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Cron Expressions Explained
A Practical Guide for Developers

📅 April 2025 ⏱ 7 min read 💻 Developer

Cron is the Unix job scheduler — and cron expressions are the five-field syntax you use to tell it when to run. Once you understand the pattern, scheduling any task becomes straightforward.

1. Anatomy of a Cron Expression

A standard cron expression has five fields separated by spaces, each representing a unit of time:

*   *   *   *   *
Minute
0–59
Hour
0–23
Day of Month
1–31
Month
1–12
Day of Week
0–7 (0&7=Sun)

Some tools (AWS EventBridge, Quartz scheduler) use a six or seven field format that adds seconds and/or year. This guide covers the standard five-field Unix cron.

2. Field-by-Field Breakdown

Minute 0–59
When within the hour to run. 0 = top of the hour, 30 = half past, 59 = one minute before the hour.
Hour 0–23
Uses 24-hour time. 0 = midnight, 12 = noon, 13 = 1 PM. Combine with minute: "30 8" = 8:30 AM.
Day of Month 1–31
Which day of the month. Be careful with 29, 30, 31 — not every month has these days. Use with caution.
Month 1–12 or JAN–DEC
Some cron implementations accept three-letter abbreviations (JAN, FEB, MAR…). Standard cron uses 1–12.
Day of Week 0–7 or SUN–SAT
0 and 7 both represent Sunday. 1=Monday, 2=Tuesday … 6=Saturday. When both day-of-month and day-of-week are set, the job runs when EITHER condition is true.

3. Special Characters

* Wildcard — every value "Every minute", "every hour", "every day"
* * * * * → runs every minute
, List — multiple values Run at specific discrete values
0 9,17 * * * → 9 AM and 5 PM daily
- Range — from to Run for every value in a range
0 9-17 * * * → every hour 9 AM through 5 PM
/ Step — every Nth Run every N units starting from a value
*/15 * * * * → every 15 minutes
@reboot Special string Runs once at system startup (not all cron implementations)
@reboot /path/to/startup.sh
@daily Special string Equivalent to 0 0 * * * — midnight every day
@daily /path/to/backup.sh
@hourly Special string Equivalent to 0 * * * * — top of every hour
@hourly /path/to/check.sh

4. Common Real-World Examples

Expression Meaning
* * * * * Every minute (usually for testing only)
0 * * * * Every hour on the hour
0 0 * * * Every day at midnight
0 8 * * 1-5 Weekdays at 8:00 AM
0 0 * * 0 Every Sunday at midnight
0 0 1 * * First of every month at midnight
0 0 1 1 * January 1st at midnight (annual)
*/5 * * * * Every 5 minutes
*/15 * * * * Every 15 minutes
0 9-17 * * 1-5 Top of each hour, 9 AM–5 PM, Monday–Friday
30 6 * * 1 Every Monday at 6:30 AM
0 0,12 * * * Midnight and noon every day
0 2 * * 0 Every Sunday at 2:00 AM (good for weekly backups)
0 3 1,15 * * 1st and 15th of each month at 3:00 AM

5. Common Gotchas

⏰
Cron uses the server's local timezone
If your server is set to UTC but your users are in EST, "0 9 * * *" runs at 4 AM EST — not 9 AM. Always verify the server timezone with `date` before deploying.
🔢
Day-of-month and day-of-week are OR, not AND
Setting "0 0 1 * 1" runs on the first of the month AND every Monday — not just Mondays that are the 1st.
📁
Environment variables may not be set
Cron runs with a minimal environment. $PATH, $HOME, and custom variables from your .bashrc are often missing. Use absolute paths for all commands and scripts.
📤
Cron emails output by default
If your cron job produces any output (stdout/stderr) and MAILTO is not set, cron tries to email it. Redirect output: `command >> /var/log/myjob.log 2>&1`
🔒
Permissions matter
The script must be executable (`chmod +x script.sh`) and the cron user must have permission to run it and access any files it touches.

6. Tools to Help

  • crontab.guru — paste any cron expression and get a plain-English description instantly
  • crontab -e — edit your personal crontab on any Unix/Linux system
  • crontab -l — list all scheduled cron jobs for the current user
  • /etc/cron.d/ — system-wide cron jobs (requires root to edit)
  • systemd timers — the modern Linux alternative to cron; more robust with built-in logging via journalctl
Quick reference: The five fields are minute, hour, day-of-month, month, day-of-week. A * means "every". A / means "every Nth". A - is a range. A , is a list.