Get PID of any command in backgrounded piped sequence of commands

For the original version of the question, when only the last command's PID was desired, the special variable $! is perfect.

foo | bar | baz &
baz_pid=$!

There's no similar easy access to the PIDs of the other processes.

It took a long time for $pipestatus (zsh) and $PIPESTATUS (bash) to be added, finally giving us access to all of the exit statuses in a pipeline, in addition to the $? for the last one that has been around since the original Bourne shell. Maybe something analogous will happen with $! eventually.

I think you could do something as suggested here.

(ls -l | echo "Hello" | df -h & echo $! >&3 ) 3>pid

Here in the above example, I have retrieved the pid of third piped process and noted it down to the file pid. I could note it down for any piped process.

A not-very-portable, Linux-specific solution could be to track the processes using the pipes that connect them. We can get the PIDs of the first (jobs -p) and last ($!) commands in the pipeline. Using either PID, this script could do the job:

#! /bin/bash

PROC=$1
echo $PROC

if [[ $(readlink /proc/$PROC/fd/1) =~ ^pipe: ]]
then
    # Assuming first process in chain...
    NEXT_FD=1
elif [[ $(readlink /proc/$PROC/fd/0) =~ ^pipe: ]]
then
    # Last process in chain...
    NEXT_FD=0
else
    # Doesn't look like a pipe.
    exit
fi

NEXT_PROC_PIPE=$(readlink /proc/$PROC/fd/$NEXT_FD)

while [[ $NEXT_PROC_PIPE =~ ^pipe: ]] 
do
    PROC=$(find /proc/*/fd -type l -printf "%p/%l\n" 2>/dev/null | awk -F'/' '($6 == "'"$NEXT_PROC_PIPE"'") && ($3 != "'$PROC'" ) {print $3}')
    NEXT_PROC_PIPE=$(readlink /proc/$PROC/fd/$NEXT_FD)
    echo $PROC
done

I use zero-based arrays here in this code. Just be careful what you run by eval.

#!/bin/bash

cmd=('sleep 10' 'sleep 2' 'sleep 5')
first=1
for c in "${cmd[@]}"; do
  ((first)) && { pipe=$c; first=0; } || pipe+='|'$c
done
shopt -u lastpipe
eval $pipe &

printf 'Pipe:\n%s\n\n' "$pipe"

shellpid=$BASHPID
parent=$(ps -o pid= --ppid $shellpid | head -n -1)
declare -a pids=()
mapfile -t pids < <(printf '%s\n' $(ps -o pid= --ppid $parent))
printf '%s\n' 'Listing the arrays:'
printf '%2s %6s %s\n' i PID command
for i in "${!cmd[@]}"; do
    printf '%2d %6d %s\n' "$i" "${pids[i]}" "${cmd[i]}"
done

printf '\n%s\n' 'ps listing:'
ps xao pid,ppid,command | head -n 1
ps xao pid,ppid,command | tail | head -n -3