Bash script to check if a script is already running.

script-already-running-check.sh

#!/bin/bash

dupe_script=$(ps -ef | grep "SCRIPT_NAME.sh" | grep -v grep | wc -l | xargs)

if [ ${dupe_script} -gt 2 ]; then
    echo -e "The SCRIPT_NAME.sh script was already running!"
    exit 0
fi

Well, y'all we're right. 3 doesn't work in usual circumstances. I was using 3 for a specific reason, which I'll explain later...

You would think it should be -gt 1, and I agree with you, logically, it should! But it's not. It's because of bash's subshells and pipes. This StackOverflow answer explains why, but I'll admit it's a bit over my head.

So because of that, we need to use 2 when we're in the script, though it will only show as 1 outside of the script. Yes, weird, I know. It bakes my noodle too. But it is what it is and it works and I won't try and fight it.

You can see what's going on in the screenshot below with some extra echos.

I've updated the gist above and changed it to 2.

Now... Why was I originally using 3? It's because I was using this in a scheduled cron job, so when it went to do the grep for the running processes, it saw an extra entry for the script as a scheduled cron job. If that's you and your use case, you'll want to continue to use 3 instead of 2.

Hope that helps explain things a bit! 😅

How about determining the script name like this?

#!/bin/bash

SCRIPT_NAME=$(basename "$0")
dupe_script=$(ps -ef | grep "$SCRIPT_NAME" | grep -v grep | wc -l | xargs)

if [ ${dupe_script} -gt 2 ]; then
    echo -e "The $SCRIPT_NAME script was already running!"
    exit 0
fi

consider looking at "pgrep", one for "-c" to count the pids and two for -f "for full path", three -u for uid running, and execute with fqfn so that the script is "/path/to/script/runtime.sh" not "runtime.sh". Now you can run multiple scripts of the same name and not get conflicts between them (eg, ~/bin/cron.sh and ~/test/cron.sh) and other user's cron.sh won't clash, or when you are editing cron.sh (eg, "emacs cron.sh")

https://man7.org/linux/man-pages/man1/pgrep.1.html