What is it ?
s6 is a small suite of programs for UNIX, designed to allow process supervision
(a.k.a service supervision),
in the line of daemontools
and runit, as well as various
operations on processes and daemons. It is meant to be a toolbox for
low-level process and service administration, providing different sets of
independent tools that can be used within or without the framework, and
that can be assembled together to achieve powerful functionality with
a very small amount of code.
Examples of things you can do by assembling together several programs
provided by s6 - besides process supervision:
- syslogd functionality,
using much less resources than the traditional syslogd.
- Reliable service readiness notification, which is the basis for
service dependency management.
- Controlled privileged gain as with
sudo, without using
any suid programs.
- The useful parts of
without having to change application code or link servers
against any specific library, and without having to switch to any
specific init system.
The s6 documentation tries to be complete and self-contained; however,
if you have never heard of process supervision before, you might be
confused at first. See the related resources section
below for pointers to more resources, and earlier approaches to process
supervision that might help you understand the basics.
- A POSIX-compliant system with a standard C development environment
- GNU make, version 3.81 or later
- skalibs version
18.104.22.168 or later. It's a build-time requirement. It's also a run-time
requirement if you link against the shared version of the skalibs
- Optional: execline version
22.214.171.124 or later. When s6 is built with execline support (which is the default),
execline is a build-time requirement, and also a run-time requirement for
certain binaries that spawn scripts interpreted with
s6 is free software. It is available under the
- See the enclosed INSTALL file for installation details.
- This page lists the differences to be aware of between
the previous versions of s6 and the current one.
All these commands exit 111 if they encounter a temporary error, and
100 if they encounter a permanent error - such as a misuse. They exit
127 if they're trying to execute into a program and cannot find it, and
126 if they fail to execute into a program for another reason.
Short-lived commands exit 0 on success.
s6-svscan and s6-supervise
are the long-lived processes maintaining the supervision tree. Other programs are
a user interface to control those processes and monitor service states.
These programs are a rewrite of the corresponding utilities from
a few extras.
Fifodir management, notification and subscription
These programs are a clean rewrite of the obsolete "pipe-tools" package; they
are now based on a properly designed notification library.
They provide a command-line interface to
inter-process notification and
Local service management and access control
suidless privilege gain
Timed lock acquisition
fd-holding, a.k.a. the sensible part of socket activation
Other components for s6-based init systems
is a package to help you create a /sbin/init binary booting a
Linux system with s6-svscan as process 1.
is a project that automates integration of s6 into Docker images.
- s6-rc is a
dependency-based service manager for s6.
- anopa is another dependency-based
service manager for s6.
- s6 is discussed on the
- There is a #s6 IRC channel on Freenode. Sometimes people are there
and answer questions.
- daemontools, the pioneering
process supervision software suite.
a derived work from daemontools with enhancements. (Note that although s6 follows
the same naming scheme, the same general design, and many of the same architecture
choices as daemontools, it is still original work, sharing no code at all with
- runit, a slightly different
approach to process supervision, with the same goals.
- perp, yet another slightly different
approach to process supervision, also with the same goals.
is another suite of system-level utilities with similarities in the design
and approach. It is written in C++, though, and is coded in quite a
different way than the previous items on this list.
Other init systems
- Felix von Leitner's minit is an
init system for Linux, with process supervision capabilities.
- suckless init is
considered by many as the smallest possible init. I disagree: suckless
init is incorrect, because it
has no supervision capabilities, and thus, killing all processes but init
can brick the machine. Nevertheless, suckless init, like many other
suckless projects, is a neat exercise in minimalism.
- sysvinit is the
traditional init system for Linux.
- Upstart is a well-known init system
for Linux, with complete service management, that comes with the Ubuntu
distribution. It includes a coffee machine and the kitchen sink.
- systemd is a problem in its own category.
- The various BSD flavors have their own style of
- MacOS X has its own init spaghetti monster called
All-in-one init systems generally feel complex and convoluted, and when most
people find out about the process supervision approach to init systems, they
usually find it much simpler.
There is a good reason for this.
Why "s6" ?
skarnet.org's small and secure
supervision software suite.
Also, s6 is a nice command name prefix to have: it identifies the origin of the
software, and it's short. Expect more use of s6- in future skarnet.org software
releases. And please avoid using that prefix for your own projects.
Take everything you read on that link with two or three salt shakers.
(This is true for anything written by the author of that document.)