Running things automatically.
WHAT cron IS
A service that runs commands on a schedule.
WHAT A CRONTAB IS
The list of scheduled commands, per user.
HOW TO EDIT IT
crontab with the edit option.
WHAT THE FIVE FIELDS ARE
Minute Hour Day of month Month Day of week
WHAT AN ASTERISK MEANS
Every value.
WHAT A SLASH MEANS
A step, such as every fifteen minutes.
WHAT THE COMMONEST MISTAKE IS
Assuming the environment matches your shell.
WHY IT MATTERS
Cron runs with a minimal environment, so commands that work when typed fail silently.
WHAT TO DO ABOUT IT
Use absolute paths for every command and file.
WHAT THE SECOND COMMONEST MISTAKE IS
Discarding all output.
WHY THAT HURTS
Failures become invisible.
WHAT TO DO INSTEAD
Send output to a log file, and check it.
WHAT TO ALWAYS TEST
The exact command, run manually, before scheduling it.
WHAT TO GUARD AGAINST
Overlapping runs, when a job takes longer than its interval.
WHAT PREVENTS IT
A lock file, or flock.
WHAT TO CHECK WHEN A CRON JOB DID NOT RUN
The cron log, and whether the crontab belongs to the right user.
