Migrating from Gentoo to Archlinux (part 3/3)

In the first part, I explained why I decided to switch away from Gentoo. In the second, I explained how to install Archlinux in a chroot, and install packages identical to those on your Gentoo installation.

In this last post, I give a few hints on how to configure Archlinux, and how to switch to your new system.

Configuring Archlinux

The general idea is the following: look at files in /etc on your Gentoo installation, and update the equivalent ones in /arch/etc/. For example, /etc/fstab can simply be copied over, so are most application specific files (say, ssh/*, screenrc, hibernate/*, sudoers, etc.). Do not overwrite system files (rc.d/*, conf.d/*), nor passwd/shadow/group files.

User configuration

One of the critical thing is to recreate the users with the same uid/gid, so that you don't need to change file ownership on your existing filesystems.

In my case, I simply copied over the relevant lines in shadow, groups, passwd, but this can be done more cleanly by using the "-u" parameter on the useradd command, e.g.:

# useradd -u 1000 -g 1002 user

Groups may have different names and meanings on Archlinux, so have a look at the list, and check what groups you need to put your users into.

Don't forget to set a root password as well.

X11 configuration

One of the differences between Gentoo and Archlinux, at least on my installation, is that Archlinux uses a set of files in /etc/X11/xorg.conf.d instead of a single /etc/X11/xorg.conf. But again, nothing that can't be fixed by moving parts of xorg.conf to the relevant files in /etc/X11/xorg.conf.d.

Do not configure the bootloader yet!

Leave that part for later. However install a kernel, and copy the relevant files (vmlinuz-linux, initramfs-linux.img) from /arch/boot/ to /boot.
I still had an legacy grub bootloader (versions 0.9x) on my Gentoo install, so I just added these lines in /etc/grub/menu.lst:

title Archlinux
root (hd0,2)
kernel /boot/vmlinuz-linux root=/dev/sda9 ro nofb
initrd  /boot/initramfs-linux.img

(read the documentation to figure out the new syntax if you already switched to the new grub 2.x on Gentoo)

Follow the guide!

For the rest, the Archlinux wiki is particularly well written, look for pages on how to configure the network, start required services, etc.

Rebooting on a rescue CD/USB stick

Now the fun part! As a live Linux system, I used systemrescuecd, installed on a USB stick (who uses CD anymore?). It would also come in handy in case you break everything and cannot boot your system.
Reboot from the USB stick, mount your root filesystem:

# mkdir /mnt/sda9 && mount /dev/sda9 /mnt/sda9

And simply swap the content of the root filesystem:

# cd /mnt/sda9
# mkdir gentoo
# mv * gentoo (this will refuse to move gentoo inside itself, which is perfectly fine, however this also moves arch to gentoo/arch, which we don't want:)
# mv gentoo/arch .

Now move the /arch root filesystem to /:

# mv arch/* .

If /boot is a separate partition, you are ready to reboot. If it is not, you want to copy the old /boot directory:

# mv boot boot.arch
# mv gentoo/boot .

And then you can reboot.

Enjoy Archlinux!

Provided you did everything properly, and configured all services, you should have a working Archlinux system on the next reboot!

It didn't work for me ,-) I did not configure the wireless connection properly, and did not know how to fix it without the help of the Archlinux wiki. So I rebooted on the rescue USB stick, moved back the Gentoo root filesystem, rebooted, fixed the configuration in the chroot, and back again. Easy!

If everything works well, you should install grub from Archlinux, so that you have a /boot directory that comes from Archlinux.

Finally, after making a backup of /gentoo/etc, just in case, you can happily:

rm -rf /gentoo