Question 1

What does "0 0 * * *" mean in cron?

Accepted Answer

This runs at midnight (00:00) every day. Breakdown: 0 (minute 0), 0 (hour 0/midnight), * (every day of month), * (every month), * (every day of week). Common variant: 0 0 * * 0 runs at midnight on Sundays only. The * wildcard means "every value" for that field. This is equivalent to the @daily shortcut.

Question 2

How do I schedule a job to run every 15 minutes?

Accepted Answer

Use "*/15 * * * *" - the */N syntax means "every N units". This runs at :00, :15, :30, :45 past every hour. Alternative: "0,15,30,45 * * * *" explicitly lists minutes. Both are equivalent, but */15 is more concise. For every N hours: "0 */N * * *". For every N days: "0 0 */N * *". The step value (/) only works with ranges or * wildcard.

Question 3

What is the difference between day of month and day of week?

Accepted Answer

Day of month (position 3) specifies date numbers 1-31. Day of week (position 5) specifies weekday names/numbers (0-7, where 0 and 7 are Sunday). Key difference: if BOTH are specified (not *), the cron triggers when EITHER condition is met (OR, not AND). Example: "0 9 1 * 1" runs at 9 AM on the 1st of every month OR every Monday - whichever comes first. Use * in one field to only use the other.

Question 4

Can I run a cron job every second?

Accepted Answer

Standard cron only supports minute granularity (minimum every 1 minute). For sub-minute schedules, use: systemd timers (supports seconds), Quartz scheduler (adds 6th field for seconds), AWS EventBridge (minimum 1 minute), or custom daemon scripts with sleep loops. If using Quartz/Spring cron: first field is seconds, so "*/5 * * * * *" runs every 5 seconds. Check your scheduler's documentation - standard Unix cron doesn't support seconds.

Question 5

Why isn't my cron job running?

Accepted Answer

Common issues: incorrect syntax (validate with parser), wrong time zone (cron uses server time, not local), permissions (user doesn't have rights to execute script), missing PATH (cron has limited environment - use absolute paths), output not captured (redirect to file to see errors), or cron daemon not running (check with "systemctl status cron"). Debug by: logging to file, using absolute paths, manually running the command, checking /var/log/syslog or /var/log/cron.

Question 6

What do @hourly, @daily, and @weekly shortcuts mean?

Accepted Answer

@hourly = "0 * * * *" (every hour at minute 0), @daily = "0 0 * * *" (midnight daily), @weekly = "0 0 * * 0" (midnight Sunday), @monthly = "0 0 1 * *" (midnight on 1st of month), @yearly/@annually = "0 0 1 1 *" (midnight Jan 1). These shortcuts improve readability but aren't supported by all cron implementations. Standard Unix cron and most modern schedulers support them. Use full expressions for maximum compatibility.

Question 7

How do I schedule a job for business days only (Monday-Friday)?

Accepted Answer

Use "0 9 * * 1-5" for 9 AM Monday through Friday. The 1-5 range in day-of-week field represents Mon-Fri (1=Monday, 5=Friday). For business hours every weekday: "0 9-17 * * 1-5" (every hour from 9 AM to 5 PM, Mon-Fri). To exclude holidays, cron alone isn't sufficient - use wrapper script that checks calendar and exits if holiday. More advanced: use external schedulers with holiday calendars (Airflow, Jenkins).

Question 8

Can I use names instead of numbers in cron expressions?

Accepted Answer

Yes, for months and weekdays. Months: JAN-DEC or 1-12. Weekdays: SUN-SAT or 0-7 (0 and 7 both represent Sunday). Case-insensitive. Examples: "0 0 * JAN MON" (midnight on Mondays in January), "0 12 1 JAN-MAR *" (noon on 1st day of Jan/Feb/Mar). Names are more readable than numbers. Three-letter abbreviations only - "JANUARY" won't work, use "JAN". Not all cron implementations support names, but most modern systems do.

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