cronjobs

Cron jobs are scheduled tasks in Unix-like operating systems managed by the cron daemon. They enable automatic execution of commands or scripts at specific times or intervals. Cron has a long history in Unix-like systems and remains widely used for routine maintenance and batch processing.

Cron jobs are defined in crontab files, which can be per-user or system-wide. A crontab line contains a time-and-date specification followed by a command. Users edit with crontab -e; crontab -l lists jobs; crontab -r removes.

The common syntax consists of five time fields: minute (0-59), hour (0-23), day of month (1-31), month (1-12), day of week (0-6, 0 or 7 Sunday). The line ends with the command to run. Special strings like @hourly, @daily, @weekly, @monthly, and @yearly can replace the five fields. Example: "0 2 * * * /usr/local/bin/backup.sh" runs daily at 02:00. Another: "30 6 * * 1-5 /usr/bin/cleanup.sh" runs at 06:30 on weekdays.

Environments are typically minimal; commands should use full paths, and scripts should set necessary environment variables. Output can be mailed or redirected to log files. Crons can be affected by system time zone and daylight saving changes.

In modern contexts, Kubernetes provides a similar concept called CronJob, which schedules Jobs in a cluster with a cron-like schedule. A CronJob defines a schedule and a job template; options include concurrencyPolicy, startingDeadlineSeconds, suspend, and history limits.

Best practices include keeping tasks idempotent, avoiding overlapping runs, monitoring execution, enabling logging, and restricting permissions.