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sh(1)
shell, the standard, job control, and restricted command interpreter
From the original Amiga UNIX 2.1 manual. Some things it mentions may not be in this release.
Also by this name: rsh(1), General commands
NAME · SYNOPSIS · DESCRIPTION · EXIT STATUS · FILES · SEE ALSO · NOTES
NAME
sh, jsh, rsh - shell, the standard, job control, and restricted command interpreter
SYNOPSIS
sh [ -acefhiknprstuvx ] [ args ] jsh [ -acefhiknprstuvx ] [ args ] rsh [ -acefhiknprstuvx ] [ args ]
DESCRIPTION
sh is a command programming language that executes commands read from a terminal or a file. The command jsh is an interface to the shell which provides all of the functionality of sh and enables Job Control (see ``Job Control,'' below). rsh is a restricted version of the standard command interpreter sh; It is used to restrict logins to 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 ASCII letters, digits, or underscores, beginning with a letter or an 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(5) 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. The exit status of a pipeline is the exit status of the last command in the pipeline. 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 (i.e., the shell waits for the pipeline to finish before executing any commands following the semicolon); an ampersand (&) causes asynchronous execution of the preceding pipeline (i.e., the shell does not wait for that pipeline to finish). The sym- bol && (||) 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 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 pat-
terns is the same as that used for file-name generation
(see ``File Name 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 exe-
cuted 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 ter-
minates. 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 executed in the current (that is, parent)
shell. The { must be followed by a space.
name () { list;}
Define a function which is referenced by name. The body
of the function is the list of commands between { and
}. The { must be followed by a space. Execution of
functions is described below (see ``Execution''). The
{ and } are unnecessary if the body of the function is a command as defined above, under ``Commands.''
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 characters up to a new-line to be ignored.
Command Substitution
The shell reads commands from the string between two grave accents (``) and the standard output from these commands may be used as all or part of a word. Trailing new-lines from the standard output are removed. No interpretation is done on the string before the string is read, except to remove backslashes (\) used to escape other characters. Backslashes may be used to escape a grave accent (`) or another backslash (\) and are removed before the command string is read. Escaping grave accents allows nested command substitution. If the command substitution lies within a pair of double quotes (" ...` ...` ... "), a backslash used to escape a double quote (\") will be removed; otherwise, it will be left intact. If a backslash is used to escape a new-line character (\new-line), both the backslash and the new-line are removed (see the later section on ``Quoting''). In addition, backslashes used to escape dollar signs (\$) are removed. Since no parameter substitution is done on the command string before it is read, inserting a backslash to escape a dollar sign has no effect. Backslashes that precede characters other than \, `, ", new-line, and $ are left intact when the command string is read.
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 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:
echo ${d:-`pwd`}
If the colon (:) is omitted from the above expressions, 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 synchro-
nously 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. The parame-
ters in this section are also referred to as environment
variables.
HOME The default argument (home directory) for the cd
command, set to the user's login directory by
login(1) from the password file [see passwd(4)].
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 parameter is set to the name of a mail
file and the MAILPATH parameter is not set, the
shell informs the user 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 param-
eters. The default value is 600 seconds (10
minutes). If set to 0, the shell will check
before each prompt.
MAILPATH
A colon (:) separated list of file names. If
this parameter is set, the shell informs the user
of the arrival of mail in any of the specified
files. Each file name can 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 new-line (see ``Blank Interpretation'').
LANG If this parameter is set, the shell will use it to
determine the current locale; see environ(5),
setlocale(3C).
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 pro-
cedure executed.
SHELL
When the shell is invoked, it scans the environ-
ment (see ``Environment,'' below) for this name.
If it is found and rsh is the file name 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.
Input/Output
A command's 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 as arguments to the invoked command. Note that parameter and command substitution occurs before word or digit is used.
<word Use file word as standard input (file descrip-
tor 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 After parameter and command substitution is
done on word, the shell input is read up to
the first line that literally matches the
resulting word, or to an end-of-file. If,
however, - is appended to <<:
1) leading tabs are stripped from word before
the shell input is read (but after parame-
ter and command substitution is done on
word),
2) leading tabs are stripped from the shell
input as it is read and before each line
is compared with word, and
3) shell input is read up to the first line
that literally matches the resulting word,
or to an end-of-file.
If any character of word is quoted (see
``Quoting,'' later), no additional processing
is done to the shell input. If no characters
of word are quoted:
1) parameter and command substitution occurs,
2) (escaped) \new-lines are removed, and
3) \ must be used to quote the characters \,
$, and `.
The resulting document becomes the standard
input.
<&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-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.
Using the terminology introduced on the first page, under ``Commands,'' if a command is composed of several simple commands, redirection will be evaluated for the entire command before it is evaluated for each simple command. That is, the shell evaluates redirection for the entire list, then each pipeline within the list, then each command within each pipeline, then each list within each command.
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.
File Name Generation
Before a command is executed, 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 /, 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.
Note that all quoted characters (see below) must be matched explicitly in a filename.
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 backslash (\) or inserting it between a pair of quote marks ('' or ""). During processing, the shell may quote certain characters to prevent them from taking on a special meaning. Backslashes used to quote a single character are removed from the word before the command is executed. The pair \new-line is removed from a word before command and parameter substitution.
All characters enclosed between a pair of single quote marks (''), except a single quote, are quoted by the shell. Backslash has no special meaning inside a pair of single quotes. A single quote may be quoted inside a pair of double quote marks (for example, "'"), but a single quote can not be quoted inside a pair of single quotes.
Inside a pair of double quote marks (""), parameter and command substitution occurs and the shell quotes the results to avoid blank interpretation and file name generation. If $* is within a pair of double quotes, the positional parameters are substituted and quoted, separated by quoted spaces ("$1 $2 ..."); however, if $@ is within a pair of double quotes, the positional parameters are substituted and quoted, separated by unquoted spaces ("$1" "$2" ... ). \ quotes the characters \, `, ", and $. The pair \new-line is removed before parameter and command substitution. If a backslash precedes characters other than \, `, ", $, and new-line, then the backslash itself is quoted by the shell.
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, the secondary prompt (i.e., the value of PS2) is issued.
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 value 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 cmd is not a Special Command. If cmd is a Special Command, then
TERM=450 cmd will modify the TERM variable in the current shell.
If the -k flag is set, all keyword arguments are placed in the environment, even if they occur after the command name. The following first 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, with the exception of signal 11 (but see also the trap command below).
Execution
Each time a command is executed, the command substitution, parameter substitution, blank interpretation, input/output redirection, and filename generation listed above are carried out. If the command name matches the name of a defined function, the function is executed in the shell process (note how this differs from the execution of shell procedures). If the command name does not match the name of a defined function, but matches one of the Special Commands listed below, it is executed in the shell process. 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 /usr/bin. The current directory is specified by a null path name, which can appear immediately after the equal sign, between two colon delimiters anywhere in the path list, or at the end of 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).
Special Commands
Input/output redirection is now permitted for these commands. File descriptor 1 is the default output location. When Job Control is enabled, additional Special Commands are added to the shell's environment (see ``Job Control'').
: 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 parame-
ter HOME is the default arg. The shell parameter
CDPATH defines the search path for the directory con-
taining arg. Alternative directory names are separated
by a colon (:). The default path is <null> (specifying
the current directory). Note that the current direc-
tory 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 / the search path is not used. Other-
wise, 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, variable names that have been
marked for export during the current shell's execution
are listed. (Variable names exported from a parent
shell are listed only if they have been exported again
during the current shell's execution.) Function names
are not exported.
getopts
Use in shell scripts to support command syntax stan-
dards [see intro(1)]; it parses positional parameters
and checks for legal options. See getopts(1) for usage
and description.
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 option causes the shell to forget
all remembered locations. If no arguments are given,
information 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. If a
command is found in a "relative" directory in the
search path, after changing to that directory, the
stored location of that command is recalculated. Com-
mands 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(1M) 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, using the
internal field separator, IFS (normally space or tab),
to delimit word boundaries, 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.
Lines can be continued using \new-line. Characters
other than new-line can be quoted by preceding them
with a backslash. These backslashes are removed before
words are assigned to names, and no interpretation is
done on the character that follows the backslash. 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 speci-
fied 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 file name 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 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 substitut-
ing.
-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 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 numeric or symbolic 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 or corresponding sym-
bolic names. 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 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 [ -[HS][a | cdfnstv] ]
ulimit [ -[HS][c | d | f | n | s | t | v] ] limit
ulimit prints or sets hard or soft resource limits.
These limits are described in getrlimit(2).
If limit is not present, ulimit prints the specified
limits. Any number of limits may be printed at one
time. The -a option prints all limits.
If limit is present, ulimit sets the specified limit to
limit. The string unlimited requests the largest valid
limit. Limits may be set for only one resource at a
time. Any user may set a soft limit to any value below
the hard limit. Any user may lower a hard limit. Only
a super-user may raise a hard limit; see su(1).
The -H option specifies a hard limit. The -S option
specifies a soft limit. If neither option is speci-
fied, ulimit will set both limits and print the soft
limit.
The following options specify the resource whose limits
are to be printed or set. If no option is specified,
the file size limit is printed or set.
-c maximum core file size (in 512-byte blocks)
-d maximum size of data segment or heap (in
kbytes)
-f maximum file size (in 512-byte blocks)
-n maximum file descriptor plus 1
-s maximum size of stack segment (in kbytes)
-t maximum CPU time (in seconds)
-v maximum size of virtual memory (in kbytes)
umask [ nnn ]
The user file-creation mask is set to nnn [see
umask(1)]. If nnn is omitted, the current value of the
mask is printed.
unset [ name ... ]
For each name, remove the corresponding variable or
function value. The variables PATH, PS1, PS2, MAIL-
CHECK, and IFS cannot be unset.
wait [ n ]
Wait for your background process whose process id is n
and report its termination status. If n is omitted,
all your shell's currently active background processes
are waited for and the return code will be 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 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 /usr/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.
-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.
-p If the -p flag is present, the shell will not set
the effective user and group IDs to the real user
and group IDs.
-r If the -r flag is present the shell is a res-
tricted shell.
-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 Com-
mands) is written to file descriptor 2.
The remaining flags and arguments are described under the set command above.
Job Control (jsh)
When the shell is invoked as jsh, Job Control is enabled in addition to all of the functionality described previously for sh. Typically Job Control is enabled for the interactive shell only. Non-interactive shells typically do not benefit from the added functionality of Job Control. With Job Control enabled every command or pipeline the user enters at the terminal is called a job. All jobs exist in one of the following states: foreground, background or stopped. These terms are defined as follows: 1) a job in the foreground has read and write access to the controlling terminal; 2) a job in the background is denied read access and has conditional write access to the controlling terminal [see stty(1)]; 3) a stopped job is a job that has been placed in a suspended state, usually as a result of a SIGTSTP signal [see signal(5)]. Every job that the shell starts is assigned a positive integer, called a job number which is tracked by the shell and will be used as an identifier to indicate a specific job. Additionally the shell keeps track of the current and previous jobs. The current job is the most recent job to be started or restarted. The previous job is the first non-current job. The acceptable syntax for a Job Identifier is of the form:
%jobid
where, jobid may be specified in any of the following formats:
% or + for the current job
- for the previous job
?<string> specify the job for which the command line
uniquely contains string.
n for job number n, where n is a job number
pref where pref is a unique prefix of the command
name (for example, if the command ls -l foo
were running in the background, it could be
referred to as %ls); pref cannot contain
blanks unless it is quoted.
When Job Control is enabled, the following commands are
added to the user's environment to manipulate jobs:
bg [%jobid ...]
Resumes the execution of a stopped job in the background. If %jobid is omitted the current job is assumed.
fg [%jobid ...]
Resumes the execution of a stopped job in the foreground, also moves an executing background job into the foreground. If %jobid is omitted the current job is assumed.
jobs [-p|-l] [%jobid ...] jobs -x command [arguments]
Reports all jobs that are stopped or executing in the background. If %jobid is omitted, all jobs that are stopped or running in the background will be reported. The following options will modify/enhance the output of jobs:
-l Report the process group ID and working directory of the jobs.
-p Report only the process group ID of the jobs.
-x Replace any jobid found in command or arguments with the corresponding process group ID, and then execute command passing it arguments.
kill [-signal] %jobid
Builtin version of kill to provide the functionality of the kill command for processes identified with a jobid.
stop %jobid ...
Stops the execution of a background job(s).
suspend
Stops the execution of the current shell (but not if it is the login shell).
wait [%jobid ...]
wait builtin accepts a job identifier. If %jobid is omitted wait behaves as described above under Special Commands.
Restricted Shell (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.
A restricted shell can be invoked in one of the following ways: (1) rsh is the file name part of the last entry in the /etc/passwd file [see passwd(4)]; (2) the environment variable SHELL exists and rsh is the file name part of its value; (3) the shell is invoked and rsh is the file name part of argument 0; (4) the shell is invoke with the -r option.
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 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 [see profile(4)] 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 a restricted shell. 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).
jsh Only
If the shell is invoked as jsh and an attempt is made to exit the shell while there are stopped jobs, the shell issues one warning:
There are stopped jobs.
This is the only message. If another exit attempt is made, and there are still stopped jobs they will be sent a SIGHUP signal from the kernel and the shell is exited.
FILES
/etc/profile $HOME/.profile /tmp/sh* /dev/null
SEE ALSO
cd(1), echo(1), getopts(1), intro(1), login(1), pwd(1), stty(1), test(1), umask(1), wait(1). dup(2), exec(2), fork(2), getrlimit(2), pipe(2), ulimit(2), setlocale(3C) in the Programmer's Reference Manual. newgrp(1M), profile(4), environ(5), signal(5) in the System Administrator's Reference Manual.
NOTES
Words used for filenames in input/output redirection are not interpreted for filename generation (see ``File Name Generation,'' above). For example, cat file1 >a* will create a file named a*.
Because commands in pipelines are run as separate processes, variables set in a pipeline have no effect on the parent shell.
If you get the error message cannot fork, too many processes, try using the wait(1) command to clean up your background processes. If this doesn't help, the system process table is probably full or you have too many active foreground processes. (There is a limit to the number of process ids associated with your login, and to the number the system can keep track of.)
Only the last process in a pipeline can be waited for.
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.