[linux] What's the best way to send a signal to all members of a process group?

Killing child process in shell script:

Many time we need to kill child process which are hanged or block for some reason. eg. FTP connection issue.

There are two approaches,

1) To create separate new parent for each child which will monitor and kill child process once timeout reached.

Create test.sh as follows,

#!/bin/bash

declare -a CMDs=("AAA" "BBB" "CCC" "DDD")
for CMD in ${CMDs[*]}; do
    (sleep 10 & PID=$!; echo "Started $CMD => $PID"; sleep 5; echo "Killing $CMD => $PID"; kill $PID; echo "$CMD Completed.") &
done
exit;

and watch processes which are having name as 'test' in other terminal using following command.

watch -n1 'ps x -o "%p %r %c" | grep "test" '

Above script will create 4 new child processes and their parents. Each child process will run for 10sec. But once timeout of 5sec reach, thier respective parent processes will kill those childs. So child won't be able to complete execution(10sec). Play around those timings(switch 10 and 5) to see another behaviour. In that case child will finish execution in 5sec before it reaches timeout of 10sec.

2) Let the current parent monitor and kill child process once timeout reached. This won't create separate parent to monitor each child. Also you can manage all child processes properly within same parent.

Create test.sh as follows,

#!/bin/bash

declare -A CPIDs;
declare -a CMDs=("AAA" "BBB" "CCC" "DDD")

CMD_TIME=15;
for CMD in ${CMDs[*]}; do
    (echo "Started..$CMD"; sleep $CMD_TIME; echo "$CMD Done";) &
    CPIDs[$!]="$RN";
    sleep 1;
done

GPID=$(ps -o pgid= $$);
CNT_TIME_OUT=10;
CNT=0;
while (true); do
    declare -A TMP_CPIDs;

    for PID in "${!CPIDs[@]}"; do
        echo "Checking "${CPIDs[$PID]}"=>"$PID;

        if ps -p $PID > /dev/null ; then
          echo "-->"${CPIDs[$PID]}"=>"$PID" is running..";
          TMP_CPIDs[$PID]=${CPIDs[$PID]};
        else
          echo "-->"${CPIDs[$PID]}"=>"$PID" is completed.";
        fi
    done

    if [ ${#TMP_CPIDs[@]} == 0 ]; then
        echo "All commands completed.";
        break;
    else
        unset CPIDs;
        declare -A CPIDs;
        for PID in "${!TMP_CPIDs[@]}"; do
            CPIDs[$PID]=${TMP_CPIDs[$PID]};
        done
        unset TMP_CPIDs;

        if [ $CNT -gt $CNT_TIME_OUT ]; then
            echo ${CPIDs[@]}"PIDs not reponding. Timeout reached $CNT sec. killing all childern with GPID $GPID..";
            kill -- -$GPID;
        fi
    fi

    CNT=$((CNT+1));
    echo "waiting since $b secs..";
    sleep 1;
done

exit;

and watch processes which are having name as 'test' in other terminal using following command.

watch -n1 'ps x -o "%p %r %c" | grep "test" '

Above script will create 4 new child processes. We are storing pids of all child process and looping over them to check if they are finished their execution or still running. Child process will execution till CMD_TIME time. But if CNT_TIME_OUT timeout reach , All children will get killed by parent process. You can switch timing and play around with script to see behavior. One drawback of this approach is , it is using group id for killing all child tree. But parent process itself belong to same group so it will also get killed.

You may need to assign other group id to parent process if you don’t want parent to be killed.

More details can be found here,

Killing child process in shell script