Cron Expression Generator

TL;DR (Quick Cron Answers)

• Cron expression (Unix): minute hour day-of-month month day-of-week → 5 fields.
• Quartz/Spring variants: often 6 or 7 fields (adds seconds, sometimes year) and special wildcard **? ** for no specific value in DOM/DOW.
• Kubernetes CronJob: spec.schedule uses standard cron format; modern clusters support spec.timeZone for local time-based schedules.
• Best practice: Always test & monitor your cron jobs; log outputs and set alerts for failures.

What is Cron?

Cron is a time-based job scheduler in Unix-like systems. It runs commands at specified intervals using cron expressions. Each line in your crontab declares when and what to run.

Where cron runs:

• System crontab: /etc/crontab (has a user field)
• User crontab: crontab -e per user

Predefined strings like @hourly, @daily, @weekly, @monthly, @reboot are shortcuts you can use instead of writing full expressions.

Cron Expression Syntax (Unix / POSIX-style)

Format (5 fields):

┌───────────── minute (0–59)
│ ┌─────────── hour (0–23)
│ │ ┌───────── day of month (1–31)
│ │ │ ┌─────── month (1–12 or JAN–DEC)
│ │ │ │ ┌───── day of week (0–6 or SUN–SAT; 0 or 7 = Sunday on many systems)
│ │ │ │ │
│ │ │ │ │
* * * * *

Wildcards & Operators

• * any value
• , list → 1,2,5
• - range → 1-5
• / step → */10
• L, W, # are non-standard extensions (commonly supported by Quartz, not by classic Unix cron).

Examples

• Every 15 minutes: */15 * * * *
• Daily at 02:00: 0 2 * * *
• Weekdays at 18:30: 30 18 * * 1-5
• First day of month at 07:00: 0 7 1 * *

Portability tip: Avoid relying on non-standard modifiers unless your scheduler documents them. Classic Unix cron won't accept ?, L, W, or #.

Quartz Cron / Spring Cron vs Unix Cron (Know the Differences)

Many application schedulers (e.g., Quartz, Spring) adopt extended cron syntax:

• Fields: seconds minutes hours day-of-month month day-of-week [year] → 6 or 7 fields
• Special ?: used in day-of-month or day-of-week to mean no specific value
• Extended modifiers like L (last), W (weekday), # (nth weekday) are commonly supported.

Quick mapping

• Unix: 0 2 * * * → daily 02:00
• Quartz/Spring (with seconds): 0 0 2 * * ? → daily 02:00 (no specific DOW)

Common pitfalls

• Copying a 5-field expression from Unix into Spring/Quartz won’t work. Add seconds (and sometimes year) or use a helper to convert.
• ? is invalid in Unix cron but required in some Quartz scenarios.

Kubernetes CronJobs (k8s)

• Define schedules via spec.schedule (standard cron format).
• Modern Kubernetes versions support spec.timeZone, so you can pin a CronJob to a specific IANA timezone (e.g., "Europe/Zurich"). Without it, schedules are evaluated in the controller’s local time (often UTC).

Example

apiVersion: batch/v1
kind: CronJob
metadata:
  name: db-backup
spec:
  schedule: '0 2 * * *' # 02:00 daily
  timeZone: 'Europe/Zurich' # ensure local-time scheduling (supported on modern clusters)
  successfulJobsHistoryLimit: 3
  failedJobsHistoryLimit: 1
  jobTemplate:
    spec:
      template:
        spec:
          restartPolicy: OnFailure
          containers:
            - name: backup
              image: alpine:3.20
              command: [ 'sh', '-lc', 'pg_dump $DB_URL > /backups/$(date +%F).sql' ]

Cron Expression Generator / Decoder (Inline)

Use the quick reference below to build or decode schedules. Copy/paste into your crontab or k8s CronJob.

Quick Patterns

Decode Cheatsheet

• Any */N → run every N units of that field.
• Both DOM and DOW set → behaviour differs by implementation; stick to one (or use ? in Quartz).

Crontab Basics

• Edit: crontab -e
• List: crontab -l
• Environment: set PATH, SHELL, MAILTO, and variables explicitly—cron runs with a minimal environment.
• Output: capture with >> /var/log/myjob.log 2>&1 or rely on system mail.

Cron Debugging checklist

1. Is the command executable when run as the same user?
2. Are absolute paths used for binaries and files?
3. Is the environment (PATH, LANG, TZ) set?
4. Is the expression valid for your scheduler flavor?
5. Are failures monitored/alerted?

Monitoring & Reliability

• Pipe outputs to logs and alert on exit codes ≠ 0.
• Consider dedicated monitors (health pings, heartbeats) that alert when scheduled runs are missing.
• In Kubernetes, watch Job statuses, set backoffLimit, and retain some history.

Cron FAQs

Cron Checklist

1. Identify your scheduler flavor (Unix, systemd timer, Quartz/Spring, k8s).
2. Normalize expressions to that flavor (5 vs 6/7 fields, special symbols).
3. Pin time zones and DST behavior explicitly.
4. Add monitoring (heartbeat + failure alerts).
5. Document ownership, runbooks, and escalation paths.