Bash: Parallel execute and retain exit code

:for

#!/usr/bin/env bash

# Wait for pids to terminate. If one pid exits with
# a non zero exit code, send the TERM signal to all
# processes and retain that exit code
#
# usage:
# :wait 123 32
function :wait(){
    local pids=("$@")
    [ ${#pids} -eq 0 ] && return $?
    
    trap 'kill -INT "${pids[@]}" &>/dev/null || true; trap - INT' INT
    trap 'kill -TERM "${pids[@]}" &>/dev/null || true; trap - RETURN TERM' RETURN TERM

    for pid in "${pids[@]}"; do
        wait "${pid}" || return $?
    done

    trap - INT RETURN TERM
}

# Run a function in parallel for each argument.
# Stop all instances if one exits with a non zero
# exit code
#
# usage:
# :for func 1 2 3
#
# env:
# FOR_PARALLEL: Max functions running in parallel
function :for(){
    local f="${1}" && shift

    local i=0
    local pids=()
    for arg in "$@"; do
        ( ${f} "${arg}" ) &
        pids+=("$!")
        if [ ! -z ${FOR_PARALLEL+x} ]; then
            (( i=(i+1)%${FOR_PARALLEL} ))
            if (( i==0 )) ;then
                :wait "${pids[@]}" || return $?
                pids=()
            fi
        fi
    done && [ ${#pids} -eq 0 ] || :wait "${pids[@]}" || return $?
}

usage

#!/usr/bin/env bash
set -ex

# import :for
source <(curl -Ls https://goo.gl/Wr4QZZ)

msg="You should see this three times"

:(){
  i="${1}" && shift

  echo "${msg}"

  sleep 1
  if   [ "$i" == "1" ]; then sleep 1
  elif [ "$i" == "2" ]; then false
  elif [ "$i" == "3" ]; then
    sleep 3
    echo "You should never see this"
  fi
} && :for : 1 2 3 || exit $?

echo "You should never see this"

usage.run

$ ./usage
+ source /dev/fd/63
++ curl -Ls https://goo.gl/Wr4QZZ
+ msg='You should see this three times'
+ :for : 1 2 3
+ local f=:
+ shift
+ local i=0
+ pids=()
+ local pids
+ for arg in "$@"
+ pids+=("$!")
+ : 1
+ '[' '!' -z ']'
+ for arg in "$@"
+ i=1
+ shift
+ echo 'You should see this three times'
You should see this three times
+ pids+=("$!")
+ sleep 1
+ '[' '!' -z ']'
+ for arg in "$@"
+ pids+=("$!")
+ '[' '!' -z ']'
+ '[' 5 -eq 0 ']'
+ :wait 24933 24934 24935
+ pids=("$@")
+ local pids
+ '[' 5 -eq 0 ']'
+ trap 'kill -INT "${pids[@]}" &>/dev/null || true; trap - INT' INT
+ trap 'kill -TERM "${pids[@]}" &>/dev/null || true; trap - RETURN TERM' RETURN TERM
+ for pid in "${pids[@]}"
+ wait 24933
+ : 3
+ i=3
+ shift
+ echo 'You should see this three times'
You should see this three times
+ sleep 1
+ : 2
+ i=2
+ shift
+ echo 'You should see this three times'
You should see this three times
+ sleep 1
+ '[' 3 == 1 ']'
+ '[' 3 == 2 ']'
+ '[' 3 == 3 ']'
+ sleep 3
+ '[' 1 == 1 ']'
+ sleep 1
+ '[' 2 == 1 ']'
+ '[' 2 == 2 ']'
+ false
+ for pid in "${pids[@]}"
+ wait 24934
+ return 1
++ kill -TERM 24933 24934 24935
++ trap - RETURN TERM
+ return 1
+ exit 1

A similar implementation posted on reddit:

#!/usr/bin/env bash
set -ex

msg="You should see this three times"

:(){
  i="${1}" && shift

  echo "${msg}"

  sleep 1
  if   [ "$i" == "1" ]; then sleep 1
  elif [ "$i" == "2" ]; then false
  elif [ "$i" == "3" ]; then
    sleep 3
    echo "You should never see this"
  fi
} && export -f : && xargs -n 1 -P 0 bash -c ': "$@"' -- <<<'1 2 3' || exit $?


echo "You should never see this"

Problems with the xargs -P alternative:

• Spawns a new shell, which means:
  • does not preserve shell options: set -ex
  • does not forward variables (see echo "${msg}" in new usage)
• The third process won't be stopped from printing You should never see this
• Does not retain exit code (returns 123 instead of 1)

$ ./usage.sh 
+ msg='You should see this three times'
+ export -f :
+ xargs -n 1 -P 0 bash -c ': "$@"' --



You should never see this
+ exit 123