Cron Syntax Explained (With Examples)

Cron runs scheduled jobs on Linux, and its syntax is deceptively small — five fields and a command. Most cron bugs come from misreading one field or misusing the */N step value. Here's the whole thing, with examples you can copy.

The five fields

* * * * *  command
| | | | |
| | | | +-- day of week (0-7, 0 and 7 = Sunday)
| | | +---- month (1-12)
| | +------ day of month (1-31)
| +-------- hour (0-23)
+---------- minute (0-59)

A * means "every". Read left to right: minute, hour, day-of-month, month, day-of-week.

Examples you'll actually use

0 2 * * *        # 02:00 every day
*/5 * * * *      # every 5 minutes
0 */6 * * *      # every 6 hours, on the hour
30 4 * * 0       # 04:30 on Sundays
0 9 1 * *        # 09:00 on the 1st of each month
15 14 * * 1-5    # 14:15 Monday to Friday

The */N gotcha

This is the single most common cron mistake:

• 5 * * * * = "at minute 5 of every hour" → runs once an hour.
• */5 * * * * = "every 5 minutes" → runs 288 times a day.

If a job that should run constantly only fires hourly (or vice versa), check for a missing */.

Where crontabs live

• Per user: crontab -l lists the current user's jobs; crontab -e edits them.
• System-wide: /etc/crontab and files in /etc/cron.d/. These add a user field between day-of-week and the command:

0 2 * * *  root  /usr/local/bin/backup.sh

Forgetting that user field is a classic reason a system cron job silently never runs.

Debugging a job that won't run

1. Is it in the right crontab? (crontab -l vs /etc/crontab).
2. Is the schedule what you think? Re-read the five fields — especially day-of-month (field 3) vs day-of-week (field 5).
3. Does the command work with cron's minimal PATH? Use absolute paths.
4. Check the logs (/var/log/syslog or journalctl) for the cron run.

Practise reading and fixing schedules

The Cron & Scheduling arc in Terminal Trials has you decode real schedules and fix a broken one (the */5 gotcha included) in a safe simulated shell — free, in your browser.