cron

Updated

September 4, 2026

Overview

cron is the classic daemon that runs scheduled jobs from per-user and system crontab files. On modern Ubuntu, systemd timers are often a better fit for service-integrated jobs, but cron remains everywhere: shared hosting, legacy scripts, and simple user schedules. This page covers how the daemon interprets schedules and where jobs live; day-to-day editing is usually via crontab.

Syntax

Jobs are not started as cron …. You install lines into a crontab; cron reads them.

Crontab schedule fields:

* * * * * command
│ │ │ │ │
│ │ │ │ └── day of week (0–7, Sun=0 or 7; some versions allow names)
│ │ │ └──── month (1–12)
│ │ └────── day of month (1–31)
│ └──────── hour (0–23)
└────────── minute (0–59)

System drop-in files under /etc/cron.d/ use an extra username field after the schedule:

* * * * * root /usr/local/bin/job.sh

Common schedule patterns

Expression Meaning
*/15 * * * * Every 15 minutes
0 * * * * Top of every hour
0 3 * * * Daily at 03:00
0 9 * * 1-5 Weekdays 09:00
0 0 1 * * Monthly, 1st at midnight
@reboot Once at cron startup (after boot)
@daily / @weekly / @hourly Nicknames (implementation-specific paths)

Special characters: * any, , list, - range, / step. Extensions like L, W, # are not portable — stick to the five standard fields unless you know your cron flavor.

Key Use Cases

  1. Periodic backups and reports
  2. Log or temp cleanup
  3. Polling scripts that are not worth a long-running daemon
  4. User-level schedules without writing systemd units

Safety

  • Jobs run with a minimal environment — always use absolute paths and set PATH/SHELL in the crontab when needed.
  • Mail/MAILTO can flood or leak output; redirect stdout/stderr intentionally.
  • Never put secrets on the command line in a world-readable system crontab; use root-owned scripts with 0600 or secret managers.
  • Overlapping long jobs: guard with flock so two copies do not run at once.

Examples with Explanations

User crontab (via crontab tool)

crontab -l
crontab -e

cron executes the commands as that user. See the crontab chapter for flags.

Example job lines

# m h dom mon dow command
0 3 * * * /usr/local/bin/backup.sh >>/var/log/backup.log 2>&1
*/15 * * * * /usr/bin/flock -n /tmp/poll.lock /usr/local/bin/poll.sh
@reboot /usr/local/bin/start-tunnel.sh

Redirects capture output; flock -n skips a run if the previous still holds the lock.

System-wide locations (Ubuntu)

Path Role
/etc/crontab System crontab (has user field)
/etc/cron.d/* Drop-in packages/admin jobs (user field)
/etc/cron.daily/ etc. Scripts run by run-parts on a schedule
/var/spool/cron/crontabs/ Per-user crontabs (do not edit by hand) 
ls /etc/cron.d
ls /etc/cron.daily

Environment inside cron

SHELL=/bin/bash
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
MAILTO=you@example.com
0 4 * * * /usr/local/bin/nightly.sh

Variables at the top of a crontab apply to later lines. HOME is usually the user’s home; do not assume interactive aliases.

Debugging “it didn’t run”

# Is cron running?
systemctl status cron          # Debian/Ubuntu package name
systemctl status crond         # some RHEL-like systems

# Logs (paths vary)
journalctl -u cron -n 50 --no-pager
grep CRON /var/log/syslog | tail

# Syntax / permission issues
crontab -l
ls -l /usr/local/bin/nightly.sh   # must be executable; readable by the job user

Common failures: relative paths, missing +x, script expects a TTY, job runs as wrong user, anacron vs laptop sleep (see anacron).

Prefer systemd timers when…

systemctl list-timers --all
# unit pair: foo.service + foo.timer

Timers give dependency ordering, better logging via the journal, and clearer failure states. Use cron when you need a portable one-liner or cannot install units.

Understanding Output

Successful jobs are silent unless they print. By default many crons mail any stdout/stderr to the user (MAILTO). If mail is unconfigured, output may vanish into a local mailbox or be discarded — always log explicitly for important jobs.

Notes & Pitfalls

  • Day-of-month and day-of-week both set: meaning is “either matches” on Vixie/cronie (easy to schedule more often than intended).
  • Timezone is the system timezone (timedatectl); DST transitions can skip or double an hour.
  • Laptops/desktops asleep at the scheduled minute miss the job (use anacron or timers with Persistent=true).
  • Percent signs % in commands are special in some crons (newline to stdin) — escape as \%.
  • SELinux/AppArmor may deny scripts that work interactively.

Additional Resources

  • man cron / man 5 crontab
  • crontab.guru — schedule explainer