Crontabs

Crontabs, short for cron tables, are sets of scheduled tasks used by the cron daemon on Unix-like systems. Each user can have their own crontab, and there are system-wide schedules stored separately in files such as /etc/crontab and the /etc/cron.d directory. The crontab mechanism allows automatic execution of commands or scripts at specified times and intervals.

A crontab entry consists of a schedule and a command. The schedule has five fields: minute (0-59), hour (0-23), day of month (1-31), month (1-12 or names), and day of week (0-7, where both 0 and 7 represent Sunday). These fields are followed by the command to run. Wildcards are allowed, using asterisks, lists (comma separated), ranges (dash), and steps (slash). For example, “0 5 * * * /path/script.sh” runs every day at 05:00. There are also special strings such as @reboot, @yearly (or @annually), @monthly, @weekly, @daily, and @hourly, which replace the five-field syntax.

When both day of month and day of week are specified, cron triggers the command if either field matches, not only when both match. Per-user crontabs are edited with crontab -e, listed with crontab -l, or removed with crontab -r. System-wide entries reside in /etc/crontab and in files under /etc/cron.d. Environment within cron is minimal; it is advisable to use absolute paths, set necessary environment variables within the crontab, and redirect output (or mail it via MAILTO) to monitor success or failure. Logs are generally written to the system log, such as /var/log/syslog or /var/log/cron, depending on the OS.