This creates a lock based on the existence of a file

lock.sh

#!/bin/bash

LOCK_FILE="/var/lock/s3cmd.lock"

# Check if another instance is already running
if [[ -e $LOCK_FILE ]]; then
  echo "Another instance of the script is already running. Exiting..."
  exit 1
else
  touch $LOCK_FILE
fi

function cleanup {
  # Remove lock file upon exit
  rm $LOCK_FILE
}

trap cleanup EXIT INT TERM

# Your code goes here