Linux Init

What Is Init in Linux, and Why Does It Matter?

Imagine you’re turning on your Linux machine. It starts whirring to life, launching services and preparing everything for you to work, but have you ever wondered who’s pulling the strings? This is where “init” (short for initialisation) comes in, acting as the conductor behind the scenes that brings every component to life in harmony. Init is the very first process that your Linux system launches, and its job is to make sure that every other critical service follows suit, all in the right order.

In this article, you’ll uncover what init really does, from booting your system to managing processes, along with the unique features of different init systems like systemd, OpenRC, and runit. By the end, you’ll see why understanding init can be a game-changer for troubleshooting and optimising your Linux experience.

The Role of Init in Linux Booting

Init is the bedrock of a Linux system’s startup process. Launched by the kernel as process ID (PID) 1, init sets off a chain reaction that brings the entire system online. Here’s a closer look at the critical stages init goes through to get your system up and running:

1. The Big Bang of Booting

• As soon as the kernel hands off control, init is born as the system’s very first process. Without it, the system can’t proceed, often resulting in a “kernel panic,” which is a system-wide freeze due to an error. Init’s successful launch is critical because every other process depends on it.

2. Setting the Stage with Configuration

• Init doesn’t launch services haphazardly; it follows a set of predefined instructions found in configuration files. Older systems used files like /etc/inittab, but modern init systems like systemd read from directories like /etc/systemd/system/. These files tell init what’s needed for each “runlevel”—essentially, a specific state of the system with predefined services and functionalities.

3. Scripting the Action: From Commands to Unit Files

• Init translates these configurations into action. On systems that use traditional init (like SysV), this means running scripts that launch specific services. On systems that use systemd, “unit files” provide more flexibility. These unit files allow administrators to specify dependencies and conditions, making it easier to manage service startup in a way that’s both efficient and reliable.

4. Orchestrating the Show

• At each runlevel, init knows which services need to start, from networking to daemon processes, and it brings them online in sequence. This orchestration ensures that the system’s essential components are ready and waiting for you when you log in.

5. Init’s Watchful Eye

• Init’s role doesn’t end after startup. It remains vigilant, keeping an eye on processes throughout the system’s uptime. If a process is orphaned (abandoned by its parent), init steps in to adopt and manage it, ensuring stability across the board. This keeps your Linux system humming along even when individual processes encounter issues.

6. Beyond Booting: Modern Init Systems Take on More

• With more advanced init systems like systemd, init has evolved to handle much more than booting alone. It manages power states (like shutdown and restart), logging, and even supervises services to ensure they restart automatically if they fail. This expanded role helps streamline Linux administration and improves system resilience.

Here's the outline of typical init levels (or runlevels) used in traditional Unix-like systems. Each level corresponds to a specific state of system functionality, from system halt to full multi-user mode.

In systemd-based systems, these traditional runlevels are mapped to targets, such as multi-user.target or graphical.target, to provide more flexibility and compatibility with different systems.

Reference:

- https://abhijit-pal.medium.com/understanding-init-in-linux-systems-d985040fa266