What is Cron?

Cron is a time-based job scheduler built into Unix and Linux operating systems. It allows you to schedule commands or scripts to run automatically at fixed times, dates, or intervals — without any manual intervention. The name comes from the Greek word chronos (time). Cron is the standard tool for automating repetitive system tasks such as backups, log rotation, report generation, and sending emails.

History

The original cron was written by Ken Thompson at Bell Labs as part of Version 7 Unix around 1979. It was a simple daemon that woke up every minute, checked a table of jobs, and ran any that were scheduled. Vixie Cron, written by Paul Vixie in 1987–1988, replaced it as the standard implementation across most Unix systems and introduced the familiar five-field syntax still used today. Vixie Cron (or its descendants) is the default cron on most Linux distributions. Modern variants include Anacron (for systems that don't run continuously), Fcron, and systemd timers as an alternative on systemd-based systems.

How Cron Works

The cron daemon (crond) runs continuously in the background. Every minute it reads the crontab files (one per user, plus system-wide crontabs in /etc/cron.d/) and executes any jobs whose scheduled time matches the current time. Each user manages their own crontab with the crontab command.

• The cron daemon starts automatically at boot and runs silently in the background
• It checks the crontab every minute and fires matching jobs as separate processes
• Output from cron jobs is emailed to the user by default (redirect to /dev/null or a log file to suppress)
• Jobs run in a minimal environment — always use absolute paths for commands and files

Cron Expression Syntax

A cron expression consists of five space-separated fields followed by the command to run:

┌───────────── minute (0–59)
│ ┌───────────── hour (0–23)
│ │ ┌───────────── day of month (1–31)
│ │ │ ┌───────────── month (1–12)
│ │ │ │ ┌───────────── day of week (0–7, 0 and 7 = Sunday)
│ │ │ │ │
* * * * *  command to execute

Field Values

• * — Every value (wildcard)
• 5 — A specific value (e.g., minute 5)
• 1-5 — A range (e.g., Monday through Friday)
• */5 — Every N units (e.g., every 5 minutes)
• 1,3,5 — A list of values (e.g., January, March, May)
• 1-5/2 — A range with step (e.g., 1, 3, 5)

Common Examples

# Every minute
* * * * * /path/to/script.sh

# Every day at midnight
0 0 * * * /path/to/backup.sh

# Every day at 3:30 AM
30 3 * * * /path/to/task.sh

# Every Monday at 9:00 AM
0 9 * * 1 /path/to/weekly-report.sh

# Every 5 minutes
*/5 * * * * /path/to/check.sh

# Every hour (at minute 0)
0 * * * * /path/to/hourly.sh

# First day of every month at midnight
0 0 1 * * /path/to/monthly.sh

# Every weekday (Mon–Fri) at 8:00 AM
0 8 * * 1-5 /path/to/workday.sh

# Every 15 minutes between 9 AM and 5 PM on weekdays
*/15 9-17 * * 1-5 /path/to/poll.sh

# Twice a day (midnight and noon)
0 0,12 * * * /path/to/twice-daily.sh

Standard vs Quartz Cron Format

There are two common cron formats you will encounter depending on the platform:

Standard (Unix/Linux) Cron — 5 Fields

The classic format used by crontab on Unix/Linux. The smallest unit is a minute.

# minute hour day-of-month month day-of-week
  *       *    *             *     *

Quartz Cron — 6 or 7 Fields

Used by the Quartz Scheduler (Java), Spring @Scheduled, AWS EventBridge, and many CI/CD tools. An extra seconds field is added at the beginning, and an optional year field can be appended at the end.

# second minute hour day-of-month month day-of-week [year]
  0      30     9    *             *     ?

Quartz also introduces extra special characters not available in standard cron:

• ? — No specific value; used in day-of-month or day-of-week when the other is specified (you cannot set both to a non-wildcard value — use ? for the one you want to ignore)
• L — Last: L in day-of-month means the last day of the month; 5L in day-of-week means the last Friday of the month
• W — Nearest weekday: 15W means the nearest weekday to the 15th of the month
• # — Nth weekday of the month: 2#1 means the first Monday of the month

Side-by-Side Comparison

# Standard cron — every day at 09:30
30 9 * * *

# Quartz cron — every day at 09:30:00
0 30 9 * * ?

# Standard cron — every Monday at 08:00
0 8 * * 1

# Quartz cron — every Monday at 08:00:00
0 0 8 ? * MON

# Standard cron — every 5 minutes
*/5 * * * *

# Quartz cron — every 5 minutes
0 */5 * * * ?

# Quartz only — last day of each month at midnight
0 0 0 L * ?

# Quartz only — first Monday of each month at 09:00
0 0 9 ? * 2#1

Which Format to Use?

• Linux/Unix system tasks → Standard 5-field cron
• Java applications (Quartz, Spring) → Quartz 6-field cron
• AWS EventBridge scheduled rules → Quartz-style cron (6 fields, no seconds, uses ?)
• GitHub Actions, GitLab CI scheduled pipelines → Standard 5-field cron
• Kubernetes CronJob → Standard 5-field cron

Special Strings

Most cron implementations support shorthand strings as an alternative to the five-field syntax:

• @yearly (or @annually) — Run once a year: 0 0 1 1 *
• @monthly — Run once a month: 0 0 1 * *
• @weekly — Run once a week: 0 0 * * 0
• @daily (or @midnight) — Run once a day: 0 0 * * *
• @hourly — Run once an hour: 0 * * * *
• @reboot — Run once at startup

Crontab Commands

# Edit your crontab in the default editor
crontab -e

# List your current crontab
crontab -l

# Remove your crontab entirely
crontab -r

# Edit another user's crontab (root only)
crontab -u username -e

Use Cases

• Backups — Run database dumps or file backups nightly
• Log rotation — Archive, compress, or delete old log files on a schedule
• Report generation — Generate and email daily/weekly reports automatically
• Cache clearing — Invalidate or rebuild caches at off-peak hours
• Health checks — Ping services every minute and alert if they're down
• Data synchronisation — Sync files between servers or pull data from APIs at intervals
• Sending emails — Queue and dispatch scheduled newsletters or digests
• CI/CD pipelines — Trigger nightly builds or test runs
• Certificate renewal — Let's Encrypt Certbot uses a cron job to renew TLS certificates

Important Rules

• Always use absolute paths for commands and files — cron runs in a minimal environment with a limited PATH
• Redirect output to avoid email floods: command >> /var/log/job.log 2>&1
• Test your script manually as the same user before adding it to crontab
• Day-of-month and day-of-week fields are combined with OR, not AND — if both are set to non-wildcards, the job runs when either condition is true
• Use flock or a lock file if you don't want overlapping runs of the same job
• Be careful with crontab -r — it deletes your entire crontab without confirmation
• Consider Anacron on laptops or servers that aren't always on — it runs missed jobs when the machine starts

Cron vs Systemd Timers

On modern Linux systems, systemd timers are an alternative to cron. They offer more precise scheduling, logging via journald and dependency management. However, cron remains more portable and is supported on virtually every Unix-like system including macOS.

For most use cases — running scripts, backups, data fetches — cron is simpler and sufficient. Use systemd timers when you need tight integration with other system services.