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PROLOG | NAME | SYNOPSIS | DESCRIPTION | OPTIONS | OPERANDS | STDIN | INPUT FILES | ENVIRONMENT VARIABLES | ASYNCHRONOUS EVENTS | STDOUT | STDERR | OUTPUT FILES | EXTENDED DESCRIPTION | EXIT STATUS | CONSEQUENCES OF ERRORS | APPLICATION USAGE | EXAMPLES | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
WAIT(1P) POSIX Programmer's Manual WAIT(1P)
This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
wait — await process completion
wait [pid...]
When an asynchronous list (see Section 2.9.3.1, Examples) is started
by the shell, the process ID of the last command in each element of
the asynchronous list shall become known in the current shell
execution environment; see Section 2.12, Shell Execution Environment.
If the wait utility is invoked with no operands, it shall wait until
all process IDs known to the invoking shell have terminated and exit
with a zero exit status.
If one or more pid operands are specified that represent known
process IDs, the wait utility shall wait until all of them have
terminated. If one or more pid operands are specified that represent
unknown process IDs, wait shall treat them as if they were known
process IDs that exited with exit status 127. The exit status
returned by the wait utility shall be the exit status of the process
requested by the last pid operand.
The known process IDs are applicable only for invocations of wait in
the current shell execution environment.
None.
The following operand shall be supported:
pid One of the following:
1. The unsigned decimal integer process ID of a command,
for which the utility is to wait for the termination.
2. A job control job ID (see the Base Definitions volume
of POSIX.1‐2008, Section 3.204, Job Control Job ID)
that identifies a background process group to be waited
for. The job control job ID notation is applicable only
for invocations of wait in the current shell execution
environment; see Section 2.12, Shell Execution
Environment. The exit status of wait shall be
determined by the last command in the pipeline.
Note: The job control job ID type of pid is only
available on systems supporting the User
Portability Utilities option.
Not used.
None.
The following environment variables shall affect the execution of
wait:
LANG Provide a default value for the internationalization
variables that are unset or null. (See the Base Definitions
volume of POSIX.1‐2008, Section 8.2, Internationalization
Variables for the precedence of internationalization
variables used to determine the values of locale
categories.)
LC_ALL If set to a non-empty string value, override the values of
all the other internationalization variables.
LC_CTYPE Determine the locale for the interpretation of sequences of
bytes of text data as characters (for example, single-byte
as opposed to multi-byte characters in arguments).
LC_MESSAGES
Determine the locale that should be used to affect the
format and contents of diagnostic messages written to
standard error.
NLSPATH Determine the location of message catalogs for the
processing of LC_MESSAGES.
Default.
Not used.
The standard error shall be used only for diagnostic messages.
None.
None.
If one or more operands were specified, all of them have terminated
or were not known by the invoking shell, and the status of the last
operand specified is known, then the exit status of wait shall be the
exit status information of the command indicated by the last operand
specified. If the process terminated abnormally due to the receipt of
a signal, the exit status shall be greater than 128 and shall be
distinct from the exit status generated by other signals, but the
exact value is unspecified. (See the kill −l option.) Otherwise, the
wait utility shall exit with one of the following values:
0 The wait utility was invoked with no operands and all process
IDs known by the invoking shell have terminated.
1‐126 The wait utility detected an error.
127 The command identified by the last pid operand specified is
unknown.
Default.
The following sections are informative.
On most implementations, wait is a shell built-in. If it is called in
a subshell or separate utility execution environment, such as one of
the following:
(wait)
nohup wait ...
find . −exec wait ... \;
it returns immediately because there are no known process IDs to wait
for in those environments.
Historical implementations of interactive shells have discarded the
exit status of terminated background processes before each shell
prompt. Therefore, the status of background processes was usually
lost unless it terminated while wait was waiting for it. This could
be a serious problem when a job that was expected to run for a long
time actually terminated quickly with a syntax or initialization
error because the exit status returned was usually zero if the
requested process ID was not found. This volume of POSIX.1‐2008
requires the implementation to keep the status of terminated jobs
available until the status is requested, so that scripts like:
j1&
p1=$!
j2&
wait $p1
echo Job 1 exited with status $?
wait $!
echo Job 2 exited with status $?
work without losing status on any of the jobs. The shell is allowed
to discard the status of any process if it determines that the
application cannot get the process ID for that process from the
shell. It is also required to remember only {CHILD_MAX} number of
processes in this way. Since the only way to get the process ID from
the shell is by using the '!' shell parameter, the shell is allowed
to discard the status of an asynchronous list if "$!" was not
referenced before another asynchronous list was started. (This means
that the shell only has to keep the status of the last asynchronous
list started if the application did not reference "$!". If the
implementation of the shell is smart enough to determine that a
reference to "$!" was not saved anywhere that the application can
retrieve it later, it can use this information to trim the list of
saved information. Note also that a successful call to wait with no
operands discards the exit status of all asynchronous lists.)
If the exit status of wait is greater than 128, there is no way for
the application to know if the waited-for process exited with that
value or was killed by a signal. Since most utilities exit with
small values, there is seldom any ambiguity. Even in the ambiguous
cases, most applications just need to know that the asynchronous job
failed; it does not matter whether it detected an error and failed or
was killed and did not complete its job normally.
Although the exact value used when a process is terminated by a
signal is unspecified, if it is known that a signal terminated a
process, a script can still reliably determine which signal by using
kill as shown by the following script:
sleep 1000&
pid=$!
kill −kill $pid
wait $pid
echo $pid was terminated by a SIG$(kill −l $?) signal.
If the following sequence of commands is run in less than 31 seconds:
sleep 257 | sleep 31 &
jobs −l %%
either of the following commands returns the exit status of the
second sleep in the pipeline:
wait <pid of sleep 31>
wait %%
The description of wait does not refer to the waitpid() function from
the System Interfaces volume of POSIX.1‐2008 because that would
needlessly overspecify this interface. However, the wording means
that wait is required to wait for an explicit process when it is
given an argument so that the status information of other processes
is not consumed. Historical implementations use the wait() function
defined in the System Interfaces volume of POSIX.1‐2008 until wait()
returns the requested process ID or finds that the requested process
does not exist. Because this means that a shell script could not
reliably get the status of all background children if a second
background job was ever started before the first job finished, it is
recommended that the wait utility use a method such as the
functionality provided by the waitpid() function.
The ability to wait for multiple pid operands was adopted from the
KornShell.
This new functionality was added because it is needed to determine
the exit status of any asynchronous list accurately. The only
compatibility problem that this change creates is for a script like
while sleep 60 do
job& echo Job started $(date) as $! done
which causes the shell to monitor all of the jobs started until the
script terminates or runs out of memory. This would not be a problem
if the loop did not reference "$!" or if the script would
occasionally wait for jobs it started.
None.
Chapter 2, Shell Command Language, kill(1p), sh(1p)
The Base Definitions volume of POSIX.1‐2008, Section 3.204, Job
Control Job ID, Chapter 8, Environment Variables
The System Interfaces volume of POSIX.1‐2008, wait(3p)
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) In the event of any discrepancy between this version and
the original IEEE and The Open Group Standard, the original IEEE and
The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 WAIT(1P)
Pages that refer to this page: bg(1p), fg(1p), jobs(1p), kill(1p), sleep(1p)