Cron

The scheduler that runs commands at fixed times.

Cron runs commands on a schedule defined by five fields: minute, hour, day of month, month, day of week: followed by the command. It wakes every minute and runs whatever matches.

Cron jobs run in a minimal environment: a short PATH, no interactive shell config, and the user's home as working directory. Absolute paths and explicit output capture (>> log 2>&1) are what separate cron jobs that work from cron jobs that fail silently.

See it in practice

$ crontab -l && grep CRON /var/log/syslog | tail -5

What is scheduled, and evidence of what actually ran. The second half is where silent failures surface.

Where you'll meet it

crontab

Run commands on a schedule, and read the five-field syntax fluently.

Environment variable

A named value processes inherit when they start.

Daemon

A background process providing a service, usually started at boot.

Learn these concepts by typing them: the interactive course builds them up lesson by lesson in a real simulated shell.