Tip: Creating throwaway appliances with febootstrap

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:
	/sbin/unix_update
	/usr/libexec/pt_chown
	/usr/sbin/build-locale-archive
	/usr/sbin/glibc_post_upgrade.x86_64
	/usr/sbin/tzdata-update
$ ls -lh supermin.d/
total 3.1M
-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
#!/bin/bash -
echo Welcome to my world
bash -i
$ 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: