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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 |
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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.1‐2017, 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.1‐2017, 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.1‐2017 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.1‐2017 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.1‐2017 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.1‐2017, Section 3.204, Job
Control Job ID, Chapter 8, Environment Variables
The System Interfaces volume of POSIX.1‐2017, wait(3p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. 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.opengroup.org/unix/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 2017 WAIT(1P)
Pages that refer to this page: bg(1p), fg(1p), jobs(1p), kill(1p), sleep(1p)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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