Scheduler

Cron — the task scheduler

Running tasks on a schedule: crontab syntax, classic cron and modern systemd timers.

systemd timers — scheduled jobs

A systemd timer is a .timer unit that starts the service of the same name on a schedule; its output goes to the journal.

at — a one-time scheduled job

Run a command once at a given time — unlike cron's recurring schedule.

flock — preventing a second run

flock holds a file lock while a command runs, so a second copy of the script waits for it or exits at once.