s6 - skarnet's small supervision suite[web search]
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 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 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 a s6-svscan-based supervision tree without replacing init
- How to replace init
- How to perform socket activation with s6
- A POSIX-compliant system with a standard C development environment
- GNU make, version 3.81 or later
- skalibs version 126.96.36.199 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.
- execline version 188.8.131.52 or later. It's a build-time and run-time requirement.
s6 is free software. It is available under the ISC license.
- The current released version of s6 is 184.108.40.206.
- Alternatively, you can checkout a copy of the
git clone git://git.skarnet.org/s6
- There's also a GitHub mirror of the s6 git repository.
- 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 or hardware error, and 100 if they encounter a permanent error - such as a misuse. Short-lived commands exit 0 on success.
- 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-svwait program
- The s6-svlisten1 program
- The s6-svlisten program
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-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.
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
suidless privilege gain
Timed lock acquisition
fd-holding, a.k.a. the sensible part of socket activation
- The s6-fdholder-store program
- The s6-fdholder-storec program
- The s6-fdholder-retrieve program
- The s6-fdholder-retrievec program
- The s6-fdholder-delete program
- The s6-fdholder-deletec program
- The s6-fdholder-list program
- The s6-fdholder-listc program
- The s6-fdholder-getdump program
- The s6-fdholder-getdumpc program
- The s6-fdholder-setdump program
- The s6-fdholder-setdumpc program
- The s6-fdholder-transferdump program
- The s6-fdholder-transferdumpc program
- 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
- 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
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.
- s6 is discussed on the supervision mailing-list.
- There is a #s6 IRC channel on Freenode. Sometimes people are there and answer questions.
- 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
- 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 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.
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.)