SH(1HAS) User Commands SH(1HAS)


NAME


sh, jsh - standard and job control shell and command interpreter

SYNOPSIS


/usr/bin/sh [-acefhiknprstuvx] [argument]...


/usr/xpg4/bin/sh [+- abCefhikmnoprstuvx]
[+- o option]... [-c string] [arg]...


/usr/bin/jsh [-acefhiknprstuvx] [argument]...


DESCRIPTION


The /usr/bin/sh utility is a command programming language that executes
commands read from a terminal or a file.


The /usr/xpg4/bin/sh utility is a standards compliant shell. This utility
provides all the functionality of ksh(1), except in cases discussed in
ksh(1) where differences in behavior exist.


The jsh utility is an interface to the shell that provides all of the
functionality of sh and enables job control (see Job Control section
below).


Arguments to the shell are listed in the Invocation section below.

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 !.

USAGE


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.h(3HEAD) 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 can 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 section 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 File Name Generation section), 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 can 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 section). 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 Lines


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 grave accents (``)
and the standard output from these commands can 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
can 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 (\") is removed. Otherwise, it is 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.
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 \, `, ", 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 can be assigned
values by set. Keyword parameters (also known as variables) can 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}
Use Default Values. If parameter is unset or
null, the expansion of word is substituted;
otherwise, the value of parameter is
substituted.


${parameter:=word}
Assign Default Values. If parameter is unset or
null, the expansion of word is assigned to
parameter. In all cases, the final value of
parameter is substituted. Only variables, not
positional parameters or special parameters, can
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 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. 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(5)).


PATH
The search path for commands (see Execution section below).


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
checks 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 checks 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 is e 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 section).


SHACCT
If this parameter is set to the name of a file writable by
the user, the shell writes an accounting record in the file
for each shell procedure executed.


SHELL
When the shell is invoked, it scans the environment (see
Environment section below) for this name.


See environ(7) for descriptions of the following environment variables
that affect the execution of sh: LC_CTYPE and LC_MESSAGES.


The shell gives default values to PATH, PS1, PS2, MAILCHECK, and IFS.
Default values for 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 Redirection
A command's input and output can be redirected using a special notation
interpreted by the shell. The following can appear anywhere in a simple-
command or can precede or follow a command and are not passed on as
arguments to the invoked command. Note: 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 EOF.
Otherwise, the file is created.


<>word
Open file word for reading and writing as standard input.


<<[-]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 EOF. If,
however, the hyphen (-) 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
EOF.
If any character of word is quoted (see Quoting section
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 is
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 is 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.

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.


Notice 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 can 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 can 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 can 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 \, `, ,
(comma), and $. The pair \newline is removed before parameter and command
substitution. If a backslash precedes characters other than \, `, ,
(comma), $, and newline, 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 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(7)) 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 can
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 can be augmented by prefixing it
with one or more assignments to parameters. Thus:

TERM=450 command


and

(export TERM; TERM=450; command


are equivalent as far as the execution of command is concerned if command
is not a Special Command. If command is a Special Command, then

TERM=450 command


modifies 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
example first prints a=b c and c:

echo a=b c

a=b c

set -k

echo a=b c

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 script
files, which require a sub-shell for invocation). 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. 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 section below).

:

No effect; the command does nothing. A zero exit code is returned.


. filename

Read and execute commands from filename and return. The search path
specified by PATH is used to find the directory containing filename.


bg [%jobid ...]

When Job Control is enabled, the bg command is added to the user's
environment to manipulate jobs. Resumes the execution of a stopped
job in the background. If %jobid is omitted the current job is
assumed. (See Job Control section below for more detail.)


break [ n ]

Exit from the enclosing for or while loop, if any. If n is specified,
break n levels.


cd [ argument ]

Change the current directory to argument. The shell parameter HOME is
the default argument. The shell parameter CDPATH defines the search
path for the directory containing argument. Alternative directory
names are separated by a colon (:). The default path is <null>
(specifying the current directory). Note: 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 argument begins with a / the search path is not used.
Otherwise, each directory in the path is searched for argument.


chdir [ dir ]

chdir changes the shell's working directory to directory dir. If no
argument is given, change to the home directory of the user. If dir
is a relative pathname not found in the current directory, check for
it in those directories listed in the CDPATH variable. If dir is the
name of a shell variable whose value starts with a /, change to the
directory named by that value.


continue [ n ]

Resume the next iteration of the enclosing for or while loop. If n is
specified, resume at the n-th enclosing loop.


echo [ arguments ... ]

The words in arguments are written to the shell's standard output,
separated by space characters. See echo(1) for fuller usage and
description.


eval [ argument ... ]

The arguments are read as input to the shell and the resulting
command(s) executed.


exec [ argument ... ]

The command specified by the arguments is executed in place of this
shell without creating a new process. Input/output arguments can
appear and, if no other arguments are given, cause the shell
input/output to be modified.


exit [ n ]

Causes the calling shell or shell script to exit with the exit status
specified by n. If n is omitted the exit status is that of the last
command executed (an EOF also causes 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.


fg [%jobid ...]

When Job Control is enabled, the fg command is added to the user's
environment to manipulate jobs. This command resumes the execution of
a stopped job in the foreground and also moves an executing
background job into the foreground. If %jobid is omitted, the current
job is assumed. (See Job Control section below for more detail.)


getopts

Use in shell scripts to support command syntax standards (see
Intro(1)). This command parses positional parameters and checks for
legal options. See getoptcvt(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 are done are indicated by an asterisk (*) adjacent to the hits
information. Cost is incremented when the recalculation is done.


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 are reported. (See Job Control section below for more
detail.)


kill [ -sig ] %job ...
kill -l

Sends either the TERM (terminate) signal or the specified signal to
the specified jobs or processes. Signals are either given by number
or by names (as given in signal.h(3HEAD) stripped of the prefix "SIG"
with the exception that SIGCHD is named CHLD). If the signal being
sent is TERM (terminate) or HUP (hangup), then the job or process is
sent a CONT (continue) signal if it is stopped. The argument job can
be the process id of a process that is not a member of one of the
active jobs. See Job Control section below for a description of the
format of job. In the second form, kill -l, the signal numbers and
names are listed. (See kill(1)).


login [ argument ... ]

Equivalent to `exec login argument....' See login(1) for usage and
description.


newgrp [ argument ]

Equivalent to exec newgrp argument. 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, 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 forth, 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 EOF is encountered.


readonly [ name ... ]

The given names are marked readonly and the values of these names can
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 [ argument ... ] ]


-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 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 can 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.


stop pid ...

Halt execution of the process number pid. (see ps(1)).


suspend

Stops the execution of the current shell (but not if it is the login
shell).


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 [ argument n [ n2 ... ]]

The command argument is to be read and executed when the shell
receives numeric or symbolic signal(s) (n). (Note: argument 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 attempt to
trap on signal 11 (memory fault) produces an error. If argument is
absent, all trap(s) n are reset to their original values. If argument
is the null string, this signal is ignored by the shell and by the
commands it invokes. If n is 0, the command argument 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 can 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 that the current limit, if any, be removed.
Any user can set a soft limit to any value less than or equal to
the hard limit. Any user can lower a hard limit. Only a user with
appropriate privileges can raise or remove a hard limit. See
getrlimit(2).

The -H option specifies a hard limit. The -S option specifies a soft
limit. If neither option is specified, ulimit sets 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)

Run the sysdef(8) command to obtain the maximum possible limits for
your system. The values reported are in hexadecimal, but can be
translated into decimal numbers using the bc(1) utility. See
swap(8).)

As an example of ulimit, to limit the size of a core file dump to 0
Megabytes, type the following:

ulimit -c 0


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 is
zero.


Invocation


If the shell is invoked through exec(2) and the first character of
argument zero is -, commands are initially read from /etc/profile and
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: 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 does 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
(see rsh(8)).


-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 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.h(3HEAD)).


Every job that the shell starts is assigned a positive integer, called a
job number which is tracked by the shell and is 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 can 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.


pref
Where pref is a unique prefix of the command name. For
example, if the command ls -l name 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 is reported. The following options 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.


Large File Behavior


See largefile(7) for the description of the behavior of sh and jsh when
encountering files greater than or equal to 2 Gbyte ( 2^31 bytes).

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 are sent a SIGHUP signal from the kernel and the
shell is exited.

FILES


$HOME/.profile


/dev/null


/etc/profile


/tmp/sh*

ATTRIBUTES


See attributes(7) for descriptions of the following attributes:

/usr/bin/sh, /usr/bin/jsh

+---------------+-----------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+-----------------+
|CSI | Enabled |
+---------------+-----------------+

/usr/xpg4/bin/sh

+---------------+-----------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+-----------------+
|CSI | Enabled |
+---------------+-----------------+

SEE ALSO


Intro(1), bc(1), echo(1), getoptcvt(1), kill(1), ksh(1), login(1),
newgrp(1), pfexec(1), pfsh(1), ps(1), pwd(1), set(1), shell_builtins(1),
stty(1), test(1), umask(1), wait(1), dup(2), exec(2), fork(2),
getrlimit(2), pipe(2), ulimit(2), setlocale(3C), signal.h(3HEAD),
passwd(5), profile(5), XPG4(7), attributes(7), environ(7), largefile(7),
rsh(8), su(8), swap(8), sysdef(8)

WARNINGS


The use of setuid shell scripts is strongly discouraged.

NOTES


Words used for filenames in input/output redirection are not interpreted
for filename generation (see File Name Generation section above). For
example, cat file1 >a* createsa 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 the input or the output of a while or until loop is redirected, the
commands in the loop are run in a sub-shell, and variables set or changed
there have no effect on the parent process:

lastline=
while read line
do

lastline=$line
done < /etc/passwd
echo "lastline=$lastline" # lastline is empty!


In these cases, the input or output can be redirected by using exec, as
in the following example:

# Save standard input (file descriptor 0) as file
# descriptor 3, and redirect standard input from the file
/etc/passwd:

exec 3<&0 # save standard input as fd 3
exec </etc/passwd # redirect input from file

lastline=
while read line
do
lastline=$line
done

exec 0<&3 # restore standard input
exec 3<&- # close file descriptor 3
echo "$lastline" # lastline


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 continues to exec the original command. Use
the hash command to correct this situation.


The Bourne shell has a limitation on the effective UID for a process. If
this UID is less than 100 (and not equal to the real UID of the process),
then the UID is reset to the real UID of the process.


Because the shell implements both foreground and background jobs in the
same process group, they all receive the same signals, which can lead to
unexpected behavior. It is, therefore, recommended that other job control
shells be used, especially in an interactive environment.


When the shell executes a shell script that attempts to execute a non-
existent command interpreter, the shell returns an erroneous diagnostic
message that the shell script file does not exist.


April 9, 2016 SH(1HAS)