Question 1

What is the origin of the cron scheduling system?

Accepted Answer

Cron was created by Ken Thompson for Version 7 Unix in 1979. The name comes from the Greek word "chronos" meaning time. The original cron daemon woke up every minute to check a single system-wide crontab file. Modern implementations like Vixie cron (written by Paul Vixie in 1987) added per-user crontab files, environment variable support, and the five-field syntax that became the de facto standard used today.

Question 2

What do the five fields in a cron expression represent?

Accepted Answer

The five fields are, in order: minute (0-59), hour (0-23), day of month (1-31), month (1-12), and day of week (0-6, where 0 is Sunday). Each field accepts a single value, a range (1-5), a step (*/15), a comma-separated list (1,3,5), or an asterisk (*) meaning every possible value. Some extended implementations add a sixth field for seconds, but the standard five-field format covers most scheduling needs.

Question 3

How do step values and combined ranges work in cron?

Accepted Answer

Step values use the slash notation — for example, */15 in the minute field means every 15 minutes (0, 15, 30, 45). You can combine steps with ranges: 1-30/5 means every 5 minutes from minute 1 through 30 (1, 6, 11, 16, 21, 26). This is useful for staggering jobs — instead of flooding at minute 0, you can spread load across an interval. The parser expands these into the full set of matching values to calculate exact run times.

CRON Expression Parser

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