Cron Expression Generator

The /5 syntax means "every 5th unit." When used in the minute field (/5 * * * *), it runs the job every 5 minutes (at 0, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, and 55 minutes past the hour). The slash (/) is the step operator: */N means "every Nth value starting from 0." You can use it in any field: */2 in the hour field means every 2 hours, */3 in the month field means every 3 months.

Cron is the daemon (background service) that runs scheduled tasks on Unix/Linux systems. Crontab (cron table) is the file that contains the schedule entries. Each line is a cron expression followed by a command. You edit your crontab with 'crontab -e', list it with 'crontab -l', and remove it with 'crontab -r'. Each user has their own crontab, and there's also a system-wide crontab at /etc/crontab.

The standard Unix/Linux cron uses 5 fields: minute, hour, day of month, month, and day of week. However, some systems use 6 or 7 fields. Quartz Scheduler (common in Java) adds a seconds field at the beginning and an optional year field at the end. AWS EventBridge uses 6 fields with a year field. Spring Boot's @Scheduled uses 6 fields with seconds. This tool uses the standard 5-field format used by crontab on Linux and macOS.

By default, cron uses the system's local timezone, not UTC. On most servers, this is set in /etc/timezone or via the TZ environment variable. You can override the timezone for cron jobs by adding TZ=UTC at the top of your crontab, or per-job: TZ=America/New_York 0 9 * * * /path/to/script.sh. Cloud services like AWS and Azure typically run cron expressions in UTC unless configured otherwise.

Cron does not wait for the previous execution to finish. It starts a new instance on schedule regardless. This can lead to overlapping runs if your job takes longer than the interval. To prevent this, use a lock file (flock command): */5 * * * * flock -n /tmp/myjob.lock /path/to/script.sh. The -n flag makes flock exit immediately if the lock is held, so only one instance runs at a time.