Killing a process group with a timeout in bash

On Linux, most programs that daemonize write their process number to a file, which can be used to send signals to the daemon. To automate deployment (in case you already do not have upstart or systemd integration with the program in question), here’s one approach to stop a daemon with potentially many child processes:

• Send SIGTERM to the parent process id (known via, say, a file written to by the program while daemonizing).
• Wait for some time so the process can handle the signal (e.g., by cleaning up its children and any other resources), while checking if the process has exited.
• If the process exited before timeout, all good, otherwise, send SIGKILL to the parent process’ process group. SIGKILL might be a bit extreme, and maybe you want to try SIGINT first, but for this post, we’ll just send SIGKILL.

Here’s the implementation in bash:

#!/bin/sh

TIMEOUT=30 # seconds
TARGET_PID=$1

if ! kill -0 $TARGET_PID 2>/dev/null; then
    echo "Process $TARGET_PID either does not exist,\
          or you are not allowed to send signals to it"
    exit 1
fi

kill $TARGET_PID 2>/dev/null

STARTED_AT=$(date +"%s") # current UNIX timestamp
# $((...)) means evaluation in arithmetic context
TO_END_AT=$(($STARTED_AT + $TIMEOUT))

while true; do

    # See below, "sending" a 0 does not actually send anything, but performs
    # error-checking, and allows us to know if the process is still alive.
    kill -0 $TARGET_PID 2>/dev/null

    if ! kill -0 $TARGET_PID 2>/dev/null; then
        echo "Process exited"
        break
    fi

    sleep 1

    if [ $(date +"%s") -gt $TO_END_AT ]; then
        echo "Timeout reached"
        break
    fi

done

if kill -0 $TARGET_PID 2>/dev/null; then
    PGID=$(ps --no-header -o '%r' $TARGET_PID)
    echo "Killing pgid $PGID"
    # To kill the entire pgroup, we must send the _negation_ of the process
    # group id to kill(1)
    kill -9 $((-$PGID)) 
fi

kill(1) allows us to check the status of a process by sending it the 0 signal, which doesn’t actually send anything to the target process, but does all the error checking a kill invocation normally would. This includes checking if we are allowed to send signals to the process in question, and the process is running at all.