How to Change Run Level on AlmaLinux: A Comprehensive Guide

AlmaLinux has become a go-to Linux distribution for businesses and individuals seeking a community-driven, open-source operating system that closely follows the Red Hat Enterprise Linux (RHEL) model. For administrators, one of the key tasks when managing a Linux system involves understanding and manipulating run levels, also known as targets in systems using systemd.

This blog post will guide you through everything you need to know about run levels in AlmaLinux, why you might want to change them, and step-by-step instructions to achieve this efficiently.

Understanding Run Levels and Targets in AlmaLinux

In traditional Linux distributions using the SysVinit system, “run levels” were used to define the state of the machine. These states determined which services and processes were active. With the advent of systemd, run levels have been replaced by targets, which serve the same purpose but with more flexibility and modern features.

Common Run Levels (Targets) in AlmaLinux

Here’s a quick comparison between traditional run levels and systemd targets in AlmaLinux:

Other specialized targets also exist, such as emergency.target for minimal recovery and troubleshooting.

Why Change Run Levels?

Changing run levels might be necessary in various scenarios, including:

• System Maintenance: Access a minimal environment for repairs or recovery by switching to rescue.target or emergency.target.
• Performance Optimization: Disable the graphical interface on a server to save resources by switching to multi-user.target.
• Custom Configurations: Run specific applications or services only in certain targets for testing or production purposes.
• Debugging: Boot into a specific target to troubleshoot startup issues or problematic services.

How to Check the Current Run Level (Target)

Before changing the run level, it’s helpful to check the current target of your system. This can be done with the following commands:

1. Check Current Target:

   systemctl get-default
   

   This command returns the default target that the system boots into (e.g., graphical.target or multi-user.target).

2. Check Active Target:

   systemctl list-units --type=target
   

   This lists all active targets and gives you an overview of the system’s current state.

Changing the Run Level (Target) Temporarily

To change the current run level temporarily, you can switch to another target without affecting the system’s default configuration. This method is useful for tasks like one-time maintenance or debugging.

Steps to Change Run Level Temporarily

1. Use the systemctl command to switch to the desired target. For example:

   • To switch to multi-user.target:

     sudo systemctl isolate multi-user.target
     

   • To switch to graphical.target:

     sudo systemctl isolate graphical.target
     

2. Verify the active target:

   systemctl list-units --type=target
   

Key Points

• Temporary changes do not persist across reboots.
• If you encounter issues in the new target, you can switch back by running systemctl isolate with the previous target.

Changing the Run Level (Target) Permanently

To set a different default target that persists across reboots, follow these steps:

Steps to Change the Default Target

1. Set the New Default Target: Use the systemctl set-default command to change the default target. For example:

   • To set multi-user.target as the default:

     sudo systemctl set-default multi-user.target
     

   • To set graphical.target as the default:

     sudo systemctl set-default graphical.target
     

2. Verify the New Default Target: Confirm the change with:

   systemctl get-default
   

3. Reboot the System: Restart the system to ensure it boots into the new default target:

   sudo reboot
   

Booting into a Specific Run Level (Target) Once

If you want to boot into a specific target just for a single session, you can modify the boot parameters directly.

Using the GRUB Menu

1. Access the GRUB Menu: During system boot, press Esc or another key (depending on your system) to access the GRUB boot menu.

2. Edit the Boot Parameters:

   • Select the desired boot entry and press e to edit it.

   • Locate the line starting with linux or linux16.

   • Append the desired target to the end of the line. For example:

     systemd.unit=rescue.target
     

3. Boot Into the Target: Press Ctrl+X or F10 to boot with the modified parameters.

Key Points

• This change is only effective for the current boot session.
• The system reverts to its default target after rebooting.

Troubleshooting Run Level Changes

While changing run levels is straightforward, you might encounter issues. Here’s how to troubleshoot common problems:

1. System Fails to Boot into the Desired Target

• Ensure the target is correctly configured and not missing essential services.
• Boot into rescue.target or emergency.target to diagnose issues.

2. Graphical Interface Fails to Start

• Check the status of the gdm (GNOME Display Manager) or equivalent service:

  sudo systemctl status gdm
  

• Restart the service if needed:

  sudo systemctl restart gdm
  

3. Services Not Starting in the Target

• Use systemctl to inspect and enable the required services:

  sudo systemctl enable <service-name>
  sudo systemctl start <service-name>
  

Advanced: Creating Custom Targets

For specialized use cases, you can create custom targets tailored to your requirements.

Steps to Create a Custom Target

1. Create a New Target File:

   sudo cp /usr/lib/systemd/system/multi-user.target /etc/systemd/system/my-custom.target
   

2. Modify the Target Configuration: Edit the new target file to include or exclude specific services:

   sudo nano /etc/systemd/system/my-custom.target
   

3. Add Dependencies: Add or remove dependencies by creating .wants directories under /etc/systemd/system/my-custom.target.

4. Test the Custom Target: Switch to the new target temporarily using:

   sudo systemctl isolate my-custom.target
   

5. Set the Custom Target as Default:

   sudo systemctl set-default my-custom.target
   

Conclusion

Changing run levels (targets) in AlmaLinux is an essential skill for administrators, enabling fine-tuned control over system behavior. Whether you’re performing maintenance, optimizing performance, or debugging issues, the ability to switch between targets efficiently is invaluable.

By understanding the concepts and following the steps outlined in this guide, you can confidently manage run levels on AlmaLinux and customize the system to meet your specific needs. For advanced users, creating custom targets offers even greater flexibility, allowing AlmaLinux to adapt to a wide range of use cases.

Feel free to share your experiences or ask questions in the comments below. Happy administering!