s6 Init Sequence - blitterated/docker-dev-env-s6 GitHub Wiki

First read this description of s6's init sequence and code

s6 initialization sequence

docker run
- /init
  - s6-overlay-suexec
    - preinit
    - stage0
      - s6-linux-init-maker (docs) - configures /run/s6/basedir/bin/init below with s6-overlay environment variable customizations.
        
        make_env() - needs more research. where is autofile()?
        
        make_image() - auto_*() ???
        
        make_scripts() - auto_script() ???
        
        make_bins()
      - exec /run/s6/basedir/bin/init - only exists in container. Created by s6-linux-init-maker.
        
        s6-linux-init (docs) - LOTS of tty stuff in here!
        
        s6-linux-init-telinit - will execute this instead if s6-linux-init's PID is not 1.

`stage0` call to `s6-linux-init-maker`

Here's the call to s6-linux-init-maker, which can be found in stage0 here.

s6-linux-init-maker -NC -D top -c "$basedir" -p "$PATH" -f /package/admin/s6-overlay-@VERSION@/etc/s6-linux-init/skel $stampopt $dumpopt $logopt $loguseropt $graceopt $syncopt -- "$basedir"

Impact on s6-linux-init-maker variable values:

| switch | variable | value | comments | | ----- | ----- | ----- | | -N | mounttype | 0 | passed in as -NC | | -C | inns | 1 | passed in as -NC | | -D | initdefault | top | | | -c | robase | /run/s6/basedir | passed as -c "$basedir". basedir set here | | -f | skeldir | /package/admin/s6-overlay-@VERSION@/etc/s6-linux-init/skel | Directory contents can be seen here | | -p | initial_path | /command:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin | passed as -p "$PATH". This picks up on what was set in /init. See here for how it's added. See here, here, and here for defaults. | | -s | env_store | /run/s6/container_environment | set due to S6_KEEP_ENV | | -B | nologger | 1 | set due to lack of S6_LOGGING |

`s6-linux-init-maker` generated init script

Dumped in container with:

cat /run/s6/basedir/bin/init

Contents:

#!/package/admin/execline/command/execlineb -S0

/package/admin/s6-linux-init/command/s6-linux-init -c "/run/s6/basedir" -m 0022 -p "/command:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin" -s "/run/s6/container_environment" -D "top" -N -C -B -- "$@"

Impact on s6-linux-init variable values:

switch	variable	value
`-c`	`basedir`	`/run/s6/basedir`
`-m`	`if (!uint0_oscan(l.arg, &mask)) dieusage()`	Default for `mask` is `0022`, `root`'s `umask`. I think this is checking that `umask` matches either `root` or another explicitly specified user's `umask`
`-p`	`path`	`/command:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin`
`-s`	`envdumpdir`	`/run/s6/container_environment`
`-D`	`initdefault`	`top`
`-N`	`mounttype`	`0`
`-C`	`inns`	`1`
`-B`	`nologger`	`1`

Other observations

switch	var name	info	links
`-d`	`slashdev`	specify a devtmpfs directory overriding `udev` and `/dev` tmpfs	using devtmpfs for /dev
`-C`	`inns`	is this used to specify an init runlevel?	tldp: Run Levels, s6-linux-init code that looks like run levels