Glossary

Cron

Cron is the Unix/Linux task scheduler that runs commands on a schedule. Configuration is stored in a crontab — a table of entries such as "every minute", "at 3:00 every Sunday", and so on.

Crontab syntax

# min hour day month weekday command
0 3 * * *  /usr/bin/php /app/backup.php   # every day at 3:00
*/15 * * * * curl https://api.example.com/ping  # every 15 min

Five fields: minutes (0–59), hours (0–23), day of month (1–31), month (1–12), day of week (0–7, where 0 and 7 are Sunday). * means "any value"; */N means "every N".

Pitfalls

Cron runs in a minimal environment — environment variables may be absent. Use full paths to commands
If a job takes longer than its interval, parallel instances will start. Use lock files or flock
Log output: command >> /var/log/job.log 2>&1