dpkg-fsys-usrunmess - undoes the merged-/usr-via-aliased-dirs mess
dpkg-fsys-usrunmess [option...]
dpkg-fsys-usrunmess is a tool to fix up filesystems that have been installed anew with recent installers with unfortunate defaults or migrated to the broken merged /usr via aliased directories layout, which is not supported by dpkg. See the dpkg FAQ.
Note: In Debian its tech-ctte has decreed that on the release after bookworm the non-usrmerged layout is not going to be supported, and thus some of its maintainers might not fix issues or intentionally remove non-usrmerged support, so you will need decide and pick your poison.
The program will perform the following overall actions:
Check whether the system needs to be switched, otherwise do nothing,
Check for dpkg database consistency and otherwise abort.
Generate and install a regression prevention package, if requested on the command-line or otherwise on the prompt.
Get the list of files and alternatives that need to be restored.
Look for untracked kernel modules files that need to be moved too.
Create a shadow hierarchy under /.usrunmess, by creating the directories symlinks or hardlinking or copying the files as needed.
Prompt for confirmation before proceeding, if requested on the command-line.
Lock the dpkg database.
Mark all packages as half-configured to force running maintainer scripts that might need to recreate files.
Replace the aliased directories with the shadow ones, by creating a backup of the old symlinked directories and renaming the shadow ones over.
Relabel MAC information for directories and symlinks if necessary.
Remove backup symlinks.
Remove old moved objects, but defer directory removal.
Remove old deferred directories that are not referenced by dpkg-query.
Remove shadow root directory.
Register a policy-rc.d to disallow service actions, which means that services might need to be restarted afterwards, ideally via a reboot.
Reconfigure all packages.
Unregister the policy-rc.d and restore the alternatives state.
Note: When running the program from some shells such
as bash (1) or zsh (1), after
executing it, you might need to request the shell to forget all
remembered executable locations with for example
hash -r.
Note: Some directories might linger after the migration in case they contain untracked files. A list is printed once the script has finished for further investigation.
Warning: Note that this operation has the potential to render the system unusable or broken in case of a sudden crash or reboot, unexpected state of the system, or possible bugs in the script. Be prepared with recovery media and consider doing backups beforehand.
Prompt at the time of no return, so that the debug output or the shadow hierarchy can be evaluated before proceeding.
Enables or disables generating and installing a regression prevention package into the system. If no option has been specified, the action to take will be prompted. The generated package contains the Protected field set to yes to protect against accidental removal of the package.
This option enables the dry-run mode, where no destructive action takes place, only the preparatory part.
Show the usage message and exit.
Show the version and exit.
This setting defines whether to enable dry-run mode.
<https://wiki.debian.org/Teams/Dpkg/FAQ#broken-usrmerge>.