How to check if script is running or not from script itself?

Question

Having below sample script sample.sh

#!/bin/bash

if ps aux | grep -o "sample.sh" >/dev/null
then
    echo "Already script running"
    exit 0
fi

echo "start script"

while true
do
    echo "script running"
    sleep 5
done

In above script i want to check if this script previously running or not if running then not run it again.

problem is check condition always become true (because to check the condition require to run script) and it always show me "Already script running" message.

Any idea how to solve it?

Answer 1

You need a proper lock. I'd do using flock like this:

exec 201> /tmp/lock.$(basename $0).file
if ! flock -n 201  ; then
  echo "another instance of $0 is running";
  exit 1
fi

# cmds 

exec 201>&- 
rm -rf  /tmp/lock.$(basename $0).file

This basically creates lock for script using a temporary file. The temporary file has particular significance other than it's used to tell whether your script has acquired a lock. When there's an instance of this program running, the next run of the same program can't run as the lock will prevent it.