adoc

init.adoc

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+[[init]]
+= init
+
+[[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.
+
+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 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.
+
+BusyBox provides its own minimalistic init implementation which
+Buildroot uses by default.
+
+[[custom-init]]
+== Custom init
+
+Is the default BusyBox `/init` too bloated for you, minimalism freak?
+
+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:
+
+....
+./run -e 'init=/count.sh'
+....
+
+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
+
+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:
+
+....
+./build -p -n
+....
+
+To restore it, run:
+
+....
+./build -t initscripts-reconfigure
+....
+
+[[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 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:
+
+....
+./run -e 'init=/init_env_poweroff.sh - asdf=qwer zxcv' -n
+....