S6

S6 is a process supervision suite designed for modularity, security, and performance. It’s not just an init system — it’s a full ecosystem of tools for service management (s6-rc), logging, and system initialization (s6-linux-init). Written by Laurent Bercot, it’s the most technically sophisticated of the non-systemd inits.

S6 is actually three layers:

s6 — core process supervision (svscan, supervise, svc)

s6-rc — dependency-based service manager

s6-linux-init — creates the /sbin/init binary

Philosophy

S6 takes the daemontools/runit model and extends it with:

Modularity — every piece is separate and replaceable
Security — services run with minimal privileges, no ambient authority
Performance — fully parallel startup, designed for speed
Reliability — automatic restart, logging, notification

Unlike runit, s6 has a real dependency-based service manager (s6-rc) with declarative service definitions. It’s the closest you can get to systemd’s feature set without the bloat.

Who uses it?

Artix Linux — available as an alternative init
Chimera Linux — uses s6 as its init system
Docker images — s6-overlay is popular for container supervision
Embedded systems, minimal Linux builds

How it works

Kernel starts /sbin/init (created by s6-linux-init-maker)
s6-linux-init mounts tmpfs, copies early services, execs into s6-svscan
s6-svscan becomes PID 1 and starts the supervision tree
rc.init script launches s6-rc with the default runlevel
s6-rc resolves dependencies and starts services in order

Key concepts

Concept	What it means
`s6-svscan`	PID 1 — root of the supervision tree
`s6-supervise`	Monitors one service directory, restarts on crash
`s6-svc`	Control tool — send commands to services
`s6-rc`	Dependency-based service manager (compiled database)
`s6-log`	Built-in logging with auto-rotation
Service directories	Like runit — each service is a directory with a `run` script
Compilation	`s6-rc` compiles service definitions into a binary database

Basic commands

# Start a service
s6-svc -u /run/service/sshd

# Stop a service
s6-svc -d /run/service/sshd

# Restart a service
s6-svc -r /run/service/sshd

# Check status
s6-svc -a /run/service/sshd

# With s6-rc (service manager):
s6-rc -d change sshd    # start
s6-rc -d stop sshd      # stop
s6-rc -d status sshd    # status

Example service

/etc/s6/sv/sshd/run:

#!/bin/sh
exec /usr/bin/sshd -D

With s6-rc, you also need a service definition file:

/etc/s6/rc/sources/sshd/type:

oneshot

/etc/s6/rc/sources/sshd/up:

#!/bin/sh
s6-svc -u /run/service/sshd

Comparison with OpenRC

	OpenRC	S6
Service format	Shell scripts	`run` scripts + s6-rc definitions
Dependencies	Manual (`need`/`use`)	Automatic via s6-rc
Supervision	No built-in	Yes — full supervision tree
Logging	syslog or separate	Built-in per-service (`s6-log`)
Boot speed	Fast	Very fast
Complexity	Low	Medium-High
Flexibility	Moderate	Very high (modular)

Should you use it?

Pick S6 if:

You want the most powerful non-systemd init
You need dependency-based service management
You like the idea of a compiled service database
You value process supervision and auto-restart
You’re comfortable with a steeper learning curve

Stick with OpenRC if:

You want something simple and well-documented
You need shell-script-based services you can debug easily
You don’t need process supervision or dependency resolution

S6