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67 lines
2.7 KiB
Plaintext
67 lines
2.7 KiB
Plaintext
=== init
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==== What is the init executable?
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When the Linux kernel finishes booting, it runs an executable as the first and only userland process.
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The default path is `/init`, but we an set a custom one with the `init=` kernel command line argument.
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This process is then responsible for setting up the entire userland (or destroying everything when you want to have fun).
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This typically means reading some configuration files (e.g. `/etc/initrc`) and forking a bunch of userland executables based on those files.
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systemd is a "popular" `/init` implementation for desktop distros as of 2017.
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BusyBox provides its own minimalistic init implementation which Buildroot uses by default.
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==== Custom init
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Is the default BusyBox `/init` too bloated for you, minimalism freak?
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No problem, just use the `init` kernel boot parameter:
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....
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./run -e 'init=/sleep_forever.out'
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....
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Remember that shell scripts can also be used for `init` https://unix.stackexchange.com/questions/174062/init-as-a-shell-script/395375#395375:
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....
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./run -e 'init=/count.sh'
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....
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Also remember that if your init returns, the kernel will panic, there are just two non-panic possibilities:
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* run forever in a loop or long sleep
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* `poweroff` the machine
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==== Disable networking
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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.
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To disable networking, use:
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....
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./build -p -n
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....
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To restore it, run:
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....
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./build -t initscripts-reconfigure
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....
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==== The init environment
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The docs make it clear https://www.kernel.org/doc/html/v4.14/admin-guide/kernel-parameters.html
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_____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
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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.
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_____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
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And you can try it out with:
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....
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./run -e 'init=/init_env_poweroff.sh - asdf=qwer zxcv' -n
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....
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