Obtaining PID of command earlier in Pipeline

I’m writing a bash script to use inotifywait to monitor a directory and kick off actions when changes are detected. Something like:

inotifywait -m ... | while read f; do something; done

Since inotifywait doesn’t terminate by itself, this script will not halt.

So my plan was to get the PID of the inotifywait process, save it to a file and have a different process kill it later, say like:

inotifywait -m ... | { echo ??PID?? > pid-file; while ... }

But I don’t know how to get the PID. Is there a simple way to achieve this? Another way is just to save the PID of the shell-script $$ to the file and kill the entire shell-script but I wanted to do some cleanup after the while loop.

I have tried using coproc and I think it will work but it seems like more complication than necessary.

Answers:

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Method 1

In a pipeline, all processes are started concurrently, there’s not one that is earlier than the others.

You could do:

(echo "$BASHPID" > pid-file; exec inotifywait -m ...) | while IFS= read -r...

Or portably:

sh -c 'echo "$$" > pid-file; exec inotifywait -m ...' | while IFS= read -r...

Also note that when the subshell that runs the while loop terminates, inotifywait would be killed automatically the next time it writes something to stdout.

Method 2

If you need the process ID in the loop, print it first.

sh -c 'echo "$$"; exec inotifywait -m ...' | {
  read inotifywait_pid
  while IFS= read -r f; do
    …
    if …; then kill "$inotifywait_pid"; break;
  done
}

Method 3

This SO answer seems applicable:

inotifywait -m file > >(while read f; do echo f; done) &

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0