An Introduction to Linux Init Systems and their evolution through time

The Evolution of Init Systems in Linux

In the landscape of Linux distributions, the init system serves as the first userspace process that the kernel starts upon boot. It is responsible for bringing the system to a usable state by spawning and managing all other processes and services. Over the years, multiple approaches to system initialization have been developed, each with its own goals, philosophies, and controversies. This article explores the historical development and evolution of init systems in Linux, highlighting their key features, challenges, and the communities and controversies they have inspired.

From the early BSD and SysV init scripts to today’s sophisticated, feature-rich systems like systemd, the evolution of init systems reflects the broader story of Linux itself: innovation through collaboration, an ongoing tension between minimalism and complexity, and a passionate user base that values both tradition and technological progress.

Although systemd is currently the most widely adopted solution—used by the majority of mainstream distributions—there is no shortage of alternatives for users and maintainers who prefer a different approach. This diversity ensures that the Linux ecosystem remains vibrant and adaptable, catering to everything from embedded devices to enterprise data centers. As technology marches on, init systems will continue to play a crucial role in shaping the performance, stability, and philosophy of Linux-based systems.

But, With That Said… What Is an Init System?

Screenshot of an init system info screen during boot. — You can usually see the output of your init system when either you boot your computer or when it shuts down.

An init system (short for “initialization system”) is the parent of all userspace processes on a Unix-like operating system, including Linux. An init system is foundational to how the entire operating system behaves after the kernel has done its part.

Bootstrapping the user space (mounting file systems, starting essential services).
Managing service lifecycles (starting, stopping, reloading services).
Handling system runlevels or targets (which define which services and features run).

Early Unix and Linux Init Systems

Traditional UNIX/BSD-Style Init

BSD init: Historically, BSD Unix systems had a single rc script that started necessary services in a fixed order.
Simplicity: The approach was straightforward—shell scripts stored in /etc/rc.* directories, each responsible for bringing up services sequentially.
Drawback: Lack of parallelization. Services started one after another, which could slow boot times.

System V Init (SysV init)

SysV init became the de facto standard in many Linux distros for over a decade, providing a stable if somewhat antiquated foundation.

Influenced by UNIX System V: SysV init introduced the concept of “runlevels” (0 through 6) to define different states of system operation.
Scripts and Directories: Each runlevel (e.g., multi-user mode, single-user mode) had corresponding directories (/etc/rc.d/rcN.d) with scripts to start or stop services in a specified order.
Advantages:
- Widely adopted in early Linux distributions (e.g., Slackware, Red Hat Linux, and Debian).
- Well-understood and stable.
Challenges:
- Boot process can be slow because tasks generally run sequentially.
- Limited service management. Reliance on shell scripts made the process flexible but also cumbersome and prone to errors.

You can access our System V guide and cheat sheet here: The System V to-the-point guide and cheat sheet.

The Push for Modernization (Early 2000s)

By the 2000s, Linux systems were becoming more complex, with more services starting at boot, networking dependencies, and modern hardware requiring faster and more dynamic management of processes. The need for a more advanced init system became apparent:

Parallelization: As multi-core systems became common, starting services in parallel could significantly reduce boot times.
Event-Driven Behavior: Systems needed to handle events (like new devices or network interfaces coming up) more gracefully, rather than relying on a fixed script order.
Service Tracking: SysV init had limited built-in capabilities for monitoring or automatically restarting failed services.

Upstart (2006)

While Upstart solved many problems of SysV init, its rise was eventually overshadowed by the emergence of systemd, partly because of licensing considerations and a narrower developer community.

Created by Canonical: Upstart was initially developed for Ubuntu to address the need for event-driven, parallel startup.
Event-Based Model: Instead of runlevels, Upstart uses events (e.g., startup, filesystem-mounted, net-device-up) to trigger service starts and stops.
Improved Concurrency: Services that do not depend on each other can start in parallel, speeding up the boot process.
Adoption:
- Ubuntu began transitioning to Upstart in 2006 (Ubuntu 6.10, “Edgy Eft”).
- Other distros like Fedora and openSUSE experimented with or partially supported Upstart before ultimateyl moving on to systemd.

Systemd (2010)

Genesis and Goals

Developed by Lennart Poettering and Kay Sievers (among others) at Red Hat.
Vision: A faster, more modern, and more integrated system to handle all aspects of system initialization and service management—from the moment the kernel boots until shutdown.
Key Features:
1. Aggressive Parallelization: Using socket activation, D-Bus activation, and other triggers to start services on-demand.
2. Unified Configuration: Service units described in .service files, replacing many legacy shell scripts.
3. Tracking and Supervision: Each service runs as a process that systemd can easily monitor and restart if it crashes.
4. Integration: Tools like journald for logging, logind for session management, networkd for networking, and timedated for time synchronization are all part of the systemd suite.

Rapid Adoption

By the mid-2010s, systemd became the dominant init system across the majority of mainstream Linux distributions.

Red Hat/Fedora: Fedora 15 (2011) officially adopted systemd. Red Hat Enterprise Linux followed suit in RHEL 7.
Debian: After significant debate, Debian chose systemd as the default init system in 2014 (for Debian 8 “Jessie”), spurring controversy among some users.
Ubuntu: Switched from Upstart to systemd in 2015 (Ubuntu 15.04 “Vivid Vervet”).
OpenSUSE and SUSE Linux Enterprise: Also adopted systemd.

Controversies

Despite the debate, systemd introduced numerous technical innovations and performance improvements, making it attractive to many Linux maintainers and enterprise users.

Monolithic vs. Modular: Critics argue that systemd does too much (logging, device management, etc.) and breaks the “do one thing well” Unix philosophy.
Complexity: Configuration can be more complex than simple SysV shell scripts, and debugging certain issues may require specialized knowledge of systemd’s architecture.
Community Forks: Distros like Devuan emerged as a fork of Debian specifically to provide a systemd-free environment, preserving SysV init or offering alternatives.

Note: see at the end of this article the “Community Debates, Dramas and Forks” section for more info about systemd’s controversies.

You can access our System V guide and cheat sheet here: The systemd to-the-point guide and cheat sheet.

Alternatives and Other Modern Init Systems

While systemd dominates, several alternatives focus on simplicity or different philosophies of service management.

OpenRC

Origin: Initially developed for Gentoo, though also used by Alpine Linux and others.
Philosophy: It extends the traditional rc-system approach with parallelization, better dependency tracking, and process supervision.
Key Points:
- Functions as a layer on top of existing init (often used with SysV init).
- Retains shell scripts but includes a dependency tree, allowing some parallel startup.

Runit

Design: A cross-platform init scheme known for simplicity and speed.
Three-Stage Boot Process: Splits system startup into three stages—one-time initialization, running services, and system shutdown.
Service Supervision: Each service is monitored continuously, with automatic restarts if needed.
Usage: Adopted by distributions like Void Linux, known for its minimalist approach.

s6

Development: Created by Laurent Bercot, focusing on a small, secure, and efficient supervision and init framework.
Philosophy: Micro-service-based approach for starting and supervising processes, with minimal overhead and maximum reliability.
Adoption: Niche but growing; used by some highly customized or embedded systems.

BusyBox init / Embedded Inits

BusyBox: Commonly used on embedded systems or small containers.
Functionality: Combines tiny versions of many common UNIX utilities into a single binary, including a minimal init.
Use Case: Favored in resource-constrained environments where advanced features of systemd are unnecessary or too large.

The Role of Init Systems in Containers

Containerization Trend: With Docker, Kubernetes, and other container technologies, many services run in isolated containers that often don’t need a full init system.
Minimal Base Images: Alpine Linux uses BusyBox with OpenRC, providing a lean environment.
Systemd in Containers: Some container images do use systemd (e.g., systemd-based environments for complex service orchestration), though it’s more common in heavier images or orchestrated environments (like Podman or systemd-nspawn containers).

Community Debates, Dramas and Forks

These forks illustrate how strongly users can feel about init systems, underscoring philosophical and technical divisions in the Linux community, and that’s an understatement… The Debian’s Systemd debate alone was a drama worthy of all the popcorn in the world, and it filled tens of thousands of forum pages (I’m not exaggerating there).

Debian’s Systemd Debate: One of the most contentious battles occurred around 2013–2014, culminating in Debian’s Technical Committee deciding to adopt Systemd as the default.
Devuan: A direct fork of Debian 8 created by the “Veteran Unix Admins” to continue providing a Systemd-free experience.
- I was in this camp, eventually, I ended up giving up and just using the regular Debian with SystemD.
Artix Linux: A fork of Arch Linux that uses OpenRC or runit (instead of systemd), appealing to Arch users who want alternatives.

Keep Learning About Linux

You can continue learning about Linux and its evolution following these links:

An Introduction to Linux Init Systems and their evolution through time

The Evolution of Init Systems in Linux

But, With That Said… What Is an Init System?