Part 6: Cron and Crontab in Linux

In this section, we will dive into the concepts of cron and crontab, two essential tools for scheduling tasks in Linux. We’ll explain their purpose, how to use them, and provide practical examples for automation.

What is `cron`?

cron is a time-based job scheduler in Unix-like operating systems. It allows users to schedule tasks (commands or scripts) to run automatically at specified intervals, such as daily, weekly, or monthly.

What is `crontab`?

crontab (short for "cron table") is a configuration file used by the cron daemon to define the schedule for running tasks. Each user on the system can have their own crontab file.

How to Use `crontab`

The crontab command is used to manage the crontab file. Commonly used options include:

crontab -e: Edit the current user’s crontab file.
crontab -l: List all active crontab entries for the current user.
crontab -r: Remove the current user’s crontab file.

Crontab Syntax

A crontab entry follows this format:

minute hour day month weekday command

Where:

minute: 0-59
hour: 0-23
day: 1-31
month: 1-12
weekday: 0-6 (Sunday = 0 or 7)
command: The script or command to execute

Special Syntax

*: Matches every value (e.g., every minute, hour, etc.)
,: Separates multiple values (e.g., 1,5,10 means at 1, 5, and 10)
-: Specifies a range (e.g., 1-5 means from 1 to 5)
/: Specifies step values (e.g., */5 means every 5 units)

Examples of Crontab Usage

Example 1: Run a Script Every Day at Midnight

0 0 * * * /path/to/script.sh

This runs script.sh at 12:00 AM every day.

Example 2: Backup Files Every Sunday at 2:30 AM

30 2 * * 0 /path/to/backup.sh

This runs backup.sh at 2:30 AM on Sundays.

Example 3: Clear Temporary Files Every Hour

0 * * * * rm -rf /tmp/*

This clears the /tmp directory at the beginning of every hour.

Example 4: Run a Command on Specific Days

0 6 1,15 * * /path/to/report.sh

This runs report.sh at 6:00 AM on the 1st and 15th of each month.

How to Show Active Crontabs

To view all active crontab entries for the current user, use:

crontab -l

Output:

# Example crontab entries
0 0 * * * /path/to/script.sh
30 2 * * 0 /path/to/backup.sh

How to Edit Crontabs

To edit the current user’s crontab file, use:

crontab -e

This opens the crontab file in the default text editor. After making changes, save and exit to update the scheduled tasks.

Switching the Default Editor

If you prefer a specific text editor, you can set it before editing crontabs:

export EDITOR=vim
crontab -e

Removing Crontab Entries

To remove all crontab entries for the current user, use:

crontab -r

Use with caution, as this deletes all scheduled tasks.

Practical Examples of Automation with Cron

Example: Monitoring Disk Space

Add an entry to check disk space and email a warning if usage exceeds 90%:

*/10 * * * * /path/to/disk_space_monitor.sh

This runs disk_space_monitor.sh every 10 minutes.

Example: Syncing Files with a Remote Server

Schedule a task to sync files with an external server every day:

0 3 * * * rsync -avz /local/dir/ user@remote:/backup/dir/

Example: Restarting a Service Automatically

Restart a web server every day at 4:00 AM:

0 4 * * * systemctl restart apache2

Creating a Shell Script to Display Active Crontabs

Script: Displaying Active Crontabs with Descriptions

#!/bin/bash

# Display header
echo "Active crontabs for $(whoami):"

# List crontabs
crontab -l | awk '{print NR ".", $0}'

Summary

Cron: Schedules tasks.
Crontab: Manages the schedule.
Syntax: Minute, hour, day, month, weekday, and command.
Commands: crontab -e, crontab -l, crontab -r.
Use cases include backups, monitoring, and automation.

Cron and crontab are essential for automating repetitive tasks in Linux. Try creating your own schedules to gain practical experience!