Testing file locking

Question

I have a script which locks a file to avoid concurrent access to it, How can I execute this same script from two different terminals synchronously, to check if it works?

Here is the script

#!/bin/bash

(
  flock -xn 200
  trap 'rm /tmp/test_lock.txt' 0
  RETVAL=$?
  if [ $RETVAL -eq 1 ]
  then
    echo $RETVAL
    echo  "file already removed"
    exit 1
  else
    echo "locked and removed"
  fi
) 200>/tmp/test_lock.txt

Answer 1

You can't execute them synchronously, but by placing a sleep 60 inside the bracketed lock section you can demonstrate to yourself that only one instance can run simultaneously (at the same time). Or not.

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You'll notice that what you have does not work. This is because you delete the lock file, which means that any new process trying to obtain the lock does so on a different file to the one that already holds the lock.