Run in parallel

run-in-parallel.sh

#!/usr/bin/env bash

NTOK=3

do_cmd() {
    echo "Command start $1"
    sleep 5
    echo "Command end $1"
    return 1
}

# Set up 2 pipes: one for the semaphore tokens, and
# another one for collecting return values.

p=$(mktemp -u)
mkfifo $p
exec 3<>$p
rm $p

p=$(mktemp -u)
mkfifo $p
exec 4<>$p
rm $p

# Fill the semaphore pipe with tokens
for ((i=0; i<$NTOK; i++)); do
    echo -n "#" >&3
done

pids=()

# Main execution loop
for i in {0..5}; do
    read -n 1 -u 3 x
    ( 
        do_cmd $i
        echo "$?" >&4        
        [[ $i -eq 5 ]] && echo "." >&4 # End
        echo -n "#" >&3
    ) &
    pids+=($!)
done

for pid in ${pids[@]}; do
    echo "waiting $pid"
    wait $pid
done

#cat <&4

while read e && [[ $e != "." ]]; do
    echo "exit: $e"
done <&4

echo "done"