Cron in one screenful of Python

cron.py

#!/usr/bin/env python

import os
import sys
import time

from crontab import CronTab


def main():
    assert len(sys.argv) == 2, 'Usage: cron.py <filename>'
    get_next_run = lambda tab, last_run: last_run + tab.next(last_run, default_utc=True)
    entries: list[tuple[CronTab, str, float]] = []
    now = time.time()
    with open(sys.argv[1]) as f:
        for idx, line in enumerate(f):
            line = line.strip()
            if not line or line.startswith('#'):
                continue
            fields = line.split(maxsplit=5)
            assert len(fields) == 6, f'Invalid entry on line {idx + 1}.'
            timespec, command = ' '.join(fields[:5]), fields[5]
            tab = CronTab(timespec)
            next_run = get_next_run(tab, now)
            entries.append((tab, command, next_run))
    while True:
        for idx, (tab, command, next_run) in enumerate(entries):
            now = time.time()
            if next_run <= now:
                os.system(command)
                next_run = get_next_run(tab, now)
                entries[idx] = (tab, command, next_run)
        now = time.time()
        delay = min(next_run - now for _, _, next_run in entries)
        delay = min(max(delay, 0), 60)
        time.sleep(delay)


if __name__ == '__main__':
    main()