telinit

The telinit command is used to change the runlevel of a system in Unix-like operating systems that use the SysVinit init system. Runlevels are preset operating states that define what system services are available. Each runlevel has its own configuration and purpose.

Overview of Runlevels

• 0: Halt the system

• 1: Single-user mode (maintenance or emergency mode)

• 2: Multi-user mode without networking (varies by distribution)

• 3: Multi-user mode with networking (text mode)

• 4: Undefined (can be user-defined)

• 5: Multi-user mode with networking and graphical interface (default for many distributions)

• 6: Reboot the system

Usage of telinit

The telinit command is a symbolic link to the init command. It sends signals to the init process to change the system runlevel. When you use telinit, you are essentially instructing the init process to transition the system to a specified runlevel.

Basic Syntax

telinit [runlevel]

Example Commands

Changing to Runlevel 1 (Single-User Mode)

Single-user mode is typically used for maintenance purposes, where you need minimal services running:

sudo telinit 1

Changing to Runlevel 3 (Multi-User Mode without GUI)

Runlevel 3 is used for full multi-user mode with networking but without a graphical interface:

sudo telinit 3

Changing to Runlevel 5 (Multi-User Mode with GUI)

Runlevel 5 is used for full multi-user mode with networking and a graphical user interface:

sudo telinit 5

Rebooting the System (Runlevel 6)

To reboot the system:

sudo telinit 6

Halting the System (Runlevel 0)

To halt the system:

sudo telinit 0

Checking Current Runlevel

To check the current runlevel of the system, use the runlevel command:

runlevel

This command outputs two characters: the previous and the current runlevel. For example:

N 5

This means there was no previous runlevel (N) and the current runlevel is 5.

Transitioning from SysVinit to Systemd

In systems using systemd, the concept of runlevels is replaced by targets, which offer more flexibility. However, systemd maintains compatibility with traditional runlevels.

Equivalent Commands in systemd

To change the runlevel (target) in systemd, use the systemctl isolate command:

• Single-User Mode (rescue.target):

  sudo systemctl isolate rescue.target

• Multi-User Mode without GUI (multi-user.target):

  sudo systemctl isolate multi-user.target

• Multi-User Mode with GUI (graphical.target):

  sudo systemctl isolate graphical.target

• Reboot the System:

  sudo systemctl reboot

• Halt the System:

  sudo systemctl poweroff

Conclusion

The telinit command is an essential tool for managing the runlevel of a Unix-like system using SysVinit. Understanding how to change runlevels is crucial for system maintenance and administration. With the transition to systemd, similar functionalities are maintained through targets, providing a more modern and flexible approach to system state management.