Cron & Timers

Must-know cold

• Cron fields: min hour dom mon dow command (5 time fields + command)
• crontab -l · crontab -e · absolute paths + redirect logs
• systemctl list-timers --all · systemctl status job.timer
• Silent failure classics: PATH, % in the command, bashisms (SHELL=/bin/sh), no logging, wrong user
• Prefer flock to prevent overlap

Cron field reference

Definition: Five time fields (min hour dom mon dow) that schedule when a cron job runs.

*  *  *  *  *  command
│  │  │  │  │
│  │  │  │  └── day of week (0–7; 0 and 7 = Sunday)
│  │  │  └───── month (1–12)
│  │  └──────── day of month (1–31)
│  └─────────── hour (0–23)
└────────────── minute (0–59)

Special strings (Vixie/cronie): @reboot @hourly @daily (@midnight) @weekly @monthly @yearly (@annually). @reboot runs when cron starts, not necessarily at kernel boot.

Commands

crontab

Definition: Edit, list, or remove the per-user cron schedule table.

Flag combos

Install from file: crontab /path/to/file replaces the whole crontab.

System cron locations

Definition: System-wide cron drop-ins and run-parts directories under /etc.

/etc/crontab line shape: m h dom mon dow USER command — six fields before the command (USER extra).

Cron environment

Definition: How cron jobs get PATH, logging, and flock single-instance guards.

Cron service

Definition: Check/restart the cron daemon and read its logs.

systemd timers

Definition: systemd calendar or monotonic schedules that start a service (often Type=oneshot, not required).

Common timer directives

Flag combos

Debug cron

Definition: Checklist for silent cron failures (env, user, locks, logs).

Common recipes

Pitfalls

• PATH is minimal (/usr/bin:/bin) and SHELL is /bin/sh. Bashisms and bare python fail. Use absolute paths or set PATH= / SHELL= in the crontab.
• % in a cron command is a newline (rest becomes stdin) unless written \%. date +%Y is a classic silent break.
• Comments are not allowed on the same line as a job.
• Output with no redirect is mailed to the owner (MAILTO=). Empty MAILTO="" or log to a file. No MTA → output often vanishes.
• crontab -r deletes all entries. crontab -e as root edits root’s crontab, not /etc/crontab.
• DOM and DOW both restricted → OR (Vixie/cronie). 0 9 1 * 1 is the 1st or Mondays, not “first Monday.” systemd OnCalendar= is not that OR.
• User crontab has 5 time fields; /etc/crontab and /etc/cron.d/* insert a USER field. Files in cron.d / cron.daily with a dot in the name are often ignored (run-parts).
• Debian/Ubuntu cron.daily often goes through anacron — “daily” ≠ 00:00 sharp.
• DST: the skipped hour never matches; the repeated hour can run twice.
• Overlapping long jobs without flock pile up. Put lock files in /run/lock or /var/lock, not world-writable /tmp.
• Enable the .timer, not usually the .service (or the job also starts at boot). AccuracySec= defaults to 1 min, so timers are not second-exact.
• Persistent= only catches up calendar timers. Check list-timers LAST/NEXT.