Unix - How Cron Jobs Work Internally in UNIX Systems

Cron is a UNIX utility used for scheduling tasks to run automatically at specified times. It is widely used for system maintenance, backups, and automation tasks.

The cron system works through a background daemon called crond. This daemon continuously runs and checks scheduled tasks stored in crontab files. It wakes up every minute to compare the current time with job schedules.

A crontab file contains time fields followed by the command to be executed. These fields specify minute, hour, day of month, month, and day of week. If all fields match the current time, the command is executed.

Cron supports both user crontabs and system crontabs. User crontabs run with the user’s permissions, while system crontabs allow specifying the user under which the command runs.

When executing a job, cron creates a non-interactive shell environment. This means environment variables are minimal, and absolute paths should be used to avoid execution failures.

Cron logs job execution details, which helps in debugging failures. Output can be mailed to users or redirected to files for monitoring.

Cron is a critical UNIX component for automation, reducing manual intervention and ensuring periodic tasks run reliably.