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WAIT
Section: POSIX Programmer's Manual (1P)Updated: 2013
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PROLOG
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.NAME
wait --- await process completionSYNOPSIS
wait [pid...]
DESCRIPTION
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.OPTIONS
None.OPERANDS
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.
-
-
STDIN
Not used.INPUT FILES
None.ENVIRONMENT VARIABLES
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.
ASYNCHRONOUS EVENTS
Default.STDOUT
Not used.STDERR
The standard error shall be used only for diagnostic messages.OUTPUT FILES
None.EXTENDED DESCRIPTION
None.EXIT STATUS
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.
CONSEQUENCES OF ERRORS
Default.The following sections are informative.
APPLICATION USAGE
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 ... \;
-
j1& p1=$! j2& wait $p1 echo Job 1 exited with status $? wait $! echo Job 2 exited with status $?
EXAMPLES
Although the exact value used when a process is terminated by a signal is unspecified, if it is known that a signal terminated a process, a script can still reliably determine which signal by using kill as shown by the following script:
-
sleep 1000& pid=$! kill -kill $pid wait $pid echo $pid was terminated by a SIG$(kill -l $?) signal.
-
sleep 257 | sleep 31 & jobs -l %%
-
wait <pid of sleep 31> wait %%
RATIONALE
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
FUTURE DIRECTIONS
None.SEE ALSO
Chapter 2, Shell Command Language, kill, sh 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()COPYRIGHT
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 .