{{tag>Alpine linux docker init s6 s6-rc alias}}
======Docker init Systems======
There are a number of init systems used on docker containers, many are the defaults that ship with the standard Linux distributions, e.g. systemd which is particularly popular on the larger comprehensive distributions.\\  
++++tldr;|
There is a lot of argument in the init world, SystemD seems to have taken over the init system in most "modern" Linux distributions.  It seems to break the Unix philosophy of do one thing well, and does many things, although it is broken into modules.  Perhaps more concerning is when graphical / windowing systems such as Gnome rely upon SystemD calls making it break with other init systems.\\
Some Unix (Linux focus) init systems:
  *sysv (System V)
    *[[https://www.kernel.org/doc/html/v6.14-rc4/filesystems/sysv-fs.html|SystemV Filesystem]]
    *[[https://docs.redhat.com/en/documentation/red_hat_enterprise_linux/4/html/reference_guide/s1-boot-init-shutdown-sysv|SysV Init Runlevels]]
  *[[https://systemd.io/|SystemD]]
  *[[https://wiki.gentoo.org/wiki/OpenRC|OpenRC]]
  *[[http://smarden.org/runit/|runit]]
  *[[https://skarnet.org/software/s6/|s6]]
  *And arguably not a full init systems:
    *[[https://supervisord.org/|supervisord]]
    *[[http://cr.yp.to/daemontools.html|daemontools]]
    ***launchd** is the process control system system used by Apple Max OS X, perhaps inspiration for systemd?
Some references (just do an online search....):
  *[[https://www.tecmint.com/best-linux-init-systems/|6 Best Modern Linux ‘init’ Systems (1992-2023)]]
  *[[https://www.hackerstack.org/linux-system-v-init-and-systemd-essentials/|Linux SysV init and Systemd essentials]]
When creating my own imaged I have generally is the S6-RC init system, most if not all have been built on the latest Apline Linux as base image. Some examples:
  *[[https://wiki.kptree.net/doku.php?id=docker_notes:docker-dns#docker_-_dns_server|DNS (ISC Bind9)]]
  *[[https://wiki.kptree.net/doku.php?id=docker_notes:docker-dhcp#docker_-_dhcp_server|DHCP server (ISC Kea)]]
  *[[https://wiki.kptree.net/doku.php?id=docker_notes:docker-deluge#docker_deluge_image_service|Torrent server (Deluge)]]
  *mailserver
    *[[https://wiki.kptree.net/doku.php?id=docker_notes:docker-mailserver#docker_mailserver|Mail server (Postfix and Dovecot)]]
    *[[https://wiki.kptree.net/doku.php?id=docker_notes:docker-mailserver#mariadb|Mail server database and web interface (mariadb php nginx)]]
    *[[https://wiki.kptree.net/doku.php?id=docker_notes:docker-mailserver#roundcube_webmail|Web Mailserver (Roundcube, postgresdb)]]

I have also used supervisord system where it was simple to follow the formula from the image supplier to add functionality. Some examples:
  *[[https://wiki.kptree.net/doku.php?id=docker_notes:docker-nextcloud#nextcloud_with_supervisord|Nextcloud (only example)]]

There are other init systems out there that I have not looked into, including a number of legacy init systems that are no longer supported. 
++++
=====s6 supervision rc system=====
I decided to build this container image based upon Alpine Linux using the [[https://www.skarnet.org/software/s6/index.html|S6]] init system. The Skarnet.org is the S6 authors web site and main repository.
S6 seems to be one of the most performant small init systems available, however it is not very easy to use.  I could really use a front-end to help with the management.

====s6 Service Driectories====
S6 [[http://www.skarnet.org/software/s6/servicedir.html|Service directories]]

The s6-rc service directories are stored at ''%%/etc/s6-overlay/s6-rc.d/%%''.
^ sub-directory                            ^ Must  ^ Comment                               |   |
| ├── <fs large>**named**</fs>                                | *1    | Service, longrun type, as required                               |   |
| │   ├── **dependencies.d**                   | *     | Mandatory sub-directory with of dependent services as sub-directories |   |
| │   │   ├── base                         |       | built in base services                |   |
| │   │   └── pre_start_script             |       | User pre start script (if required)   |   |
| │   ├── producer-for                     |*1  | file required for logging                  |   |
| │             └ <code>        named-log</code>  | *1    | file content with name of logging service |   |
| │   ├── run                              |* | File containing the service run script  |   |
| │             └ <code>        #!/command/execlineb -P
        fdmove -c 2 1
        /usr/sbin/named -f -4 </code>  |* | file content with service to run. Note service must run in foreground|  
| │   ├── timeout-down                     |      |optional file with timeout down, integer in milliseconds   |
| │             └ <code>        180000</code>  | *1    | file content with name of logging service |   |
| │   ├── timeout-up                       |       |optional file with timeout up, integer in milliseconds   |
| │             └ <code>        30000</code>  | *1    | file content with name of logging service |   |
| │   └── type                             |*       |mandatory file containing service type, in this case longrun   |
| │             └ <code>        longrun</code>  | *1    | file content with type of service |   |
| ├── named-log                            |       |                                       |   |
| │   ├── consumer-for                     |       |                                       |   |
| │   ├── dependencies.d                   |       |                                       |   |
| │   │   └── named-log-prepare            |       |                                       |   |
| │   ├── pipeline-name                    |       |                                       |   |
| │   ├── run                              |       |                                       |   |
| │   └── type                             |       |                                       |   |
| ├── named-log-prepare                    |       |                                       |   |
| │   ├── dependencies.d                   |       |                                       |   |
| │   │   ├── base                         |       |                                       |   |
| │   │   └── pre_start_script             |       |                                       |   |
| │   ├── type                             |       |                                       |   |
| │   └── up                               |       |                                       |   |
| └── pre_start_script                     |       |                                       |   |
|                       ├── dependencies.d |       |                                       |   |
|                             │   └── base |       |                                       |   |
|                                 ├── type |       |                                       |   |
|                                   └── up |       |                                       |   |
====s6 commands====
++++Basic S6 commands:|
  *''s6-svstat /run/service/servicename'' or ''s6-svstat /run/s6-rc/servicedirs/deluged'' - check a daemon status, longrun only!
  *''s6-rc -u change servicename'' - enable and start a daemon (up)
  *''s6-rc -d change servicename'' - disable and stop a daemon (down)
  *''s6-rc-db list all|services|oneshots|longruns|bundles'' - lists the nominated services in the active s6-rc-db
  *''s6-rc-db type servicename'' - Prints the type of servicename: oneshot, longrun or bundle
  *''s6-rc-db dependencies servicename'' - lists the dependencies in the servicename directory, i.e. in servicesname/dependencies.d 
  *''s6-rc-db dependencies servicename'' - lists all the dependencies of servicename
  *''s6-rc-db -d|-u timeout servicename'' - lists the timeout, -d for timeout down and -u for timeout up 

BTW dont spell dependencies as dependancies and wonder why things don't work, lol the English language sucks.
++++

=====docker builtin basic init=====
In the docker run command the ''%%--init%%'' flag can be used to [[https://docs.docker.com/reference/cli/docker/container/run/#init|specify an init process]] is used.  This causes docker to use [[https://github.com/krallin/tini|tini]] as an init system on process 1, instead of the first docker command.  Tini is a very basic init system specifically for containers that reaps zombie processes and preforms signal forwarding.

In docker compose add the directive ''init: true'' see example below:
++++docker-compose.yml snippet|
<code>services:                                                                                                           
    ntfy:
      image: binwiederhier/ntfy
      init: true
      container_name: ntfy </code>
++++

  *[[https://blog.phusion.nl/2015/01/20/docker-and-the-pid-1-zombie-reaping-problem/|Docker and the PID 1 zombie reaping problem]]
  *[[https://github.com/docker-library/official-images?tab=readme-ov-file#init|docker library #init]]
  *[[https://www.baeldung.com/ops/docker-init-parameter|How to Use –init Parameter in Docker Run Command]]

=====alias in S6 init=====
I like the alias command to change some default CLI command flags, in particular ls => ls -la, which I normally call ll.  I had no success using the alias command in S6 init scripts. I found the following solution, by using the s6 pre_start_script to add a link ''ll.sh'' script in to the ''/bin'' directory.

<code bash>#get my favourite alias working in Alpine sh has been a pain
/bin/ln -s /app/scripts/ll.sh /bin/ll</code>

Where ''ll.sh'' is:
<code bash>#!/bin/sh
exec ls -la "$@"</code>
=====References=====
  *[[https://github.com/just-containers/s6-overlay| just-containers/s6-overlay]] (**Version 3.2.1.0** as of 2025-05-07, last checked 2025-06-22), note only a minor change since previous version
  *[[https://skarnet.org/software/s6-rc/s6-rc-compile.html|The s6-rc-compile program]] Describes the functionality of the S6-rc system
  *[[https://forum.level1techs.com/t/easy-to-follow-beginner-guide-on-s6-starter-pack/178411|Easy to follow Beginner Guide on s6 Starter Pack]]

It seems that the authors source pages [[https://skarnet.org|skarnet.org]] are often down or otherwise slow and reliable.  An alternative mirror is [https://skarnet.github.io/|skarnet.github.io]]

----

Other Supervisor software discussions:
  *[[https://ahmet.im/blog/minimal-init-process-for-containers/|Choosing an init process for multi-process containers]] - Interesting discussion on different init systems, current at that time. (Seems relevant now too.)
  *[[https://github.com/just-containers/s6-overlay/releases|s6-overlay]]
  *supervisord: [[http://supervisord.org/|Supervisor: A Process Control System]]
  *[[https://stackoverflow.com/questions/33117068/use-of-supervisor-in-docker|Use of Supervisor in docker]]
<- docker_notes:docker-compose|Back ^ docker_notes:index|Start page ^ docker_notes:docker-reverse-proxy|Next ->