Question

I was wondering how, if possible, I can create a simple job management in BASH to process several commands in parallel. That is, I have a big list of commands to run, and I’d like to have two of them running at any given time.

I know quite a bit about bash, so here are the requirements that make it tricky:

• The commands have variable running time so I can’t just spawn 2, wait, and then continue with the next two. As soon as one command is done a next command must be run.
• The controlling process needs to know the exit code of each command so that it can keep a total of how many failed

I’m thinking somehow I can use trap but I don’t see an easy way to get the exit value of a child inside the handler.

So, any ideas on how this can be done?

---

Well, here is some proof of concept code that should probably work, but it breaks bash: invalid command lines generated, hanging, and sometimes a core dump.

# need monitor mode for trap CHLD to work
set -m
# store the PIDs of the children being watched
declare -a child_pids

function child_done
{
    echo "Child $1 result = $2"
}

function check_pid
{
    # check if running
    kill -s 0 $1
    if [ $? == 0 ]; then
        child_pids=("${child_pids[@]}" "$1")
    else
        wait $1
        ret=$?
        child_done $1 $ret
    fi
}

# check by copying pids, clearing list and then checking each, check_pid
# will add back to the list if it is still running
function check_done
{
    to_check=("${child_pids[@]}")
    child_pids=()

    for ((i=0;$i<${#to_check};i++)); do
        check_pid ${to_check[$i]}
    done
}

function run_command
{
    "$@" &
    pid=$!
    # check this pid now (this will add to the child_pids list if still running)
    check_pid $pid
}

# run check on all pids anytime some child exits
trap 'check_done' CHLD

# test
for ((tl=0;tl<10;tl++)); do
    run_command bash -c "echo FAIL; sleep 1; exit 1;"
    run_command bash -c "echo OKAY;"
done

# wait for all children to be done
wait

Note that this isn’t what I ultimately want, but would be groundwork to getting what I want.

---

Followup: I’ve implemented a system to do this in Python. So anybody using Python for scripting can have the above functionality. Refer to shelljob

Answer 1

GNU Parallel is awesomesauce:

$ parallel -j2 < commands.txt
$ echo $?

It will set the exit status to the number of commands that failed. If you have more than 253 commands, check out --joblog. If you don’t know all the commands up front, check out --bg.