When an asynchronous list (see Asynchronous Lists ) 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 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.
The following operand shall be supported:
The following environment variables shall affect the execution of wait:
The standard error shall be used only for diagnostic messages.
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:
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 IEEE Std 1003.1-2001 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 IEEE Std 1003.1-2001 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 IEEE Std 1003.1-2001 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.
Shell Command Language , kill() , sh , the System Interfaces volume of IEEE Std 1003.1-2001, wait(), waitpid()