Cron is the standard Linux scheduler for running commands at set intervals. This guide covers crontab syntax, common schedules, best practices for reliable cron jobs, and debugging when they fail.
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Each cron job line has 5 time fields followed by the command: minute hour day-of-month month day-of-week command. Fields: minute (0-59), hour (0-23), day (1-31), month (1-12), weekday (0-7, 0 and 7 are Sunday). Use * for "every". Examples: 0 2 * * * = 2am every day. */5 * * * * = every 5 minutes. 0 9 * * 1 = 9am every Monday. 0 0 1 * * = midnight on the first of every month.
Edit your crontab: crontab -e (opens in your default editor). List current jobs: crontab -l. Remove all jobs: crontab -r (destructive — be careful). Edit another user's crontab: sudo crontab -u ubuntu -e. System-wide cron jobs live in /etc/cron.d/ (files), /etc/cron.daily/, /etc/cron.hourly/, /etc/cron.weekly/, /etc/cron.monthly/ (directories — drop scripts in, cron runs them at the appropriate interval).
By default, cron emails output to the local system mailbox which nobody reads. Redirect output: 0 2 * * * /home/ubuntu/backup.sh >> /home/ubuntu/cron.log 2>&1. The >> appends to the log, 2>&1 captures both stdout and stderr. For quiet jobs: command > /dev/null 2>&1 (discard all output). Use logrotate to rotate cron logs: create /etc/logrotate.d/myapp with: /home/ubuntu/cron.log { rotate 7 daily compress missingok notifempty }.
Cron runs with a minimal environment — $PATH does not include /usr/local/bin where npm, python3, and other tools usually live. Always use absolute paths: /usr/bin/python3 instead of python3, /usr/local/bin/node instead of node. Find absolute paths with: which python3, which node. Alternatively, source your bash profile at the start of your cron script: #!/bin/bash — source /home/ubuntu/.bashrc.
If a cron job is not running: check cron is active: systemctl status cron. Check the system log for cron errors: grep CRON /var/log/syslog. Run the command manually as the cron user to check for errors. Verify file permissions: the script must be executable (chmod +x). Check if the script path is correct. Test your crontab syntax at crontab.guru. Add set -x to your script to enable debug output.
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