s6
============================================================================

s6 is an init replacement and service manager, similar to runit.
The overview is good:
https://skarnet.org/software/s6/overview.html

For every service/daemon controlled by s6, an `sv-supervise` process watches it.
`s6-svscan` is a supervisor of supervisors — it watches a flock of `s6-supervise` processes.
Configuration of a daemon is handled by its controlling `sv-supervise` process, based out of a "service directory".
All service directories controlled by `s6-svscan` are collected under a "scan directory".
The `s6-svc` command controls a `sv-supervise` process, and lets us signal its daemon.
The `s6-svscanctl` command controls a set of `sv-supervise` processes, and start or stop entire process trees.

The scan directory is typically '/service/' (or /var/svc.d/ on Alpine).
The service directories might be under '/etc/service' or '/var/service' or '/etc/sv'.

Note that the "scan directory" is set as the argument to `s6-svscan` or (if not supplied) its current directory.

A service directory generally contains at least a `run` script, which may be as simple as:

	#!/command/execlineb -P
	fdmove -c 2 1
	myservice myarg

`run` files are written in a shell-ish language called `execline`.
http://skarnet.org/software/execline/
(Maybe the service scripts can be just sh scripts too?)

A service directory typically also contains a `finish` script.


Links
----------------------------------------------------------------------------

- http://www.skarnet.org/software/s6/index.html
- http://www.skarnet.org/software/s6/servicedir.html
- https://github.com/skarnet/s6-rc/tree/master/examples/source
- http://jaytaylor.com/notes/node/1484870107000.html