How to get the PID of a process in a pipeline

Question

Consider the following simplified example:

my_prog|awk \'...\' > output.csv &
my_pid=\"$!\" #Gives the PID for awk instead of for my_prog
sleep 10
kill $my         


        

Answer 1

My solution was to query jobs and parse it using perl.
Start two pipelines in the background:

$ sleep 600 | sleep 600 |sleep 600 |sleep 600 |sleep 600 &
$ sleep 600 | sleep 600 |sleep 600 |sleep 600 |sleep 600 &

Query background jobs:

$ jobs
[1]-  Running                 sleep 600 | sleep 600 | sleep 600 | sleep 600 | sleep 600 &
[2]+  Running                 sleep 600 | sleep 600 | sleep 600 | sleep 600 | sleep 600 &

$ jobs -l
[1]-  6108 Running                 sleep 600
      6109                       | sleep 600
      6110                       | sleep 600
      6111                       | sleep 600
      6112                       | sleep 600 &
[2]+  6114 Running                 sleep 600
      6115                       | sleep 600
      6116                       | sleep 600
      6117                       | sleep 600
      6118                       | sleep 600 &

Parse the jobs list of the second job %2. The parsing is probably error prone, but in these cases it works. We aim to capture the first number followed by a space. It is stored into the variable pids as an array using the parenthesis:

$ pids=($(jobs -l %2 | perl -pe '/(\d+) /; $_=$1 . "\n"'))
$ echo $pids
6114
$ echo ${pids[*]}
6114 6115 6116 6117 6118
$ echo ${pids[2]}
6116
$ echo ${pids[4]}
6118

And for the first pipeline:

$ pids=($(jobs -l %1 | perl -pe '/(\d+) /; $_=$1 . "\n"'))
$ echo ${pids[2]}
6110
$ echo ${pids[4]}
6112

We could wrap this into a little alias/function:

function pipeid() { jobs -l ${1:-%%} | perl -pe '/(\d+) /; $_=$1 . "\n"'; }
$ pids=($(pipeid))     # PIDs of last job
$ pids=($(pipeid %1))  # PIDs of first job

I have tested this in bash and zsh. Unfortunately, in bash I could not pipe the output of pipeid into another command. Probably because that pipeline is ran in a sub shell not able to query the job list??