Since I have been experimenting with different init systems I wanted to write my thoughts and experiences down. I have been using Slackware during this time to try out other inits, so most of these breakdowns for non-SystemD will be coming from Slackware's defaults for the init but I have tweaked and adjusted them as I went along with my normal workflow.
But for those who do not know, what is an init system? When a Linux kernel finishes booting, the first user space process it starts is PID 1, traditionally called init. This process has three core responsibilities.
Because PID 1 is the parent of every other process, the design of the init system heavily influences boot speed, reliability, and the overall system administration experience. Over the past three decades, several init systems have been created, each trying to solve perceived shortcomings of its predecessor. Below I walk through the most influential ones, focusing on their development history, distribution adoption, and the unique features that set them apart. I also would like to note that while I will list things that make them unique, the items might not be exclusive to that specific init but that they did that specific item first or better than others.
Derived from AT&T UNIX System V (early 1990s). First widespread Linux implementation, 1993‑1994 (by Linus Torvalds and early maintainers). Core design is simple, deterministic run level model (0–6). Implementation is typically a small C binary (/sbin/init) that reads /etc/inittab and executes scripts in /etc/rc.d/rc?.d/.
What makes SysV unique?
Why it fell out of favor, the static ordering leads to slow parallel boot, and the lack of supervision makes modern 'always on' services cumbersome.
Created by Scott James Remnant at Canonical in 2005. The goal was to replace the static run level model with an event driven approach that could start / stop services as soon as their dependencies became available. The first production release was Ubuntu 6.06 “Dapper Drake” that used Upstart as the default init.
What makes Upstart unique?
Services could be started as soon as a required resource appeared (start ssh when network up event fires). Upstart shipped a /sbin/init that could emulate the classic SysV runlevels, easing migration. Built in ability to automatically restart a crashed daemon (respawn stanza). Upstart’s event model heavily influenced SystemD’s early design (SystemD’s unit concept and targets). Canonical’s decision to adopt SystemD (citing faster boot times, richer dependency handling, and a unified ecosystem) ended Upstart’s mainstream relevance. The codebase is still maintained but only for legacy Ubuntu derivatives.
Created by Lennart Poettering and Kay Sievers (both at Red Hat) in 2010. The goral was to provide a feature rich, parallel, and reliable init system that also standardizes service management across Linux distributions The first stable release, SystemD v1 (2010), was adopted by Fedora Linux.
What makes SystemD unique?
Roughly 90% of the top 100 Linux distributions by download count ship SystemD as PID 1.
Forked from the init scripts used by Gentoo in 2006, officially released as a separate project in 2009. The goal of this project is to provide dependency based service management without the heavy weight components of SystemD, staying compatible with the traditional SysV script layout.
What makes OpenRC unique?
Authored by Guido van Rossum (yes, the Python creator) and Bjørn Nilsen, first released in 2004. With the philosophy of 'One process per service, supervision, and fast boot' a tiny, three stage init system (run, runsvdir, sv). runsvdir starts a directory of service directories (/etc/service). runsv, one per service, watches run (the service start script) and finish (shutdown script). sv, a command line tool to control services (sv start foo). All scripts are plain shell, runit itself is about 4 KB (C source compiled).
What makes Runit unique?
Developed by skaware (Laurent Bercot) as a collection of tiny, composable Unix tools. First released in 2011 the goal is to provide a modular, secure, and POSIX compliant process supervision suite that can be used as a full init system or as a per service supervisor. All tools are small (a few kilobytes each) and can be combined as needed.
What makes s6 unique?
So what should we use? Well it is not so simple, there is a use case for everything. I feel, like every user should have the right to choose their init. And below is a breakdown of the use case or what I feel each init does best.
Future trends and outlook seems to show continued dominance of SystemD, but container oriented distributions (Alpine, distroless images) increasingly ship OpenRC or runit to keep image size <10MB. However it is unclear how it will be post age verification in 2026. cgroup v2 will become the default for most non SystemD supervisors (OpenRC, runit) via optional helper tools. The pressures of standardization in the Linux space is both a positive and a negative for users. Freedesktop.org spec continues to evolve, making it easier for alternative init systems to adopt compatible socket activation and D‑Bus interfaces without copying SystemD code. And SystemD as well as other init systems are undergoing regular security reviews, the trend is toward formal verification of supervision logic.
If you need a full featured, modern init system with fast boot, robust supervision, and deep integration with the Linux kernel, SystemD remains the default choice across most distributions. If you prioritize minimalism, script based configuration, or run in container heavy environments, consider OpenRC, runit, or s6. Each offers a different balance of size, features, and complexity. For those who want a dependency aware but still script centric approach without moving to SystemD’s code base, OpenRC is the sweet spot. In the meanwhile I will continue testing and ask that you do the same, I have said before and will say again, users should have the right to choose. I personally do not mind SystemD, OpenRC has grown on me in the last month. Tinkering and testing is what makes Linux and it's community great and vibrant. So test your init system, tinker, and learn what makes something right for you.