crontab

Crontab refers to the tables used by the cron daemon to schedule automated commands on Unix-like systems. Each user can have a personal crontab, and a system-wide configuration may reside in /etc/crontab and in files under /etc/cron.d. The cron daemon (often named crond) reads these schedules and executes the specified commands at the requested times.

Each line in a user crontab defines a job with five time fields followed by a command: minute, hour, day of month, month, and day of week. Values can be numbers or ranges, with * meaning every value. Special strings like @hourly, @daily, @weekly, @monthly, @yearly, and @reboot provide shorthand scheduling. The line ends with the command to run. Comments can be added by starting a line with a #.

Administrative commands include crontab -e to edit the current user’s crontab, crontab -l to list entries, and crontab -r to remove them. The system-wide /etc/crontab requires an extra user field to specify which user should run each job. The cron service must be running for scheduled tasks to execute.

Environment in cron is typically limited; PATH and other variables may be minimal. It is common practice to use absolute paths and to redirect output to log files or mail. A MAILTO variable can direct job email notifications to a user account, depending on system configuration.

Examples: 0 2 * * * /usr/local/bin/backup.sh runs daily at 2 a.m.; */15 * * * * /usr/bin/cleanup --all can run every 15 minutes and may be redirected to a log file. Troubleshooting can involve checking the cron daemon logs, ensuring the script has executable permissions, and confirming the schedule matches the system clock.

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