crond

crond is the daemon that executes scheduled commands on Unix-like systems under the cron scheduling framework. It runs in the background, reads crontab files, and launches commands at their designated times. Cron supports both system-wide and per-user schedules: system-wide entries live in /etc/crontab and in files under /etc/cron.d, while per-user crontabs are edited with crontab -e and reside in the user’s account. Each crontab line specifies when to run a command and what to run. The five time fields indicate minute (0-59), hour (0-23), day of month (1-31), month (1-12 or names), and day of week (0-7, with 0 and 7 as Sunday); a line may use wildcards, ranges, lists, or step values. In system crontabs, a sixth field is the user account under which the command should run; per-user crontabs omit this field.

Special strings such as @reboot, @yearly (also @annually), @monthly, @weekly, @daily, and @hourly provide shorthand schedules. Cron typically runs with a minimal environment; scripts should use absolute paths, and output may be mailed to the owner or redirected via MAILTO.

Cron implementations vary; common ones include Vixie Cron, ISC Cron, BusyBox crond, cronie, and others. crond is started at boot time by init systems or systemd and is controlled as a background service. Security considerations include limiting rights of cron jobs, validating scripts, and monitoring cron logs.