recalculated. Commands
for which this will be done are indicated by an asterisk
(*) adjacent to the hits information. cost will be
incremented when the recalculation is done.
newgrp [arg ...]
Equivalent to exec newgrp arg .... See newgrp(1) for
usage and description.
pwd
Print the current working directory. See pwd(1) for
usage and description.
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 arguments are given, a list of all readonly names
is printed.
return [n]
Causes a function to exit with the return value specified
by n. If n is omitted, the return status is that of the
last command executed.
set [--aefhkntuvx [arg ...]]
-a Mark variables which are modified or created for
export.
-e Exit immediately if a command exits with a non-zero
exit status.
-f Disable filename generation.
-h Locate and remember function commands as functions
are defined (function commands are normally located
when the function is executed).
-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
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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 arguments are positional parameters and are
assigned, in order, to $1, $2, .... If no arguments are
given, 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] ...
The command arg is 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.
Any attempt to set a trap on a signal that was ignored on
entry to the current shell is ineffective. An attempt to
trap on signal 11 (memory fault) produces an error. If
arg is absent, all trap(s) n are reset to their original
values. If arg is the null string, this signal is
ignored by the shell and by the commands it invokes. If
n is 0, 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.
type [name ...]
For each name, indicate how it would be interpreted if
used as a command name.
ulimit [-f] [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.
If no flag 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.
unset[name ...]
For each name, remove the corresponding variable or
function. The variables PATH, PS1, PS2, MAILCHECK, and
IFS cannot be unset.
wait [n]
Wait for the specified process and report its termination
status. If n is not given, all currently active child
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processes 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 read initially
from /etc/profile and 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, commands are read
from string.
-s If the -s flag is present or if no arguments
remain, commands are read from the standard
input. Any remaining arguments specify the
positional parameters. Shell output (except
for special commands, see ``Special Commands,''
above) 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, 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
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have access to the full power of the standard 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 provide 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-interactively, 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).
EXAMPLE
sh -x script1
will execute each command in script1, echoing the command
just before executing it.
FILES
/bin/sh
/etc/profile
$HOME/.profile
/tmp/sh*
/dev/null
SEE ALSO
acctcom(1), cd(1), echo(1), env(1), login(1), newgrp(1),
pwd(1), test(1), umask(1), acctcms(1M), exec(2), fork(2),
pipe(2), ulimit(2), umask(2), wait(2), dup(3), signal(3),
a.out(4), profile(4), environ(5),
``Bourne Shell Reference'' in Oreo User Interface.
CAVEATS
If a command is executed, and a command with the same name
is installed in a directory in the search path before the
directory where the original command was found, the shell
will continue to exec the original command. Use the hash
command to correct this situation.
If you move the current directory or one above it, pwd may
not give the correct response. Use the cd command with a
full pathname to correct this situation.
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BUGS
Filename pattern matching is not done on redirected I/O
filenames.
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