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Jack

Posted on Originally published at anethoth.com

The Ultimate Cron Expression Cheat Sheet (With Examples)

#beginners #linux #tutorial #devops

Cron expressions are one of those things every developer needs but nobody memorizes. Bookmark this — you'll thank yourself later.

The Format 

┌───────────── minute (0-59)
│ ┌───────────── hour (0-23)
│ │ ┌───────────── day of month (1-31)
│ │ │ ┌───────────── month (1-12)
│ │ │ │ ┌───────────── day of week (0-7, Sun=0 or 7)
│ │ │ │ │
* * * * *
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Special Characters

Char Meaning Example
* Any value * * * * * = every minute
, List 1,15 * * * * = minute 1 and 15
- Range 1-5 * * * * = minutes 1 through 5
/ Step */10 * * * * = every 10 minutes

Common Schedules You'll Actually Use

Every X minutes 

*/5 * * * *    # Every 5 minutes
*/15 * * * *   # Every 15 minutes
*/30 * * * *   # Every 30 minutes
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Hourly 

0 * * * *      # Top of every hour
30 * * * *     # Half past every hour
0 */2 * * *    # Every 2 hours
0 */6 * * *    # Every 6 hours
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Daily 

0 0 * * *      # Midnight
0 9 * * *      # 9:00 AM
0 9 * * 1-5    # 9:00 AM, weekdays only
30 2 * * *     # 2:30 AM (good for backups)
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Weekly 

0 0 * * 0      # Midnight Sunday
0 0 * * 1      # Midnight Monday
0 9 * * 1      # 9 AM Monday (weekly report)
0 17 * * 5     # 5 PM Friday (weekly digest)
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Monthly 

0 0 1 * *      # Midnight, 1st of month
0 9 1 * *      # 9 AM, 1st of month
0 0 15 * *     # Midnight, 15th of month
0 0 1 */3 *    # Quarterly (Jan, Apr, Jul, Oct)
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Yearly 

0 0 1 1 *      # Midnight, January 1st
0 9 1 1 *      # 9 AM, January 1st
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Common Mistakes

❌ * * * * * doesn't mean "once a day"

It means every single minute. That's 1,440 times per day.

❌ Day of week starts at 0 (Sunday)

* * * * 1 = Monday, not Tuesday. Both 0 and 7 mean Sunday.

❌ No second-level precision

Standard cron doesn't support seconds. If you need sub-minute scheduling, use a different tool.

❌ Month and DOW are OR, not AND

0 0 15 * 5 runs on the 15th AND every Friday, not only on Fridays that fall on the 15th.

Validate Before Deploying

Never push a cron expression without checking it first:

# Describe what a cron expression does
curl "https://cronping.anethoth.com/api/v1/cron/describe?expr=*/15+*+*+*+*"

# See the next 5 run times
curl "https://cronping.anethoth.com/api/v1/cron/next?expr=0+9+*+*+1&count=5"

# Validate syntax
curl "https://cronping.anethoth.com/api/v1/cron/validate?expr=0+0+31+2+*"
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These are free, no signup required. CronPing Cron API docs

Pro Tips

  1. Add randomness for backups: Use RANDOM=$((RANDOM % 60)) sleep to avoid thundering herd
  2. Log everything: 0 * * * * /path/script.sh >> /var/log/cron.log 2>&1
  3. Use flock to prevent overlap: * * * * * flock -n /tmp/job.lock /path/script.sh
  4. Monitor your crons: A cron that fails silently is worse than no cron at all. Use a dead man's switch to alert when jobs don't run.
  5. Test in a non-production environment first: crontab -l to review, crontab -e to edit

Bookmark this. Star it. Whatever helps you find it when you need it at 3 AM.

What cron schedule do you use most often? Drop it in the comments 👇

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