This is an update to a previous posting, but using the new febootstrap 3.x cross-distro toolchain to make building supermin appliances even simpler.
Firstly we create a supermin appliance containing a few packages and their dependencies. Note that I’m not minimizing this appliance so it’s a bit bigger (3.1MB!) than the ones we would generate in reality:
$ mkdir supermin.d
$ febootstrap --names 'bash' 'coreutils' -o supermin.d
febootstrap: warning: some host files are unreadable by non-root
febootstrap: warning: get your distro to fix these files:
$ ls -lh supermin.d/
-rw-rw-r--. 1 rjones rjones 2.7M Dec 10 18:23 base.img
-rw-rw-r--. 1 rjones rjones 462K Dec 10 18:23 hostfiles
The purpose of these two files is explained in the febootstrap documentation.
This won’t boot without a /init script, and we can easily provide one:
$ cat init
echo Welcome to my world
$ chmod +x init
$ echo init | cpio -o -H newc --quiet > supermin.d/init.img
The 3 files in supermin.d/ are a supermin appliance, and could be packaged up in a Fedora, Debian or Ubuntu package.
When you actually want to come and launch this appliance, you use febootstrap-supermin-helper to reconstruct the appliance:
$ febootstrap-supermin-helper -f ext2 supermin.d x86_64 \
kernel initrd root
(Note that “kernel”, “initrd” and “root” are output files in that command)
You’d usually arrange for those files to be cached, since febootstrap-supermin-helper takes a few seconds to run (8 seconds on my laptop) and by caching it you can get reconstruction time down to a fraction of a second.
Now to boot, run qemu or qemu-kvm like this:
$ qemu-kvm -kernel kernel -initrd initrd -hda root
After a few seconds you’ll get to the shell: