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Ciro Santilli
2018-02-13 16:09:16 +00:00
parent d97d461605
commit 64130ecfb9
22 changed files with 316 additions and 569 deletions
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init.adoc
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@@ -1,30 +1,20 @@
[[init]]
= init
=== init
[[what-is-the-init-executable]]
== What is the init executable?
==== What is the init executable?
When the Linux kernel finishes booting, it runs an executable as the
first and only userland process.
When the Linux kernel finishes booting, it runs an executable as the first and only userland process.
The default path is `/init`, but we an set a custom one with the `init=`
kernel command line argument.
The default path is `/init`, but we an set a custom one with the `init=` kernel command line argument.
This process is then responsible for setting up the entire userland (or
destroying everything when you want to have fun).
This process is then responsible for setting up the entire userland (or destroying everything when you want to have fun).
This typically means reading some configuration files (e.g.
`/etc/initrc`) and forking a bunch of userland executables based on
those files.
This typically means reading some configuration files (e.g. `/etc/initrc`) and forking a bunch of userland executables based on those files.
systemd is a "popular" `/init` implementation for desktop distros as of
2017.
systemd is a "popular" `/init` implementation for desktop distros as of 2017.
BusyBox provides its own minimalistic init implementation which
Buildroot uses by default.
BusyBox provides its own minimalistic init implementation which Buildroot uses by default.
[[custom-init]]
== Custom init
==== Custom init
Is the default BusyBox `/init` too bloated for you, minimalism freak?
@@ -34,25 +24,20 @@ No problem, just use the `init` kernel boot parameter:
./run -e 'init=/sleep_forever.out'
....
Remember that shell scripts can also be used for `init`
https://unix.stackexchange.com/questions/174062/init-as-a-shell-script/395375#395375:
Remember that shell scripts can also be used for `init` https://unix.stackexchange.com/questions/174062/init-as-a-shell-script/395375#395375:
....
./run -e 'init=/count.sh'
....
Also remember that if your init returns, the kernel will panic, there
are just two non-panic possibilities:
Also remember that if your init returns, the kernel will panic, there are just two non-panic possibilities:
* run forever in a loop or long sleep
* `poweroff` the machine
[[disable-networking]]
== Disable networking
==== Disable networking
The default BusyBox init scripts enable networking, and there is a 15
second timeout in case your network is down or if your kernel / emulator
setup does not support it.
The default BusyBox init scripts enable networking, and there is a 15 second timeout in case your network is down or if your kernel / emulator setup does not support it.
To disable networking, use:
@@ -66,18 +51,12 @@ To restore it, run:
./build -t initscripts-reconfigure
....
[[the-init-environment]]
== The init environment
==== The init environment
The docs make it clear
https://www.kernel.org/doc/html/v4.14/admin-guide/kernel-parameters.html
The docs make it clear https://www.kernel.org/doc/html/v4.14/admin-guide/kernel-parameters.html
_____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
The kernel parses parameters from the kernel command line up to “–”; if
it doesn’t recognize a parameter and it doesn’t contain a ‘.’, the
parameter gets passed to init: parameters with ‘=’ go into init’s
environment, others are passed as command line arguments to init.
Everything after “–” is passed as an argument to init.
The kernel parses parameters from the kernel command line up to “–”; if it doesn’t recognize a parameter and it doesn’t contain a ‘.’, the parameter gets passed to init: parameters with ‘=’ go into init’s environment, others are passed as command line arguments to init. Everything after “–” is passed as an argument to init.
_____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
And you can try it out with: