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init.md

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+# init
+
+## 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
+
+Is the default BusyBox `/init` too bloated for you, minimalism freak?
+
+No problem, just use the `init` kernel boot parameter:
+
+    ./runqemu -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>:
+
+    ./runqemu -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
+
+### BusyBox init is fine, but networking timeout gets on my nerves
+
+I know, right?
+
+Add this line to `rootfs_post_build_script`:
+
+    rm -f "${1}/etc/init.d/"S*network
+
+To restore it, run:
+
+    ./run -t initscripts-reconfigure