sh(1) UNIX System V(Essential Utilities) sh(1)
NAME
sh, jsh, rsh - command interpreters: standard shell, job control shell,
restricted shell
SYNOPSIS
sh [ -acefhiknprstuvx ] [ args ]
jsh [ -acefhiknprstuvx ] [ args ]
/usr/lib/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). /usr/lib/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 following 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 (that is, the shell waits
for the pipeline to finish before executing any commands following the
semicolon); an ampersand (&) causes asynchronous execution of the
preceding pipeline (that is, 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 newlines may appear in a list, instead of semicolons,
10/89 Page 1
sh(1) UNIX System V(Essential Utilities) sh(1)
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 executed in the current (that is, parent) shell. The {
must be followed by a space.
name () { list;}
Define a function that is referenced by name. The body of the
function is the list of commands between { and }. The list may
appear on the same line as the {. If it does, the { and list must
be separated by a space. The } may not be on the same line as
list; it must be on a newline. 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 recognized only as the first word of a command
and when not quoted:
if then else elif fi case esac for while until do done { }
Page 2 10/89
sh(1) UNIX System V(Essential Utilities) sh(1)
Comments
A word beginning with # causes that word and all the following characters
up to a newline to be ignored.
Command Substitution
The shell reads commands from the string between two back quotes (``) and
the standard output from these commands may be used as all or part of a
word. Trailing newlines 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 back quote (`) or another backslash (\) and are
removed before the command string is read. Escaping back quotes 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 newline character (\newline), both the
backslash and the newline are removed (see the later section on
``Quoting''). In addition, backslashes used to escape dollar signs (\$)
are removed. Because 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 \, `, ",
newline, 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.
10/89 Page 3
sh(1) UNIX System V(Essential Utilities) sh(1)
${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.
* Expands to the positional parameters, beginning with 1.
@ Expands to the positional parameters, beginning with 1,
except when expanded within double quotes, in which case each
positional parameter expands as a separate field.
# 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. $ reports the process ID
of the parent shell in all shell constructs, including
pipelines, and in parenthesized sub-shells.
! The process number of the last background command invoked.
The following parameters are used by the shell. The parameters 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 /usr/lib/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 parameters. The default value is 600
seconds (10 minutes). If set to 0, the shell will check
before each prompt.
Page 4 10/89
sh(1) UNIX System V(Essential Utilities) sh(1)
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 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 newline
(see ``Blank Interpretation''). The user can modify IFS to
allow additional field separators, but space, tab and newline
are always included in the list of field separators.
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 procedure executed.
SHELL When the shell is invoked, it scans the environment (see
``Environment,'' below) for this name. If it is found and
rsh is 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. The original
whitespace characters (space, tab, and newline) are always considered
internal field separators.
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 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 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,
10/89 Page 5
sh(1) UNIX System V(Essential Utilities) sh(1)
however, - is appended to <<:
1) leading tabs are stripped from word before the shell
input is read (but after parameter 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) \newlines 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 (that is,
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.
Page 6 10/89
sh(1) UNIX System V(Essential Utilities) sh(1)
Redirection of output is not allowed in the restricted shell.
Filename 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
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.
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:
; & ( ) | ^ < > newline space tab
A character may be quoted (that is, 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 \newline 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 filename 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 \newline is removed before parameter and
command substitution. If a backslash precedes characters other than \,
`, ", $, and newline, then the backslash itself is quoted by the shell.
10/89 Page 7
sh(1) UNIX System V(Essential Utilities) sh(1)
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 (that is, 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
When a command is run in the background (cmd &) under sh, it can receive
INTERRUPT and QUIT signals but ignores them by default. [A background
process can override this default behavior via trap or signal. For
details, see the description of trap, below, or signal(2).] When a
command is run in the background under jsh, however, it does not receive
INTERRUPT or QUIT signals. Otherwise signals have the values inherited
by the shell from its parent, with the exception of three signals: 11
(SIGSEV), 14 (SIGALARM), and 18 (SIGCHILD).
Page 8 10/89
sh(1) UNIX System V(Essential Utilities) sh(1)
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 (that is, a
shell script). A sub-shell is spawned to read it. A parenthesized
command is also executed in a sub-shell.
For shell script files, in order for the ``set user ID on execution''
and/or the ``set group ID on execution'' mode to be effective, the first
line of the file must be
#! /sbin/sh
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.
10/89 Page 9
sh(1) UNIX System V(Essential Utilities) sh(1)
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>
(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 / the search
path is not used. Otherwise, each directory in the path is
searched for arg. The cd command may not be executed by
/usr/lib/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 standards [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.
Commands for which this will be done are indicated by an asterisk
Page 10 10/89
sh(1) UNIX System V(Essential Utilities) sh(1)
(*) 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, and so on, with leftover words
assigned to the last name. Lines can be continued using \newline.
Characters other than newline 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 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 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.
10/89 Page 11
sh(1) UNIX System V(Essential Utilities) sh(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 executed in order of signal number or corresponding
symbolic names. Any attempt to set a trap on a signal that was
ignored on entry to the current shell is ineffective. An error
results when an attempt is made to trap on any of the following
three signals: (1) signal 11 (SIGSEV-segmentation fault); (2)
signal 14 (SIGALRM-alarm clock); and (3) signal 18 (SIGCHILD-child
status changed). 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 specified, 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)
Page 12 10/89
sh(1) UNIX System V(Essential Utilities) sh(1)
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, MAILCHECK, 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 restricted 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 Commands) 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
10/89 Page 13
sh(1) UNIX System V(Essential Utilities) sh(1)
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)]. Jobs in the foreground
can be stopped by INTERRUPT or QUIT signals from the keyboard; background
jobs cannot be stopped by these signals.
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
Page 14 10/89
sh(1) UNIX System V(Essential Utilities) sh(1)
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 (/usr/lib/rsh) Only
/usr/lib/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 /usr/lib/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 filename part of the last entry in the /etc/passwd file [see
passwd(4)]; (2) the environment variable SHELL exists and rsh is the
filename part of its value; (3) the shell is invoked and rsh is the
filename part of argument 0; (4) the shell is invoked with the -r option.
When a command to be executed is found to be a shell procedure,
/usr/lib/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.
10/89 Page 15
sh(1) UNIX System V(Essential Utilities) sh(1)
The net effect of these rules is that the writer of the [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 (that is,
/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), csh(1), echo(1), getopts(1), intro(1), ksh(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
rsh(1) in the Network User's and Administrator's Guide
NOTES
Words used for filenames in input/output redirection are not interpreted
for filename generation (see ``Filename Generation,'' above). For
example, cat file1 >a* will create a filenamed 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
Page 16 10/89
sh(1) UNIX System V(Essential Utilities) sh(1)
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.
Prior to Release 4, the rsh command invoked the restricted shell. This
restricted shell command is /usr/lib/rsh and it can be executed by using
the full pathname. Beginning with Release 4, the rsh command is the
remote shell. See rsh(1) in the Network User's and Administrator's
Guide.
10/89 Page 17