Schedule Commands and Tasks Using Cron

Automating tasks without manual intervention is essential for efficient system management. Whether it's regular backups, database optimization, or nightly data processing, scheduling scripts to run automatically is a common requirement. UNIX and Linux systems offer multiple tools for this purpose, and cron is a popular choice. In this post, we'll explore how to leverage cron to schedule commands and tasks effectively.

Cron: Scheduling Tasks Automatically

What is Cron?

Cron is a helpful tool that lets your computer run commands at specific times. Imagine setting a reminder on your phone, but for your computer. It's like having a reliable assistant that can do tasks for you without you needing to remember.

How Does Cron Work?

Cron looks at a special file called a "crontab" to see what tasks you want to schedule. You can think of a crontab as a to-do list for your computer. Cron checks this list regularly and runs the commands when the time is right.

Creating a Crontab

You don't need to edit the crontab file directly. Instead, use the crontab -e command. This will open the file in your default text editor, making it easier to manage.

Understanding Crontab Entries

A crontab entry looks like this:

* * * * * command to run

The five asterisks represent minutes, hours, day of the month, month, and day of the week. You can replace these with specific numbers or ranges to define when the command should run. For example:

• 0 1 * * * runs a command every day at 1:00 AM

• */15 * * * * runs a command every 15 minutes

Important Tips

• Testing: Try your commands in the terminal first to make sure they work correctly before adding them to cron.

• Logging: Cron can create a log file (usually found in /var/log/cron) to help you troubleshoot any issues.

• Permissions: Make sure the user running the cron job has the necessary permissions to execute the commands.

These are basic understanding you need to effectively use cron to automate tasks on your Linux or Unix system.

Understanding Crontab Entries: Your To-Do List for Cron

In this section we see crontab entries with more details.

Breaking down a Crontab Entry:

• The Schedule: The first five parts of a line define when the task should run. They look like numbers or asterisks (*) separated by spaces.

  • Think of the asterisks as wildcards. Use them for "any" in that specific spot (e.g., * * * * * runs a task every minute).

  • You can also use numbers (e.g., 10 * * * * runs a task at 10:00 AM every day).

  • There are ways to use ranges and intervals too! We'll cover those in a bit.

Here's a table to help you understand what each part means:

Example:

This line 45 10 * * 1-5 tells cron to run a task at 10:45 AM every Monday through Friday.

Tips for Crontab Entries:

• Don't use all asterisks (*) unless you want something to run every minute (mostly for testing).

• You can use ranges like 1-7 for days of the week (remember Sunday is 0).

• Separate multiple values with commas (e.g., 10,20,30 for the 10th, 20th, and 30th minute).

• There are ways to use "step values" to run things every other hour, etc. We'll explore those in more detail later.

The "Command" Part:

The last part of the line is the actual command you want to run. It can be anything you could type in the terminal.

Important:

• Make sure the command has the full path (e.g., /bin/date instead of just date) if it's not in your system's default path.

• If your command is a script, make sure it's executable (use chmod +x scriptname.sh).

We'll cover more advanced topics like environment variables and handling missed tasks in future discussions!

Managing Your Crontabs: Adding, Editing, and Removing

In this section we discuss managing crontabs with more details including:

• Adding a crontab:

  • crontab filename: This inserts the contents of filename as your crontab, replacing anything there before.

• Editing your crontab:

  • crontab -e: This opens a copy of your crontab in your default text editor for you to make changes.

• Viewing your crontab:

  • crontab -l: This simply shows you the contents of your current crontab.

• Removing your crontab:

  • crontab -r: This completely removes your crontab, leaving you with no tasks scheduled.

As the Administrator (Root):

If you're the administrator (root), you can also manage other users' crontabs:

• crontab -r username: This removes another user's crontab (e.g., crontab -r hamed removes hamed's crontab).

• crontab -e username: This opens another user's crontab for editing (e.g., crontab -e hamed edits hamed's crontab).

Important Note (Linux only):

Linux uses a slightly different command for editing another user's crontab with a separate file:

• crontab -u username filename: This lets you replace another user's crontab with the contents of a specific file (e.g., crontab -u jsmith crontab.new replaces John Smith's crontab with the contents of crontab.new).

Be Careful!

If you accidentally start editing your crontab directly in the terminal (without any arguments), don't press Ctrl+D to exit! This will erase your entire crontab. Use Ctrl+C instead to cancel safely.

Additional Tips:

• Spreading out tasks scheduled with cron across different times can help avoid overloading your system if multiple machines run the same task simultaneously.

• Cron logs its activities, which you can usually find in a separate system log file.

We'll explore more advanced cron topics like random delays and log management in the future!

System-Wide Cron Jobs

While you can schedule tasks for your own user account, there are also ways to set up tasks that run for the entire system. These tasks are usually handled by system administrators.

System-wide Crontab Files

There are two main places where system-wide cron jobs are typically stored:

1. /etc/crontab: This file contains cron jobs that can be run as any user. It's usually managed directly by system administrators.

2. /etc/cron.d: This directory holds additional crontab files. Each file often represents a specific program or service that needs to run scheduled tasks.

These system-wide crontab files work a bit differently than the ones you create for your own user. They include an extra part at the beginning to specify which user should run the command.

Special Cron Directories

Many Linux systems also have a set of directories for quick and easy scheduling of common tasks:

• /etc/cron.hourly: Files in this directory run once every hour.

• /etc/cron.daily: Files in this directory run once every day.

• /etc/cron.weekly: Files in this directory run once every week.

These directories are a convenient way to schedule simple tasks without needing to create full crontab entries.

Remember: Editing system-wide crontab files requires extra caution, as mistakes can affect the entire system. It's generally recommended to leave these files to system administrators unless you have specific instructions to do otherwise.

Who Can Use Cron?

To keep your system secure, you can control who is allowed to use cron to schedule tasks. This is done using special files called cron.allow and cron.deny.

The Cron Allow and Deny Files

• cron.allow: This file lists users who are specifically allowed to use cron. If this file exists, only users listed here can schedule tasks.

• cron.deny: This file lists users who are not allowed to use cron. If cron.allow doesn't exist, cron.deny is checked instead.

How it works:

• If both files are missing, usually anyone can use cron.

• If cron.allow exists, only users in that list can use cron.

• If cron.allow doesn't exist but cron.deny does, everyone except the users in cron.deny can use cron.

Important:

Even if you restrict who can use cron, it's still important to make sure only root can change the crontab files. This prevents unauthorized users from sneaking in tasks.

Typical Setup:

Most systems are set up to allow everyone to use cron by default. However, it's often a good idea to restrict this to specific users for security reasons.

Remember:

• These files are usually located in /etc/cron.{allow,deny} or /var/cron/{allow,deny} depending on your system.

• Editing these files requires administrator privileges.

• Always make sure the ownership and permissions of the crontab files are correct.