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Explanation of Runlevels

In Linux systems, runlevels define the state or mode of the system, determining which services and processes are running. Each runlevel corresponds to a specific system configuration. Traditionally, Linux used SysVinit (System V initialization) for managing these runlevels.

Here’s a brief overview of the common runlevels:

Command Overview: SysVinit Runlevels

Runlevel SysVinit Command (init) Description
0 sudo init 0 Halt the system (shutdown).
1 sudo init 1 Single-user mode for system recovery or maintenance.
2 sudo init 2 Multi-user mode without a GUI (rarely used).
3 sudo init 3 Multi-user mode with networking (no GUI).
4 sudo init 4 Custom runlevel (rarely used).
5 sudo init 5 Multi-user mode with GUI (graphical interface).
6 sudo init 6 Reboot the system.

Command Overview for init

Action SysVinit Command (init)
Check the current runlevel runlevel
Switch to a specific runlevel sudo init <runlevel>
Reboot the system sudo init 6
Shutdown the system sudo init 0
Single-user mode for recovery sudo init 1
Multi-user mode with networking sudo init 3
Graphical mode (GUI) sudo init 5

Explanation:

  • sudo init <runlevel>: The command to change the system's runlevel. For example:

    • sudo init 3: Switches the system to multi-user mode with networking (runlevel 3).
    • sudo init 0: Shuts down the system (runlevel 0).
    • sudo init 6: Reboots the system (runlevel 6).

  • runlevel: This command shows the current and previous runlevel of the system.

Important Notes:

  • The init command works only on legacy systems that use the SysVinit system. Modern Linux systems generally use systemd, which replaces runlevels with targets and uses the systemctl command.