Understanding Linux Runlevels

In Linux, a runlevel is a mode of operation that defines what services and processes are running on the system. Each runlevel is assigned a specific purpose, such as system shutdown, single-user mode, or multi-user mode.

Common Runlevels

0: Halt the system (shutdown).
1: Single-user mode (maintenance mode).
2: Multi-user mode without networking.
3: Multi-user mode with networking (text-based).
4: Undefined (custom use).
5: Multi-user mode with networking and graphical interface (GUI).
6: Reboot the system.

How to Change Runlevels

To change the runlevel in Linux, you can use the telinit or systemctl command, depending on your system's init system.

Using `telinit` Command

For systems using SysVinit, you can change the runlevel with the telinit command:

sudo telinit

Example: To switch to runlevel 3 (multi-user mode with networking):

sudo telinit 3

Using `systemctl` Command

For systems using systemd, you can change the runlevel by switching the target:

sudo systemctl isolate .target

Example: To switch to graphical mode (runlevel 5 equivalent):

sudo systemctl isolate graphical.target

To switch to multi-user mode (runlevel 3 equivalent):

sudo systemctl isolate multi-user.target

To switch to single-user mode (runlevel 1 equivalent):

sudo systemctl isolate rescue.target

Checking the Current Runlevel

To check the current runlevel, use the following command:

runlevel
or
systemctl get-default
or
who -r

The output will show the previous and current runlevels.