CronExpression — cron expression builder

What is a cron expression?

A cron expression is a five-field text pattern that tells a cron scheduler when to run a job. The fields are minute, hour, day of month, month and day of week, separated by spaces and read left to right. For example, 0 8 * * 1-5 runs at 08:00 every Monday through Friday.

The cron daemon wakes once per minute and runs every job whose expression matches the current time, which puts a floor of one minute on standard cron's resolution. Quartz-style schedulers — used by Spring, and by many Java applications — prepend a sixth seconds field, so 0/30 * * * * ? fires every 30 seconds. Toggle + Seconds above to compose those.

Four special characters do most of the work: * matches every value, , separates a list, - defines a range, and / sets a step. Sunday is 0 in the day-of-week field, though most implementations also accept 7.

The rule that catches people out: when both day of month and day of week are restricted, standard cron applies OR logic. 0 0 13 * 5 runs on the 13th of every month and on every Friday — not only on Friday the 13th. Checking the computed run times above is the fastest way to catch this class of mistake before it reaches production.

Cron jobs also run with a minimal environment: your shell's PATH, aliases and exported variables are not loaded, which is the usual reason a command that works in a terminal fails under cron. Use absolute paths, and redirect output to a log file with >> /var/log/myjob.log 2>&1. The cron FAQ covers time zones, DST, overlapping runs and other edge cases in more depth.