s6 is described as 'Small suite of programs for UNIX, designed for process supervision. It can be used as an init system. You can also use all its programs separately' and is an app in the os & utilities category. There are more than 10 alternatives to s6 for a variety of platforms, including Linux, BSD, Mac, FreeBSD and Docker apps. The best s6 alternative is systemd, which is both free and Open Source. Other great apps like s6 are OpenRC, runit, Launchd and sysvinit.
systemd is a suite of basic building blocks for a Linux system. It provides a system and service manager that runs as PID 1 and starts the rest of the system.
systemd is not portable, is not truly modular and is not small code-wise.
OpenRC is a dependency-based init system that works with the system provided init program, normally /sbin/init. It is not a replacement for /sbin/init. OpenRC is 100% compatible with Gentoo init scripts, which means you can probably find one for the daemons you want to start in...
runit is a cross-platform Unix init scheme with service supervision, a replacement for sysvinit, and other init schemes. It runs on GNU/Linux, *BSD, MacOSX, Solaris, and can easily be adapted to other Unix operating systems.
a free system array created by Apple inc. For BSD - including macOS as macOS is based on the Darwin kernel of BSD
The Sysvinit package contains programs for controlling the startup, running, and shutdown of the system.
The nosh package is a suite of system-level utilities for initializing and running a BSD or Linux system, and for managing daemons.
Finit is a small SysV init replacement with process supervision similar to that of daemontools and runit. Its focus is on small and embedded GNU/Linux systems, although fully functional on standard server and desktop installations.
hummingbird is an init system designed for speed. It does nothing more than start the system and stop the system by default. There are three main "runlevels", fs which is called directly after the pseudo filesystems necessary for the system to run have been mounted...
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People who use s6 are precisely the ones who want to go away from systemd.