s6 - skarnet's small supervision suite
Software
skarnet.org
s6
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
socket
activation
[1]
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.
high-level overview
of s6
Why another supervision suite?
Isn't
runit
good enough?
What is
instant notification
? What does the
ftrigr library
do exactly?
How to run an s6-svscan-based supervision tree
without replacing init
How to
replace init
How to perform
socket activation
with s6
Installation
Requirements
A POSIX-compliant system with a standard C development environment
GNU make, version 3.81 or later
skalibs
version
2.14.5.1 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
library.
(Optional, but really recommended for full functionality):
execline
version
2.9.8.1 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
execlineb
The following optional dependencies are also supported:
If you're using
musl
and
want nsswitch-like functionality:
nsss
version
0.2.1.1 or later (build-time and boot-time)
Licensing
s6 is free software. It is available under the
ISC license
The current released version of s6 is
2.14.0.1
You can access its checksum
here
Alternatively, you can checkout a copy of the
s6
git repository
git clone git://git.skarnet.org/s6
There's also a
GitHub mirror
, or a
SourceHut mirror
of the s6 git repository.
Compilation
See the enclosed INSTALL file for installation details.
Upgrade notes
This page
lists the differences to be aware of between
the previous versions of s6 and the current one.
Reference
If you prefer to read this documentation as man pages, it is now possible!
There is a
project
that
ports the s6 documentation to a set of man pages.
Commands
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.
The
s6-svscan
program
The
s6-svscanctl
program
The
s6-supervise
program
The
s6-svc
program
The
s6-svok
program
The
s6-svstat
program
The
s6-svperms
program
The
s6-svlink
program
The
s6-svunlink
program
The
s6-svwait
program
The
s6-svlisten1
program
The
s6-svlisten
program
The
s6-notifyoncheck
program
The
s6-svdt
program
The
s6-svdt-clear
program
The
s6-permafailon
program
The
s6-background-watch
program
Daemontools-like utilities
These programs are a rewrite of the corresponding utilities from
daemontools
, with
a few extras.
The
s6-envdir
program
The
s6-envuidgid
program
The
s6-fghack
program
The
s6-setlock
program
The
s6-setsid
program
The
s6-setuidgid
program
The
s6-applyuidgid
program
The
s6-softlimit
program
The
s6-tai64n
program
The
s6-tai64nlocal
program
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
The
s6-mkfifodir
program
The
s6-cleanfifodir
program
The
s6-ftrig-notify
program
The
s6-ftrig-wait
program
The
s6-ftrig-listen1
program
The
s6-ftrig-listen
program
The
s6-ftrigrd
internal program
Local service management and access control
The
s6-ipcclient
program
The
s6-ipcserver
program
The
s6-ipcserver-socketbinder
program
The
s6-ipcserverd
program
The
s6-ioconnect
program
The
s6-ipcserver-access
program
The
s6-connlimit
program
The
s6-accessrules-cdb-from-fs
program
The
s6-accessrules-fs-from-cdb
program
suidless privilege gain
The
s6-sudo
program
The
s6-sudoc
program
The
s6-sudod
program
Logging
The
s6-log
program
The
s6-socklog
program
The
ucspilogd
program
Management of user supervision trees
The
s6-usertree-maker
program
Management of dynamic instances
An
overview
of dynamic instantiation in s6
The
s6-instance-maker
program
The
s6-instance-create
program
The
s6-instance-delete
program
The
s6-instance-control
program
The
s6-instance-status
program
The
s6-instance-list
program
fd-holding, a.k.a. the sensible part of socket activation
The
s6-fdholder-daemon
program
The
s6-fdholderd
program
The
s6-fdholder-store
program
The
s6-fdholder-retrieve
program
The
s6-fdholder-delete
program
The
s6-fdholder-list
program
The
s6-fdholder-getdump
program
The
s6-fdholder-setdump
program
The
s6-fdholder-transferdump
program
Libraries
s6/s6.h
, the main entry point
The
ftrigw
library interface
The
ftrigr
library interface
The
s6lock
library interface
The
accessrules
library interface
The
s6-fdholder
library interface
Definitions
What is a
fifodir
What is a
service directory
What is a
scan directory
What is a
local service
Why are the
libftrigw and libftrigr
needed
Related resources
s6 manual pages
flexibeast
is doing the ungrateful
but valuable work of
providing the s6 documentation
as a set of man pages
Other components for s6-based init systems
s6-linux-init
is a package to help you create a
/sbin/init
binary booting a
Linux system with s6-svscan as process 1.
s6-overlay
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.
66
is another
service manager working on top of s6.
s6 discussion
s6
is discussed on the
supervision
mailing-list.
There is a
#s6
IRC channel on OFTC. Sometimes people are there
and answer questions.
Similar work
daemontools
, the pioneering
process supervision software suite.
daemontools-encore
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
daemontools.)
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.
nosh
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
(This list hasn't been updated in a long while. I'm keeping it for reference.)
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 came with earlier versions of the Ubuntu
distribution. It is now deprecated.
systemd
is a problem in its own category.
The various BSD flavors have their own style of
init
MacOS X has its own init spaghetti monster called
launchd
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.
Miscellaneous
Why "s6" ?
karnet.org's
mall and
ecure
upervision
oftware
uite.
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.
Footnotes
[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.)