no-duplicate-processes

#!/usr/bin/env python3

# let's say you have a cron job that updates some cache and you'd like it to
# be updated often. You can measure the time it takes and set the cron frequency
# similar to that amount of time, but what if this machine is running slower one day?
# then, the last iteration of your job will still be running when the next
# iteration begins. What happens if they're both overwriting a file?
# What if the combined load of those two jobs slows down the computer even more,
# and then there are 3 jobs, and so on?

import os
import sys
import atexit
import hashlib
import subprocess

if len(sys.argv) != 2:
    print("Exactly one argument required! Put your command in quotes.")
    sys.exit(1)

command = sys.argv[1]
cmd_hash = hashlib.sha1(command.encode()).hexdigest()
lock_file_dirname = os.path.join('/tmp', os.getenv('USER', default="anybody"))
lock_file_path = os.path.join(lock_file_dirname, f'command-lock-{cmd_hash}')

if os.path.isfile(lock_file_path):
    sys.exit(0)

if not os.path.isdir(lock_file_dirname):
    os.makedirs(lock_file_dirname)

open(lock_file_path, 'w').close()
atexit.register(os.remove, lock_file_path)
subprocess.run(command, shell=True)