Killing child process in shell script

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 processes 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 behaviour.
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.

Following is one more example which monitors php script and kills if it reaches timeout.

1) test.sh

#!/bin/bash
LOG='log.txt'
trap 'echo "LineNo.$LINENO" >> $LOG; exit 1;' ERR SIGINT SIGTERM;
CMD="php test.php;"
echo $CMD
eval $CMD &>> $LOG &
GPID=$(ps -o pgid= $$);
CPID=$!
echo "PIDs: $GPID - $CPID "
CNT=0;
CNT_TIME_OUT=10;
while (true); do
if ps -p $CPID > /dev/null ; then
echo "$CPID is running..";
if [ $CNT -gt $CNT_TIME_OUT ]; then
echo "Timeout reached $CNT_TIME_OUT sec. killing $GPID.. breaking..";
kill -- -$GPID;
fi
else
echo "$CPID is completed. Breaking..";
break;
fi
CNT=$((CNT+1));
echo "waiting since $b secs..";
sleep 1;
done
exit;

1) test.php

<?php
$i=0;
$l=300;
while($i<$l) {
#throw new \Exception("Testing");
$date = new DateTime();
$date->add(DateInterval::createFromDateString('yesterday'));
echo $date->format('Y-m-d H:i:s') . " => $i\n";
sleep(1);
$i++;
echo "End => $i\n";
}
die;
?>

Thanks.

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