Traditionally on most Linux distributions, regular (human) users are managed via entries in /etc/passwd, /etc/shadow, /etc/group and /etc/gshadow. With the advent of systemd-homed it might be desirable to convert an existing, traditional user account to a systemd-homed managed one. Below is a brief guide how to do that.
Before continuing, please read up on these basic concepts:
This is a manual process, and possibly a bit fragile. Hence, do this at your own risk, read up beforehand, and make a backup first. You know what's at stake: your own home directory, i.e. all your personal data.
Here's the step-by-step guide:
Preparations: make sure you run a distribution that has systemd-homed enabled and properly set up, including the necessary PAM and NSS configuration updates. Make sure you have enough disk space in /home/ for a (temporary) second copy of your home directory. Make sure to backup your home directory. Make sure to log out of your user account fully. Then log in as root on the console.
Rename your existing home directory to something safe. Let's say your user ID is foobar. Then do:
mv /home/foobar /home/foobar.saved
Have a look at your existing user record, as stored in /etc/passwd and related files. We want to use the same data for the new record, hence it's good looking at the old data. Use commands such as:
getent passwd foobar getent shadow foobar
This will tell you the /etc/passwd and /etc/shadow entries for your user. For details about the fields, see the respective man pages passwd(5) and shadow(5).
The fourth field in the getent passwd foobar output tells you the GID of your user‘s main group. Depending on your distribution it’s a group private to the user, or a group shared by most local, regular users. Let‘s say the GID reported is 1000, let’s then query its details:
getent group 1000
This will tell you the name of that group. If the name is the same as your user name your distribution apparently provided you with a private group for your user. If it doesn‘t match (and is something like users) it apparently didn’t. Note that systemd-homed will always manage a private group for each user under the same name, hence if your distribution is one of the latter kind, then there's a (minor) mismatch in structure when converting.
Save the information reported by these three commands somewhere, for later reference.
Now edit your /etc/passwd file and remove your existing record (i.e. delete a single line, the one of your user's account, leaving all other lines unmodified). Similar for /etc/shadow, /etc/group (in case you have a private group for your user) and /etc/gshadow. Most distributions provide you with a tool for that, that adds safe synchronization for these changes: vipw, vipw -s, vigr and vigr -s.
At this point the old user account vanished, while the home directory still exists safely under the /home/foobar.saved name. Let's now create a new account with systemd-homed, using the same username and UID as before:
homectl create foobar --uid=$UID --real-name=$GECOS
In this command line, replace $UID by the UID you previously used, i.e. the third field of the getent passwd foobar output above. Similar, replace $GECOS by the GECOS field of your old account, i.e the fifth field of the old output. If your distribution traditionally does not assign a private group to regular user groups, then consider adding --member-of= with the group name to get a modicum of compatibility with the status quo ante: this way your new user account will still not have the old primary group as new primary group, but will have it as auxiliary group.
Consider reading through the homectl(1) manual page at this point, maybe there are a couple of other settings you want to set for your new account. In particular, look at --storage= and --disk-size=, in order to change how your home directory shall be stored (the default luks storage is recommended).
Your new user account exists now, but it has an empty home directory. Let‘s now migrate your old home directory into it. For that let’s mount the new home directory temporarily and copy the data in.
homectl with foobar -- rsync -aHAXv --remove-source-files /home/foobar.saved/ .
This mounts the home directory of the user, and then runs the specified rsync command which copies the contents of the old home directory into the new. The new home directory is the working directory of the invoked rsync process. We are invoking this command as root, hence the rsync runs as root too. When the rsync command completes the home directory is automatically unmounted again. Since we used --remove-source-files all files copied are removed from the old home directory as the copy progresses. After the command completes the old home directory should be empty. Let's remove it hence:
rmdir /home/foobar.saved
And that's it, we are done already. You can log out now and should be able to log in under your user account as usual, but now with systemd-homed managing your home directory.