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init command in Linux with examples

Last Updated : 15 Jul, 2025

The init process is the parent of all processes in Linux, identified by the process ID (PID) of 1. It is the first process that starts when a computer boots up and continues to run until the system shuts down. The term init stands for "initialization," and its primary role is to create and manage processes based on instructions from configuration scripts, specifically those stored in the '/etc/inittab' file. As the last step of the kernel boot sequence, 'init' is crucial for the initialization and overall functioning of the system.

The 'init' command initializes the system by executing scripts that set up the operating environment. These scripts, known as "init scripts" or "rc scripts" (short for run command scripts), are used in both Linux and UNIX systems to start and stop services and perform various other tasks during system initialization.

The configuration file '/etc/inittab' is used to control the behavior of the init process, defining which processes to start at specific runlevels and other system startup settings.

init [OPTIONS...] {COMMAND}

This syntax allows you to specify options and commands to control the system's initialization process, including changing runlevels, rebooting, or shutting down the system.

Run Levels is the state of init where a group of processes is defined to start at the startup of OS. The process spawned by init for each of this run levels are defined in the file /etc/inittab. Each runlevel has a certain number of services stopped or started. Conventionally seven runlevel exist numbers from zero to six.

Runlevel

Does not configure network interfaces, start daemons, 

or allow non-root logins

• Runlevel 0: Halt
  This runlevel stops all processes and shuts down the system. It is used when powering off the machine safely. 
• Runlevel 1: Single-User Mode
  In this mode, the system boots into a single-user environment, usually for maintenance tasks. Networking is disabled, and only the root user can log in.
• Runlevel 2: Multi-User Mode
  This runlevel supports multiple users but does not start networking services or daemons. It is often used for specific server setups that do not require network access.
• Runlevel 3: Multi-User Mode with Networking
  Runlevel 3 is a fully operational state, supporting multiple users with all network services and daemons running. This is a common default runlevel for servers.
• Runlevel 4: Undefined/User-Definable
  Runlevel 4 is generally unused by default but is available for custom user configurations. Administrators can define this runlevel to suit specific needs.
• Runlevel 5: X11 (Graphical Interface)
  Similar to runlevel 3, but with the addition of the graphical display manager (X Window System). This runlevel is typically used for desktop systems.
• Runlevel 6: Reboot
  Runlevel 6 restarts the system by rebooting. It is used when you need to restart all services or reinitialize the system completely.

The 'init' process is a fundamental component of the Linux operating system, responsible for initializing the system, managing services, and controlling the overall boot process. Through runlevels, it offers a structured way to manage system states, allowing administrators to customize which services run at various stages of the system's lifecycle. Understanding and utilizing 'init' effectively can significantly enhance system performance, security, and reliability, making it an indispensable tool for Linux system administration.

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