Replace a Linux installation with Gentoo in-place

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This document gives an idea of the steps that need to be taken to replace a running linux system with Gentoo. Based on Jordan Ritter's article on replacing RHEL9 with Gentoo.

There is no guarantee that this will work. It has only been tested once. A long time ago. Backup all your data and make sure you have other means of returning the system back to a functioning state.
These instructions were only tested on an amd64 debian box with a 3.2.46-1+deb7u1 kernel of unknown configuration, so you might have to change some steps. Please think twice before replacing the kernel.

Prepare the system

We're going to need bzip2 for the stage3.

root #apt-get install bzip2

We're also about to overwrite fstab, so save it to /tmp.

root #cp /etc/fstab /tmp/fstab

Mark all present files as if they were modified at the beginning of time (which was in 1970) so that we can remove them later on. Modprobe is temporarily removed as it might potentially slow down this proccess.

root #mv /sbin/modprobe /sbin/.modprobe
root #find / -print0 | xargs -0 touch -d "1970-01-01 00:00 +0000" -m
root #mv /sbin/.modprobe /sbin/modprobe

Overwrite root with Gentoo

Extract the latest stage3 tarball over the current system.

root #tar xjpf /tmp/stage3* -C /

Gentoo's /lib and /usr/lib are symlinks and tar if tar warned you about this, you need to fix them.

root #mv /lib /lib_ && ln -s /lib64 /lib
root #mv /usr/lib /usr/lib_ && ln -s /usr/lib64 /usr/lib

Recover fstab from /tmp as it has been overwritten by tar. You can also copy it from /etc/mtab.

root #cp /tmp/fstab /etc/fstab

Bring the Gentoo system to a bootable state

You will want to be able to log in to your new system even if network/sshd fails, so set a root password.

root #passwd

Enable your network interfaces.

root #cd /etc/init.d
root #ln -s net.lo net.eth0
root #vim /etc/conf.d/net
root #rc-update add net.eth0

Prevent any network device renaming surprises.

root #ln -s /dev/null /etc/udev/rules.d/80-net-name-slot.rules

Fetch dhcpcd if you're going to be using DHCP. Otherwise, set up your interfaces manually.

root #emerge-webrsync
root #emerge dhcpcd

Set the timezone.

root #TIMEZONE=Continent/City
root #cp /usr/share/zoneinfo/$TIMEZONE /etc/localtime
root #echo $TIMEZONE > /etc/timezone

Enable sshd for remote access

root #rc-update add sshd

Feel free to test the system and finally reboot. You might have to use -f.

root #reboot

Remove anything that was left behind by the previous OS

If everything is now working as expected, go ahead and delete all the old files. /home and /root are spared.

root #rm -rf /lib_ /usr/lib_
root #find /bin /sbin /etc /lib /usr /var -newermt "1970-01-01" ! -newermt "1970-01-02" -print0 | xargs -0 rm -rf
`find -newerXY` needs a reasonably new `find` (>=4.3.3). If you do not have it, refer to the link above for alternate predicates