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About bash : Wait-for-a-process-to-finish

Question Detail

Is there any builtin feature in Bash to wait for a process to finish?

The wait command only allows one to wait for child processes to finish.
I would like to know if there is any way to wait for any process to finish before proceeding in any script.

A mechanical way to do this is as follows but I would like to know if there is any builtin feature in Bash.

while ps -p `cat $PID_FILE` > /dev/null; do sleep 1; done

Question Answer

To wait for any process to finish

Linux (doesn’t work on Alpine, where ash doesn’t support tail --pid):

tail --pid=$pid -f /dev/null

Darwin (requires that $pid has open files):

lsof -p $pid +r 1 &>/dev/null

With timeout (seconds)

Linux:

timeout $timeout tail --pid=$pid -f /dev/null

Darwin (requires that $pid has open files):

lsof -p $pid +r 1m%s -t | grep -qm1 $(date -v+${timeout}S +%s 2>/dev/null || echo INF)

There’s no builtin. Use kill -0 in a loop for a workable solution:

anywait(){

    for pid in "[email protected]"; do
        while kill -0 "$pid"; do
            sleep 0.5
        done
    done
}

Or as a simpler oneliner for easy one time usage:

while kill -0 PIDS 2> /dev/null; do sleep 1; done;

As noted by several commentators, if you want to wait for processes that you do not have the privilege to send signals to, you have find some other way to detect if the process is running to replace the kill -0 $pid call. On Linux, test -d "/proc/$pid" works, on other systems you might have to use pgrep (if available) or something like ps | grep "^$pid ".

I found “kill -0” does not work if the process is owned by root (or other), so I used pgrep and came up with:

while pgrep -u root process_name > /dev/null; do sleep 1; done

This would have the disadvantage of probably matching zombie processes.

This bash script loop ends if the process does not exist, or it’s a zombie.

PID=<pid to watch>
while s=`ps -p $PID -o s=` && [[ "$s" && "$s" != 'Z' ]]; do
    sleep 1
done

EDIT: The above script was given below by Rockallite. Thanks!

My orignal answer below works for Linux, relying on procfs i.e. /proc/. I don’t know its portability:

while [[ ( -d /proc/$PID ) && ( -z `grep zombie /proc/$PID/status` ) ]]; do
    sleep 1
done

It’s not limited to shell, but OS’s themselves do not have system calls to watch non-child process termination.

FreeBSD and Solaris have this handy pwait(1) utility, which does exactly, what you want.

I believe, other modern OSes also have the necessary system calls too (MacOS, for example, implements BSD’s kqueue), but not all make it available from command-line.

From the bash manpage

   wait [n ...]
          Wait for each specified process and return its termination  status
          Each  n  may be a process ID or a job specification; if a
          job spec is given, all processes  in  that  job's  pipeline  are
          waited  for.  If n is not given, all currently active child processes
          are waited for, and the return  status  is  zero.   If  n
          specifies  a  non-existent  process or job, the return status is
          127.  Otherwise, the return status is the  exit  status  of  the
          last process or job waited for.

All these solutions are tested in Ubuntu 14.04:

Solution 1 (by using ps command):
Just to add up to Pierz answer, I would suggest:

while ps axg | grep -vw grep | grep -w process_name > /dev/null; do sleep 1; done

In this case, grep -vw grep ensures that grep matches only process_name and not grep itself. It has the advantage of supporting the cases where the process_name is not at the end of a line at ps axg.

Solution 2 (by using top command and process name):

while [[ $(awk '$12=="process_name" {print $0}' <(top -n 1 -b)) ]]; do sleep 1; done

Replace process_name with the process name that appears in top -n 1 -b. Please keep the quotation marks.

To see the list of processes that you wait for them to be finished, you can run:

while : ; do p=$(awk '$12=="process_name" {print $0}' <(top -n 1 -b)); [[ $b ]] || break; echo $p; sleep 1; done

Solution 3 (by using top command and process ID):

while [[ $(awk '$1=="process_id" {print $0}' <(top -n 1 -b)) ]]; do sleep 1; done

Replace process_id with the process ID of your program.

Okay, so it seems the answer is — no, there is no built in tool.

After setting /proc/sys/kernel/yama/ptrace_scope to 0, it is possible to use the strace program. Further switches can be used to make it silent, so that it really waits passively:

strace -qqe '' -p <PID>

Blocking solution

Use the wait in a loop, for waiting for terminate all processes:

function anywait()
{

    for pid in "[email protected]"
    do
        wait $pid
        echo "Process $pid terminated"
    done
    echo 'All processes terminated'
}

This function will exits immediately, when all processes was terminated. This is the most efficient solution.

Non-blocking solution

Use the kill -0 in a loop, for waiting for terminate all processes + do anything between checks:

function anywait_w_status()
{
    for pid in "[email protected]"
    do
        while kill -0 "$pid"
        do
            echo "Process $pid still running..."
            sleep 1
        done
    done
    echo 'All processes terminated'
}

The reaction time decreased to sleep time, because have to prevent high CPU usage.

A realistic usage:

Waiting for terminate all processes + inform user about all running PIDs.

function anywait_w_status2()
{
    while true
    do
        alive_pids=()
        for pid in "[email protected]"
        do
            kill -0 "$pid" 2>/dev/null \
                && alive_pids+="$pid "
        done

        if [ ${#alive_pids[@]} -eq 0 ]
        then
            break
        fi

        echo "Process(es) still running... ${alive_pids[@]}"
        sleep 1
    done
    echo 'All processes terminated'
}

Notes

These functions getting PIDs via arguments by [email protected] as BASH array.

Had the same issue, I solved the issue killing the process and then waiting for each process to finish using the PROC filesystem:

while [ -e /proc/${pid} ]; do sleep 0.1; done

There is no builtin feature to wait for any process to finish.

You could send kill -0 to any PID found, so you don’t get puzzled by zombies and stuff that will still be visible in ps (while still retrieving the PID list using ps).

Use inotifywait to monitor some file that gets closed, when your process terminates. Example (on Linux):

yourproc >logfile.log & disown
inotifywait -q -e close logfile.log

-e specifies the event to wait for, -q means minimal output only on termination. In this case it will be:

logfile.log CLOSE_WRITE,CLOSE

A single wait command can be used to wait for multiple processes:

yourproc1 >logfile1.log & disown
yourproc2 >logfile2.log & disown
yourproc3 >logfile3.log & disown
inotifywait -q -e close logfile1.log logfile2.log logfile3.log

The output string of inotifywait will tell you, which process terminated. This only works with ‘real’ files, not with something in /proc/

On a system like OSX you might not have pgrep so you can try this appraoch, when looking for processes by name:

while ps axg | grep process_name$ > /dev/null; do sleep 1; done

The $ symbol at the end of the process name ensures that grep matches only process_name to the end of line in the ps output and not itself.

Rauno Palosaari’s solution for Timeout in Seconds Darwin, is an excellent workaround for a UNIX-like OS that does not have GNU tail (it is not specific to Darwin). But, depending on the age of the UNIX-like operating system, the command-line offered is more complex than necessary, and can fail:

lsof -p $pid +r 1m%s -t | grep -qm1 $(date -v+${timeout}S +%s 2>/dev/null || echo INF)

On at least one old UNIX, the lsof argument +r 1m%s fails (even for a superuser):

lsof: can't read kernel name list.

The m%s is an output format specification. A simpler post-processor does not require it. For example, the following command waits on PID 5959 for up to five seconds:

lsof -p 5959 +r 1 | awk '/^=/ { if (T++ >= 5) { exit 1 } }'

In this example, if PID 5959 exits of its own accord before the five seconds elapses, ${?} is 0. If not ${?} returns 1 after five seconds.

It may be worth expressly noting that in +r 1, the 1 is the poll interval (in seconds), so it may be changed to suit the situation.

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