I 100% blame Mr Ellis for this..
Good question .. to understand this, we need to understand the boot process!
tl;dr Once a kernel "sorts out" the hardware it does one last thing and that is to start process 1, which is 🥁 ... the init process!
Note: if this init process unexpectedly dies (without telling the kernel to reboot or power off), then you'll get a kernel panic! So don't attempt to kill init PEOPLE!
[ 10.032969] ---[ end Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000200 ]---
The job of init is to set up the Linux environment by doing the following:
- Creating a virtual filesystem
- Mounting various devices
- Mounting root filesystems
- Perhaps do some networking
- Make a lovely brew
- Start another process, that may start others (such as systemd)
- Then wait to be told to power off or reboot etc.. but never die!
The ginit is actually a few things.. but the main part is some go code that will execute as /init and set up the environment for you.. in most cases it will then drop to a busybox shell. There is where you can modify it to do what ever you like!
The ginit.sh script will build the code/busybox and create a initrd container image, it will then extract the initrd.tar.gz to the local filesystem for you. It will also grab you a local copy of the latest Ubuntu Kernel.
The image2disk.sh <name:tag> script will pull a docker image and convert it to a raw disk you can then start with ginit etc..
qemu-system-x86_64 -nographic \
-kernel ./linux \
-append "entrypoint=/docker-entrypoint.sh root=/dev/sda console=ttyS0" \
-initrd ./initramfs.cpio.gz \
-hda ./disk.img \
-m 1G
Note: the -append is where we pass things to be read at runtime.. the k/v entrypoint=.. is where you pass what you want to start from the disk image!
Just cross your fingers it works first time?