# dpkg manual page - dpkg-fsys-usrunmess(8)
#
# Copyright © 2020-2021 Guillem Jover <guillem@debian.org>
#
# This is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
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#
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# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
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# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <https://www.gnu.org/licenses/>.

=encoding utf8

=head1 NAME

dpkg-fsys-usrunmess - undoes the merged-/usr-via-aliased-dirs mess

=head1 SYNOPSIS

B<dpkg-fsys-usrunmess> [B<option>...]

=head1 DESCRIPTION

B<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 B<not> supported by dpkg.
See the dpkg FAQ.

B<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:

=over

=item *

Check whether the system needs to be switched, otherwise do nothing,

=item *

Check for dpkg database consistency and otherwise abort.

=item *

Generate and install a regression prevention package, if requested on the
command-line or otherwise on the prompt.

=item *

Get the list of files and alternatives that need to be restored.

=item *

Look for untracked kernel modules files that need to be moved too.

=item *

Create a shadow hierarchy under F</.usrunmess>, by creating the directories
symlinks or hardlinking or copying the files as needed.

=item *

Prompt for confirmation before proceeding, if requested on the command-line.

=item *

Lock the dpkg database.

=item *

Mark all packages as half-configured to force running maintainer scripts
that might need to recreate files.

=item *

Replace the aliased directories with the shadow ones, by creating a backup
of the old symlinked directories and renaming the shadow ones over.

=item *

Relabel MAC information for directories and symlinks if necessary.

=item *

Remove backup symlinks.

=item *

Remove old moved objects, but defer directory removal.

=item *

Remove old deferred directories that are not referenced by dpkg-query.

=item *

Remove shadow root directory.

=item *

Register a B<policy-rc.d> to disallow service actions, which means that
services might need to be restarted afterwards, ideally via a reboot.

=item *

Reconfigure all packages.

=item *

Unregister the B<policy-rc.d> and restore the alternatives state.

=back

B<Note>: When running the program from some shells such as L<bash(1)> or
L<zsh(1)>, after executing it, you might need to request the shell to
forget all remembered executable locations with for example C<hash -r>.

B<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.

B<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.

This program was introduced in dpkg 1.20.6.

=head1 OPTIONS

=over

=item B<-p>, B<--prompt>

Prompt at the time of no return, so that the debug output or the shadow
hierarchy can be evaluated before proceeding.

=item B<--prevention>

=item B<--no-prevention>

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 B<Protected> field set to B<yes> to
protect against accidental removal of the package.

=item B<-n>, B<--no-act>

=item B<--dry-run>

This option enables the dry-run mode, where no destructive action takes place,
only the preparatory part.

=item B<-?>, B<--help>

Show the usage message and exit.

=item B<--version>

Show the version and exit.

=back

=head1 ENVIRONMENT

=over

=item B<DPKG_USRUNMESS_NOACT>

This setting defines whether to enable dry-run mode.

=back

=head1 SEE ALSO

L<https://wiki.debian.org/Teams/Dpkg/FAQ#broken-usrmerge>.
