How Does Cron Schedule Jobs Internally?

Startup Process

When a cron daemon starts, it reads the crontab files. These files have the scheduled jobs for users on the system. The daemon scans each user's crontab, collecting information about the tasks to run and their execution times.

After reading the crontab files, cron does a job analysis. It examines each job entry, interpreting the time and date fields to determine when the job should run next. This analysis helps cron create a picture of all the tasks it needs to manage.

Job Queue Management

Based on the analysis, cron creates an event list. This list has all the jobs that need to run, along with their next scheduled run times. The event list acts as a central store for all the tasks cron needs to manage.

To make job execution more efficient, cron sorts the jobs in the event list by their execution time. This sorting puts the jobs that need to run soonest at the front of the queue. This organization allows cron to quickly identify which job it needs to run next without searching through the entire list each time.

Time-based Execution

Cron uses sleep intervals between jobs to manage its workload. After sorting the job queue, cron calculates the time until the next job needs to run. It then enters a sleep state for that duration, saving system resources until it's time to execute the next task.

When the sleep interval ends, cron wakes up to execute the job. It checks the current time to ensure accuracy, then runs the task in the background using the privileges of the user who created it. After executing the job, cron recalculates its next run time and places it back in the right position in the event list.

This process of sleeping, waking up, executing jobs, and re-sorting the queue continues in a loop, allowing cron to manage and execute scheduled tasks over time.

Parsing Crontab Entries

Cron's job scheduling algorithm starts by parsing crontab entries. It reads the time and date fields in each entry, which have five fields: minute, hour, day of the month, month, and day of the week. These fields can have specific values, ranges, or special characters like asterisks for "any value."

To calculate the next run times, cron looks at the current time and compares it with the schedule in the crontab entry. It finds the nearest future time that matches all the fields in the entry. This happens for each job in the crontab, helping cron keep an updated schedule of upcoming tasks.

Main Execution Loop

The core of cron's scheduling algorithm is its main execution loop. This loop manages and runs jobs based on their scheduled times.

Cron first looks at the next job in the queue. It checks the job at the front of the sorted event list, which is the task with the nearest execution time.

After finding the next job, cron calculates the time until the job should run. It then sleeps for this duration. This sleep period helps cron save system resources while waiting for the next job's execution time.

When cron wakes up, it runs the job that's due. Cron runs these jobs in the background, allowing it to continue its main loop without waiting for the job to finish. The job runs with the permissions of the user who scheduled it, maintaining system security.

After running a job, cron recalculates its next run time based on its crontab entry and puts it back into the event list in the right position. This ensures that recurring jobs continue to run on schedule.

This main execution loop - checking the queue, sleeping, running jobs, and updating the schedule - continues indefinitely, allowing cron to manage and run scheduled tasks over time.