Which Runlevel Number Is Defined As User Definable

Which Runlevel Number Is Defined As User Definable

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In Linux-based operating systems, runlevels are predefined states that determine the system’s behavior and services that are running. Each runlevel has a specific purpose, such as single-user mode, multi-user mode, or graphical interface mode. However, some runlevels are user-definable, allowing administrators to configure custom behaviors.

This topic explores which runlevel number is user-definable, how runlevels work, and how they can be modified to suit specific needs.

Understanding Runlevels in Linux

A runlevel is a mode of operation in a Unix-based system that defines which services and processes are active. The concept of runlevels originated from the System V (SysV) init system, which was widely used before the adoption of systemd in modern Linux distributions.

Standard Runlevels in SysV Init

The traditional runlevels in Linux are numbered from 0 to 6, each with a predefined function:

  • Runlevel 0 – Halt (shuts down the system)
  • Runlevel 1 – Single-user mode (maintenance mode)
  • Runlevel 2 – Multi-user mode (without networking)
  • Runlevel 3 – Full multi-user mode (with networking, but no graphical interface)
  • Runlevel 4 – User-definable runlevel
  • Runlevel 5 – Multi-user mode with graphical interface
  • Runlevel 6 – Reboot (restarts the system)

Among these, runlevel 4 is the user-definable runlevel. Unlike other runlevels with fixed purposes, runlevel 4 does not have a predefined function, making it a flexible option for system administrators to configure as needed.

Why Is Runlevel 4 User-Definable?

The reason runlevel 4 is designated as user-definable is that Linux distributions generally do not assign it a specific function. While runlevels 0, 1, 2, 3, 5, and 6 serve specific roles, runlevel 4 is left open for customization.

Use Cases for Runlevel 4

Since runlevel 4 is customizable, system administrators can configure it for various purposes, such as:

  • Custom Maintenance Mode – A modified version of single-user mode with additional services.
  • Alternative Multi-User Mode – A specialized configuration for a server environment.
  • Testing and Debugging – A separate runlevel for troubleshooting system issues.
  • Secure Environment – A restricted mode for running only essential services.

How to Configure Runlevel 4

Checking the Current Runlevel

To check which runlevel the system is currently running on, use the following command:

runlevel

This will output two numbers. The first number represents the previous runlevel, while the second indicates the current runlevel.

Changing to Runlevel 4

To manually switch to runlevel 4, use the command:

init 4

Or, on some systems:

telinit 4

This will transition the system to runlevel 4, provided that it has been configured.

Customizing Runlevel 4

To modify runlevel 4, follow these steps:

  1. Create or Modify the Init Script
    • The configuration for each runlevel is stored in /etc/inittab in SysV init systems.
    • Edit the file using a text editor:
    sudo nano /etc/inittab
    • Locate the section for runlevel 4 and modify the associated script.
  2. Set Up Startup Scripts
    • Runlevels are managed by scripts stored in /etc/rc.d/ or /etc/init.d/.
    • The directory /etc/rc.d/rc4.d/ contains scripts for runlevel 4.
    • You can add, remove, or modify scripts in this directory to define custom services.
  3. Enable Custom Services
    • Use the chkconfig or update-rc.d command to enable or disable services in runlevel 4.
    • Example:
    sudo chkconfig --level 4 custom-service on
    • This ensures the service starts automatically when runlevel 4 is activated.

Runlevels in Modern Linux Systems (systemd)

With the adoption of systemd, traditional runlevels have been replaced by target units. The equivalent of runlevel 4 in systemd is the multi-user.target or a custom-defined target.

Checking the Current Target

To check the current target in a systemd-based system, use:

systemctl get-default

Changing to a Custom Target

To switch to a custom-defined target (equivalent to runlevel 4), use:

systemctl isolate custom.target

Creating a Custom Target

To define a custom target, create a .target file in /etc/systemd/system/. For example:

sudo nano /etc/systemd/system/mycustom.target

Then, add the following content:

[Unit]Description=My Custom TargetRequires=multi-user.targetAllowIsolate=yes

Save the file and reload systemd:

sudo systemctl daemon-reload

To set it as the default boot target:

sudo systemctl set-default mycustom.target

The user-definable runlevel in Linux systems is runlevel 4, allowing system administrators to configure a custom environment based on their needs. Whether used for specialized server configurations, security measures, or testing, runlevel 4 provides flexibility in traditional SysV init systems.

In modern Linux distributions using systemd, the concept of runlevels has evolved into targets, but the principle remains the same-users can create custom boot configurations to control system behavior.

Understanding runlevels and targets is crucial for effective system administration, ensuring that Linux systems are optimized for specific use cases.