Classic Fork-join parallelism in BASH, blocking and nestable

bash_parallel.source

# Classic Fork-join parallelism. bash_parallel's can be nested arbitrarily
# Silent by default; set BASH_PARALLEL_VERBOSE=1 for verbose output on stderr
function bash_parallel
{
    function bash_parallel_echo
    {
        if [[ $BASH_PARALLEL_VERBOSE == 1 ]] ; then
            echo "$@" 1>&2
        fi
    }

    # In subshell to allow nesting
    (
        i=0
        unset BASH_PARALLEL_PIDS  # allow nesting
        unset BASH_PARALLEL_CMDS  # allow nesting

        for cmd in "$@"
        do
            ( $cmd ) &  # in subshell to avoid environmental pollution
            BASH_PARALLEL_PIDS[$i]=$!
            BASH_PARALLEL_CMDS[$i]="$cmd"

            bash_parallel_echo "bash_parallel: started PID ${BASH_PARALLEL_PIDS[$i]}: $cmd"
            i=$(($i + 1))
        done

        bash_parallel_echo "bash_parallel: waiting for PIDs ${BASH_PARALLEL_PIDS[@]}"

        # bash_parallel_wait: builtin wait does not take OR of return values
        function bash_parallel_wait
        {
            local bash_parallel_ret=0
            for bash_parallel_pid in ${BASH_PARALLEL_PIDS[@]}
            do
                # Wait for completion, obtain return code and OR together
                wait $bash_parallel_pid
                bash_parallel_ret_tmp=$?
                bash_parallel_ret=$(( $bash_parallel_ret | $bash_parallel_ret_tmp ))
                bash_parallel_echo "bash_parallel: $bash_parallel_pid returned with exit code $bash_parallel_ret_tmp"
            done

            ( exit $bash_parallel_ret )
        }

        bash_parallel_wait
    )
}