s6 is a process supervision suite that also provides tools for service management (s6-rc) and system initialization/shutdown (s6-linux-init).
s6 software is all designed to be very modular and can be mixed and matched with other things. However, s6 software is also designed to work well with one another. In Artix, we take full advantage of what s6 offers and use all of the available tools to provide an /sbin/init, PID1, process supervisor suite, and service manager for users.
s6-linux-init is what actually initiates your system. It mounts a tmpfs onto /run and copies the /etc/s6/current/run-image directory into /run. /sbin/init then execs into the s6-svscan program (provided by s6) which functions as your PID1 for the lifetime of the machine. Once this is done, the stage 1 part of init is finished. In addition to being PID1, s6-svscan is also the root of your process supervision tree. It monitors every service found and appropriately spawns an s6-supervise for every service.
Additionally, during bootup the rc.init script in the /etc/s6/current/scripts directory gets executed. s6 itself does not do service management, but only process supervision. In rc.init, the actual service manager, s6-rc is started. From that point, we launch the default runlevel defined by s6-rc and begin the desired services.
On shutdown, every service in s6-rc is brought down, followed by killing all s6-supervise processes and other processes, then unmounting everything, and finally killing s6-svscan.
Note: s6-linux-init on artix is compiled with the option to print messages to /dev/console. By default, this is tty1. You can change /dev/console to another location using a kernel parameter (see your bootloader's configuration) if you'd like.
Recovery: s6-linux-init always provides a getty service on tty12 that you can use for recovery purposes (in case s6-rc crashes, etc.). As long as the system correctly boots, it will be there.
Install the s6-base package.
s6 service packages are named package_name-s6 and, when installed, will be available in /etc/s6/sv.
The s6 software suite comes with many different binaries, but in general you only need to directly interact with a small subset of them. Below are some of the more interesting programs.
s6-db-reload - a helper script for updating and reloading the s6-rc database
s6-rc - the main program for controlling and managing services
s6-rc-db - a tool for analyzing a compiled service database
s6-svc - a tool to directly send commands to an s6-supervise process
s6-svstat - a tool for checking the current states of a process monitored by s6-supervise (an s6-rc longrun)
Most of the files associated with the s6 software packages are installed in the /etc/s6 directory.
/etc/s6/current - the base directory for s6-linux-init
/etc/s6/current/scripts - various startup/shutdown scripts executed by s6-linux-init
/etc/s6/config - conf files for specific s6-rc services
/etc/s6/rc - where compiled databases are stored; the current live database is always symlinked to /etc/s6/rc/compiled
/etc/s6/rc.local - file for executing arbitrary shell commands on bootup (any shell scripts in /etc/local.d suffixed with *.start are executed on bootup and those with *.stop are executed on shutdown)
/etc/s6/skel - contains the default startup/shutdown scripts that come with artix linux
/etc/s6/sv - default directory for script packages from Artix
/etc/s6/adminsv - directory for custom user services as well as script packages from Artix that allow for editing
/etc/s6/fallsv - emergency fallback that is used if the s6-rc-compile of sv and adminsv fails
A key concept to using s6-rc is to understand the notion of "bundles." You can take their namesake literally. A bundle in s6-rc is any collection of services, oneshots, and even other bundles. These are quite similar to openrc's runlevels and are used in similar ways in Artix. The package, s6-scripts, which contains essential startup oneshots and daemons for an Artix system internally uses many different bundles for convenience, but for users the main bundle they should concern themselves with is the default bundle. This is started by s6-rc and in general users will want to add their services to this bundle.
Note: There is a bundle within default called boot. This is a collection of startup/shutdown boot oneshots and daemons deemed essential for a working system. These are mostly provided by the s6-scripts package with some optional dependencies that install themselves into the appropriate directory.
Also note that s6-rc manages dependencies so it is not necessary to manually start all needed dependencies. When service foo is started, all of its dependencies (if they are not already up) are automatically started. Here are some handy commands.
# s6-rc -d change service_name
# s6-rc -u change service_name
# s6-svc -r /run/service/service_name
# s6-rc -a list
# s6-rc-db list all
# s6-svstat /run/service/service_name
Within every directory of a bundle, there is a contents.d folder which contains empty files named after services that are within the bundle. So in order to add services to a bundle, you just touch empty files named after the services in the directory. To add services to the default bundle (so it starts on boot), you would do:
# touch /etc/s6/adminsv/default/contents.d/service1 # touch /etc/s6/adminsv/default/contents.d/service2 # s6-db-reload
The s6-db-reload command is a symlink to the hook Artix uses to handle s6-rc database upgrades. It is executed whenever any *-s6 package in Artix is installed to ensure services are immediately available in the new database. This consists of 3 main steps. First, it compiles a new database with a unique name generated by the date command with s6-rc-compile. Then, it executes s6-rc-update to update the live database to the newly compiled database. Finally, it atomically updates the symlink to /etc/s6/rc/compiled so on the next boot, the system executes the newest database.
s6-rc has three types of services: longrun, oneshot, and bundle. Most *-s6 packages are longrun services (AKA daemons). Oneshot services do exactly what their name implies: execute once on bootup and optionally on shutdown. Bundles are simply a collection of longruns, oneshots, and even other bundles. Every source directory will have a mandatory type file that contains a single line defining what type the service is. Longrun services must contain a file called run, oneshots must contain a file called up, and bundles contain a subdirectory named contents.d.
In Artix, a typical service script comes with two different parts: the daemon itself and the logger daemon that uses the s6-log tool. The scripts for the actual service foo will be installed in the /etc/s6/sv directory as foo-srv. Additionally, a small logger daemon for that specific service will be installed in foo-log. foo-log will catch any output from foo-srv and save it in /var/log/foo directory. The s6-log logger daemon is run as the s6log user and group. To give a user permission to view logs, just add him to the s6log group. s6-log does log rotation for you and has many different configurable options.
When interacting with s6-rc, the name of foo doesn't change (i.e. you still do # s6-rc -u change foo). This starts both foo-srv and foo-log and handles any of the dependencies for you.
This is a tree of a longrun directory structure (aka /etc/s6/sv/foo-srv). In many scripts, only producer-for, type, and run exist.
foo-srv ├── dependencies.d (optional subdir containing names of dependencies) ├── notification-fd (optional) ├── producer-for (required for foo-log) ├── run └── type
A tree for a logger daemon longrun (aka /etc/s6/sv/foo-log) looks like this.
foo-log ├── consumer-for ├── notification-fd ├── pipeline-line ├── run └── type