Crontab Next Runs

What Is Crontab Execution Time Calculation?

cron is the scheduled-task tool built into Linux that runs scripts or commands automatically at set intervals without manual intervention. This tool calculates the next trigger times from a standard five-field cron expression (minute hour day month weekday), so you can verify whether your crontab expression is correct. For an expression like 0 */6 * * *, just enter the first five fields to generate the execution time list.

crontab expression fields

A standard crontab expression has five fields: minute, hour, day, month, and weekday:

• Minute (0 - 59): first field, the minute of each hour to run
• Hour (0 - 23): second field, the hour of each day to run
• Day (1 - 31): third field, the day of each month to run
• Month (1 - 12): fourth field, the month of each year to run
• Weekday (0 - 7): fifth field, the day of each week to run (both 0 and 7 mean Sunday)

Each field supports the asterisk * (any value), a concrete value (e.g. 5), a list (e.g. 1,15), a range (e.g. 8-18), and a step (e.g. */10, 0-30/5). When both day-of-month and weekday are restricted, cron treats them as an OR (either one matching is enough).

crontab examples

• * * * * * — every minute
• */10 * * * * — every 10 minutes
• 0 * * * * — every hour on the hour
• 30 21 * * * — daily at 21:30
• 3,15 * * * * — at minute 3 and 15 of every hour
• 3,15 8-11 * * * — at minute 3 and 15 of hours 8 to 11
• 0 0 * * 0 — every Sunday at midnight
• 0 0 1 * * — on the 1st of every month at midnight

cron service commands

• service crond start — start the cron service
• service crond stop — stop the cron service
• service crond restart — restart the cron service
• service crond reload — reload the configuration
• crontab -l — list the current crontab
• crontab -e — edit the crontab
• crontab -r — remove the crontab

How to use

1. Enter a five-field cron expression and the number of future times to compute
2. Click "Calculate", or click one of the quick presets below
3. Review the next N execution times in your local time zone to check whether they match expectations