Cron Expression Tester

While cron is ubiquitous, modern alternatives offer significant advantages:

• GitHub Actions: Scheduled workflows with version control
• Kubernetes CronJobs: Container-based with better isolation
• AWS EventBridge: Serverless with built-in retry logic
• Temporal/Airflow: Complex workflows with dependencies
• systemd timers: Better logging and dependency management

Use traditional cron only for:

• Simple system maintenance on single servers
• Legacy systems that can't be migrated
• Quick local development tasks

Most workflow tools support cron syntax, making migration easier!

Most cron failures are due to environment or path issues:

• PATH not set: Cron uses minimal PATH, commands may not be found
• Wrong permissions: Script must be executable (chmod +x)
• Relative paths: Always use absolute paths in scripts
• Missing shebang: Start scripts with #!/bin/bash
• Cron not running: Check with systemctl status cron

Debug tip: Redirect output to a log file: * * * * * /path/to/script.sh >> /tmp/cron.log 2>&1

Cron uses the system timezone by default, which causes issues:

• Jobs run at unexpected times after DST changes
• 2 AM jobs may run twice or not at all during DST transitions
• Different servers may run jobs at different UTC times

Best practices:

• Use UTC for all cron jobs to avoid DST issues
• Set system timezone to UTC on servers
• For Kubernetes: use spec.timeZone: "UTC"
• Restart cron after timezone changes

Monthly patterns:

• 1st of month: 0 0 1 * *
• 15th of month: 0 0 15 * *
• Last day: Complex, use script logic or multiple entries
• 1st and 15th: 0 0 1,15 * *

Quarterly patterns:

• First day of quarter: 0 0 1 1,4,7,10 *
• Last day needs multiple entries:
• 0 0 31 3,12 * (Mar 31, Dec 31)
• 0 0 30 6,9 * (Jun 30, Sep 30)

Both run every 5 minutes, but there's a subtle difference:

• */5 * * * * - Every 5 minutes starting from 0
• 0,5,10,15,20,25,30,35,40,45,50,55 * * * * - Same result, but explicit

Step values are cleaner for:

• Every 2 hours: 0 */2 * * *
• Every 3 days: 0 0 */3 * *
• Every 10 minutes: */10 * * * *

Note: */5 in hour field means hours 0,5,10,15,20, not "every 5 hours"!

When both day fields are set, cron runs on EITHER condition (OR logic):

• 0 0 13 * 5 runs on 13th AND on Fridays
• NOT just "Friday the 13th"
• This is often unexpected behavior

Solutions:

• Use only one day field when possible
• For "2nd Tuesday": Use date logic in your script
• Some systems support: 0 0 8-14 * 2 for 2nd Tuesday

Concurrent execution can corrupt data or waste resources:

• Use lock files: Create a PID file at start, remove at end
• flock command: flock -n /tmp/job.lock command
• systemd timers: Better handling of overlaps
• Job queues: Use Redis/RabbitMQ for complex workflows

Example with flock:

This skips execution if previous instance is still running.

Many cron implementations support special strings:

• @reboot - Run at startup
• @yearly or @annually - 0 0 1 1 *
• @monthly - 0 0 1 * *
• @weekly - 0 0 * * 0
• @daily or @midnight - 0 0 * * *
• @hourly - 0 * * * *

Note: Not all systems support these. Kubernetes CronJobs don't support @ syntax.