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:

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.




s6 is free software. It is available under the ISC license.



Upgrade notes



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.

Supervision system

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.

Daemontools-like utilities

These programs are a rewrite of the corresponding utilities from daemontools, with 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 synchronization.

Local service management and access control

suidless privilege gain


Timed lock acquisition

fd-holding, a.k.a. the sensible part of socket activation



Related resources

Other components for s6-based init systems

s6 discussion

Similar work

Other init systems

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" ?'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 software releases. And please avoid using that prefix for your own projects.


[1] 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.)