Crontab - Complete Guide

Cron expression helper translates between human-readable schedules and cron syntax (the standard for scheduling tasks on Unix systems). Cron uses a compact 5-field format to specify when jobs run: minute, hour, day of month, month, and day of week.

How It Works

Each cron field accepts: specific values (5), ranges (1-5), lists (1,3,5), steps (/5), and wildcards (). The scheduler checks if current time matches all fields. Special strings like @daily and @hourly provide shortcuts. The helper parses expressions and shows upcoming execution times.

Technical Details

Standard cron: "minute hour day month weekday" (0-59, 0-23, 1-31, 1-12, 0-7). Extended format adds seconds and year fields. Day-of-week: 0/7=Sunday, 1=Monday. Month/day names allowed in some implementations. Timezone handling varies by system - use UTC when possible.

Common Use Cases

1. Scheduled Backups: Run database backups every night at 2 AM.
   • Example: 0 2 * * * /scripts/backup.sh (daily at 2:00 AM)
2. Report Generation: Generate weekly reports every Monday morning.
3. Cache Clearing: Clear application caches every hour.
4. Health Checks: Run system health checks every 5 minutes.
5. Data Sync: Synchronize data between systems at scheduled intervals.

Code Examples

Common Cron Examples

# Every minute
* * * * *

# Every hour at minute 0
0 * * * *

# Daily at midnight
0 0 * * *

# Every Monday at 9 AM
0 9 * * 1

# Every 15 minutes
*/15 * * * *

# First day of month at midnight
0 0 1 * *

# Weekdays at 6 PM
0 18 * * 1-5

# Every 6 hours
0 */6 * * *

# Twice daily (9 AM and 6 PM)
0 9,18 * * *

Format: minute (0-59) hour (0-23) day (1-31) month (1-12) weekday (0-7, 0/7=Sun)

Parse Cron in Node.js

// Using cron-parser library
import parser from 'cron-parser';

const interval = parser.parseExpression('0 9 * * 1-5');

// Get next execution times
console.log(interval.next().toString());
console.log(interval.next().toString());

// Using node-cron to schedule
import cron from 'node-cron';

cron.schedule('*/5 * * * *', () => {
  console.log('Running every 5 minutes');
});

// Validate cron expression
cron.validate('0 * * * *');  // true
cron.validate('invalid');    // false

cron-parser calculates execution times. node-cron schedules actual jobs in Node.js applications.

Tips & Best Practices

• Use crontab.guru to visualize and validate expressions
• Add logging to cron jobs - silent failures are hard to debug
• Use full paths in cron commands (cron has minimal PATH)
• Redirect output: command >> /var/log/job.log 2>&1
• Test expressions by running frequently first, then adjusting

Common Mistakes to Avoid

• Confusing minute vs hour field order (minute is first)
• Forgetting day-of-week is 0-6 or 0-7, not 1-7
• Not accounting for timezone differences
• Using */60 (invalid) instead of * or 0 for hour boundaries

When to Use This Tool

Use cron expressions for any scheduled task: backups, report generation, cache clearing, data synchronization, health checks, or any recurring job that needs to run at specific times.

Frequently Asked Questions

How do I run a job every 5 minutes?

*/5 * * * * - the */5 means "every 5th value" in the minutes field. This runs at :00, :05, :10, :15, etc. For every 5 hours: 0 */5 * * *

What's the difference between * and ?

In standard cron, * means "any value". Some systems (Quartz, AWS) use ? as "no specific value" for day-of-month or day-of-week when the other is specified, avoiding conflicts.

How do I schedule for specific days of the week?

The 5th field: 0=Sunday, 1=Monday, ..., 6=Saturday (7 also = Sunday in some systems). "0 9 * * 1-5" runs weekdays at 9 AM. "0 0 * * 0" runs Sundays at midnight.

Can I specify "last day of month"?

Standard cron doesn't support this directly. Workarounds: run daily and check date in script, use L in extended cron (Quartz), or use specific dates (28-31) with logic. Some CI/CD tools support custom syntax.

What timezone does cron use?

System cron uses the system timezone. For predictable scheduling across servers, set TZ in crontab (TZ=UTC) or use UTC for server time. Cloud schedulers often let you specify timezone.

Get Started

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Last updated: June 27, 2026