Chapter 11: Automation and Scheduling

Learn to automate tasks by scheduling scripts with cron and at, making your scripts run automatically at specified intervals.

In this chapter, we’ll explore methods for automating tasks in Bash using scheduling tools like cron and at. These tools allow you to set scripts to run at specific times or intervals, reducing the need for manual execution and ensuring tasks are performed on schedule.

Introduction to Cron Jobs

The cron daemon is a built-in service that schedules and executes scripts or commands at specified intervals. cron jobs are defined in a crontab file and can run hourly, daily, weekly, or even by the minute.

To edit your personal crontab, use the following command:

crontab -e

Cron Syntax and Format

Each line in a crontab file represents a cron job, with the following format:

minute hour day month day_of_week command

Each field specifies when the command should run:

minute: 0–59
hour: 0–23
day: 1–31
month: 1–12
day_of_week: 0–7 (0 and 7 represent Sunday)

Use * in any field to represent "every" (e.g., * for every minute, hour, etc.).

Example Cron Jobs

# Run a script every day at 2:30 AM
30 2 * * * /path/to/script.sh

# Run a script every Monday at 5:00 PM
0 17 * * 1 /path/to/script.sh

# Run a script every 15 minutes
*/15 * * * * /path/to/script.sh

These examples show how to configure cron jobs to run at specific times or intervals, such as daily, weekly, or every 15 minutes.

Using `at` for One-Time Scheduling

The at command schedules a one-time task for a specified time in the future. To use at, enter:

echo "/path/to/script.sh" | at 3:00 PM

This command schedules script.sh to run at 3:00 PM. at provides flexibility for scheduling individual tasks without recurring them.

Viewing and Managing Scheduled Jobs

To view and manage scheduled jobs, use the following commands:

crontab -l: Lists all scheduled cron jobs for the current user.
atq: Lists all scheduled at jobs.
atrm job_id: Removes a specific at job by its ID.

Example: Automating a Backup Script

Suppose you have a backup script, backup.sh, that you want to run daily at midnight. You could schedule it with cron as follows:

# Open your crontab for editing
crontab -e

# Add the following line to schedule the backup script
0 0 * * * /path/to/backup.sh

This crontab entry runs backup.sh every day at midnight, automating the backup process without manual intervention.

Summary and Next Steps

In this chapter, we explored task automation with cron and at for scheduling scripts. These tools enable you to automate routine tasks efficiently. In the next chapter, we’ll look at advanced scripting techniques, including optimizing and refining your scripts for even greater functionality.