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arenasbob2024-cell

Posted on • Originally published at viadreams.cc

Cron Expressions: Complete Guide for DevOps and Backend Developers

#devops #linux #automation #beginners

Cron expressions are the universal language for scheduling tasks. Whether you're scheduling database backups, sending reports, or deploying code, you need cron. Here's your complete reference.

The 5-Field Format 

┌───────────── minute (0-59)
│ ┌───────────── hour (0-23)
│ │ ┌───────────── day of month (1-31)
│ │ │ ┌───────────── month (1-12)
│ │ │ │ ┌───────────── day of week (0-6, Sun=0)
* * * * *
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Patterns You'll Use Daily 

*/5 * * * *     Every 5 minutes
0 * * * *       Every hour (on the hour)
0 0 * * *       Daily at midnight
0 9 * * 1-5     Weekdays at 9 AM
0 0 1 * *       First of every month
0 0 * * 0       Every Sunday
30 2 * * *      Daily at 2:30 AM
0 */4 * * *     Every 4 hours
0 9,17 * * *    At 9 AM and 5 PM
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Special Characters

Character Meaning Example
* Any value * * * * * (every minute)
, List 1,15 * * * * (minute 1 and 15)
- Range 0 9-17 * * * (9 AM to 5 PM)
/ Step */10 * * * * (every 10 minutes)
L Last 0 0 L * * (last day of month)
W Weekday 0 0 15W * * (nearest weekday to 15th)
# Nth weekday 0 0 * * 5#2 (2nd Friday)

Platform Differences

Linux crontab 

# Edit crontab
crontab -e

# List jobs
crontab -l

# Example: backup daily at 2 AM
0 2 * * * /usr/local/bin/backup.sh >> /var/log/backup.log 2>&1
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GitHub Actions 

on:
  schedule:
    - cron: '0 9 * * 1-5'  # UTC timezone only!
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Kubernetes CronJob 

apiVersion: batch/v1
kind: CronJob
metadata:
  name: daily-report
spec:
  schedule: "0 9 * * *"
  jobTemplate:
    spec:
      template:
        spec:
          containers:
          - name: report
            image: report-generator:latest
          restartPolicy: OnFailure
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AWS EventBridge 

cron(0 9 ? * MON-FRI *)
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Note: AWS uses ? for "no specific value" and 6 fields (including year).

Common Mistakes

  1. Timezone confusion — Most cron systems use UTC. Always specify timezone: 
CRON_TZ=America/New_York
0 9 * * * /path/to/script.sh
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  1. Day-of-week numbering — Standard: 0=Sun, 6=Sat. Some systems: 1=Mon, 7=Sun.

  2. Overlapping runs — If job takes 10 minutes but runs every 5:

# Use flock to prevent overlapping
*/5 * * * * flock -n /tmp/myjob.lock /path/to/script.sh
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  1. Missing PATH — Cron has minimal environment: 
PATH=/usr/local/bin:/usr/bin:/bin
0 * * * * node /app/script.js
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Debugging Cron Jobs 

# Check cron logs
grep CRON /var/log/syslog

# Redirect output to a log file
0 * * * * /path/to/script.sh >> /var/log/myjob.log 2>&1

# Send output via email (if mail is configured)
MAILTO=admin@example.com
0 0 * * * /path/to/daily-report.sh
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Try It Online

Parse, validate, and understand cron expressions with DevToolBox's Cron Parser — human-readable descriptions, next 10 run times, visual schedule.

What's the most complex cron expression in your infrastructure? Share below!

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