Cron Expressions Demystified: A Beginner's Guide

Cron is the time-based job scheduler built into every Unix-like system. Whether you're scheduling database backups, clearing cache, or sending email digests, cron is the tool for the job. The only catch: its syntax looks like line noise until you know how to read it.

This guide will take you from zero to confidently writing cron expressions.

The Five Fields

A standard cron expression has five fields, separated by spaces:

┌───────────── minute (0-59)
│ ┌───────────── hour (0-23)
│ │ ┌───────────── day of month (1-31)
│ │ │ ┌───────────── month (1-12)
│ │ │ │ ┌───────────── day of week (0-6, Sunday=0)
│ │ │ │ │
* * * * *

Each field defines when the job should run. The job fires when all five fields match the current time.

Special Characters

Character	Meaning	Example
*	Any value	* * * * * — every minute
,	List of values	0,30 * * * * — at :00 and :30
-	Range	0 9-17 * * * — every hour, 9am–5pm
/	Step interval	*/5 * * * * — every 5 minutes

These characters can be combined. For example, 1-30/5 in the minute field means "every 5 minutes during the first half-hour" (minutes 1, 6, 11, 16, 21, 26).

Reading Cron Expressions

The trick is to read each field left to right. Let's decode a few:

30 2 * * *

"At minute 30 of hour 2, every day of the month, every month, every day of the week" — or simply: every day at 2:30 AM.

0 */6 * * *

"At minute 0, every 6th hour" — every 6 hours (midnight, 6am, noon, 6pm).

0 9 * * 1-5

"At 9:00 AM, only on days of the week 1 through 5" — weekdays at 9am.

0 0 1 * *

"At midnight on the 1st day of every month" — monthly at midnight.

Common Schedules

Here are the cron expressions developers reach for most often:

Expression	Schedule
* * * * *	Every minute
*/5 * * * *	Every 5 minutes
*/15 * * * *	Every 15 minutes
0 * * * *	Every hour (on the hour)
0 0 * * *	Daily at midnight
0 9 * * 1-5	Weekdays at 9am
0 0 * * 0	Weekly (Sunday midnight)
0 0 1 * *	Monthly (1st at midnight)
0 0 1 1 *	Yearly (January 1st at midnight)

Real-World Examples

Database Backup Every Night

# Run pg_dump at 3:30 AM daily
30 3 * * * /usr/local/bin/pg_dump mydb > /backups/mydb_$(date +\%Y\%m\%d).sql

Clear Temp Files Weekly

# Every Sunday at 4 AM
0 4 * * 0 find /tmp -type f -mtime +7 -delete

Health Check Every 5 Minutes During Business Hours

# Mon-Fri, 8am-6pm, every 5 min
*/5 8-18 * * 1-5 curl -s https://api.example.com/health >> /var/log/health.log

Monthly Report on the 1st

# 1st of each month at 7 AM
0 7 1 * * /opt/scripts/generate-report.sh | mail -s "Monthly Report" team@example.com

Common Pitfalls

• Timezone: Cron uses the system timezone by default. For servers in UTC, adjust your times accordingly. Some cron implementations support a CRON_TZ variable.
• Output: By default, cron emails the output to the crontab owner. Redirect output to avoid inbox floods: command > /dev/null 2>&1
• PATH: Cron runs with a minimal PATH. Use absolute paths for commands (e.g., /usr/bin/python3, not just python3).
• Percent signs: In crontab files, % is interpreted as a newline. Escape it as \% if your command uses date formatting.

Editing Crontabs

Use crontab -e to edit your user's crontab, and crontab -l to list it:

# List current cron jobs
crontab -l

# Edit crontab (opens in $EDITOR)
crontab -e

# Edit another user's crontab (requires root)
sudo crontab -u www-data -e