dpkg/man/update-alternatives.pod

# dpkg manual page - update-alternatives(1)
#
# Copyright © 1997-1998 Charles Briscoe-Smith
# Copyright © 1999 Ben Collins <bcollins@debian.org>
# Copyright © 2000 Wichert Akkerman <wakkerma@debian.org>
# Copyright © 2003 Adam Heath <doogie@debian.org>
# Copyright © 2005 Scott James Remnant <scott@netsplit.com>
# Copyright © 2006-2015 Guillem Jover <guillem@debian.org>
# Copyright © 2008 Pierre Habouzit <madcoder@debian.org>
# Copyright © 2009-2011 Raphaël Hertzog <hertzog@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
# the Free Software Foundation; either version 2 of the License, or
# (at your option) any later version.
#
# This is distributed in the hope that it will be useful,
# 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.
#
# 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

update-alternatives - maintain symbolic links determining default commands

=head1 SYNOPSIS

B<update-alternatives>
[I<option>...] I<command>

=head1 DESCRIPTION

B<update-alternatives>
creates, removes, maintains and displays information about the symbolic
links comprising the alternatives system.

It is possible for several programs fulfilling the same or similar
functions to be installed on a single system at the same time.
For example, many systems have several text editors installed at once.
This gives choice to the users of a system, allowing each to use a
different editor, if desired, but makes it difficult for a program
to make a good choice for an editor to invoke if the
user has not specified a particular preference.

The alternatives system aims to solve this problem.
A generic name in the filesystem is
shared by all files providing interchangeable functionality.
The alternatives system and the system administrator
together determine which actual file is referenced by this generic name.
For example, if the text editors
L<ed(1)>
and
L<nvi(1)>
are both installed on the system, the alternatives system will cause
the generic name
I</usr/bin/editor>
to refer to
I</usr/bin/nvi>
by default.
The system administrator can override this and cause
it
to refer to
I</usr/bin/ed>
instead,
and the alternatives system will not alter this setting until explicitly
requested to do so.

The generic name is not a direct symbolic link to the selected alternative.
Instead, it is a symbolic link to a name in the
I<alternatives>
I<directory>,
which in turn is a symbolic link to the actual file referenced.
This is done so that the system administrator's changes can be confined
within the
I<%CONFDIR%>
directory: the FHS (q.v.) gives reasons why this is a Good Thing.

When each package
providing a file with a particular functionality is
installed, changed or removed,
B<update-alternatives>
is called to update information about that file in the alternatives system.
B<update-alternatives>
is usually called from the following Debian package maintainer scripts,
B<postinst>
(configure) to install the alternative and from
B<prerm> and B<postrm>
(remove) to remove the alternative.
B<Note>: In most (if not all) cases no other maintainer script actions should
call B<update-alternatives>, in particular neither of B<upgrade> nor
B<disappear>, as any other such action can lose the manual state of an
alternative, or make the alternative temporarily flip-flop, or completely
switch when several of them have the same priority.

It is often useful for a number of alternatives to be synchronized,
so that they are changed as a group; for example, when several versions
of the
L<vi(1)>
editor are installed, the manual page referenced by
I</usr/share/man/man1/vi.1>
should correspond to the executable referenced by
I</usr/bin/vi>.
B<update-alternatives>
handles this by means of
I<master>
and
I<slave>
links; when the master is changed, any associated slaves are changed
too.
A master link and its associated slaves make up a
I<link>
I<group>.

Each link group is, at any given time,
in one of two modes: automatic or manual.
When a group is in automatic mode, the alternatives system will
automatically decide, as packages are installed and removed,
whether and how to update the links.
In manual mode, the alternatives system will retain the choice of
the administrator and avoid changing the links (except when something is
broken).

Link groups are in automatic mode when they are first introduced to
the system.
If the system administrator makes changes to the system's
automatic settings,
this will be noticed the next time
B<update-alternatives>
is run on the changed link's group,
and the group will automatically be switched to manual mode.

Each alternative has a
I<priority>
associated with it.
When a link group is in automatic mode,
the alternatives pointed to by members of the group
will be those which have the highest priority.

When using the
B<--config>
option,
B<update-alternatives>
will list all of the choices for the link group
of which given
I<name>
is the master alternative name.
The current choice is marked with a ‘*’.
You will then be prompted for your choice regarding this link group.
Depending on the choice made, the link group might no longer be in
I<auto>
mode.
You will need to use the
B<--auto>
option in order to return to the automatic mode (or you can rerun
B<--config>
and select the entry marked as automatic).

If you want to configure non-interactively you can use the
B<--set>
option instead (see below).

Different packages providing the same file need to do so
B<cooperatively>.
In other words, the usage of
B<update-alternatives>
is
B<mandatory>
for all involved packages in such case.
It is not possible to
override some file in a package that does not employ the
B<update-alternatives>
mechanism.

=head1 TERMINOLOGY

Since the activities of
B<update-alternatives>
are quite involved, some specific terms will help to explain its
operation.

=over

=item generic name (or alternative link)

A name, like
I</usr/bin/editor>,
which refers, via the alternatives system, to one of a number of
files of similar function.

=item alternative name

The name of a symbolic link in the alternatives directory.

=item alternative (or alternative path)

The name of a specific file in the filesystem, which may be made
accessible via a generic name using the alternatives system.

=item alternatives directory

A directory, by default
I<%CONFDIR%/alternatives>,
containing the symlinks.

=item administrative directory

A directory, by default
I<%ADMINDIR%/alternatives>,
containing
B<update-alternatives>'
state information.

=item link group

A set of related symlinks, intended to be updated as a group.

=item master link

The alternative link in a link group which determines how the other links in the
group are configured.

=item slave link

An alternative link in a link group which is controlled by the setting of
the master link.

=item automatic mode

When a link group is in automatic mode,
the alternatives system ensures that the links in the group
point to the highest priority alternative
appropriate for the group.

=item manual mode

When a link group is in manual mode,
the alternatives system will not make any changes
to the system administrator's settings.

=back

=head1 COMMANDS

=over

=item B<--install> I<link name path priority> [B<--slave> I<link name path>]...

Add a group of alternatives to the system.
I<link>
is the generic name for the master link,
I<name>
is the name of its symlink in the alternatives directory, and
I<path>
is the alternative being introduced for the master link.
The arguments after B<--slave> are the generic name, symlink name in the
alternatives directory and the alternative path for a slave link.
Zero or more
B<--slave>
options, each followed by three arguments,
may be specified.
Note that the master alternative must exist or the call will fail.
However if a slave alternative doesn't exist, the corresponding
slave alternative link will simply not be installed (a warning will still
be displayed).
If some real file is installed where an alternative link
has to be installed, it is kept unless B<--force> is used.

If the alternative name specified exists already
in the alternatives system's records,
the information supplied will be added as a new
set of alternatives for the group.
Otherwise, a new group, set to automatic mode,
will be added with this information.
If the group is in automatic mode,
and the newly added alternatives' priority is higher than
any other installed alternatives for this group,
the symlinks will be updated to point to the newly added alternatives.

=item B<--set> I<name> I<path>

Set the program
I<path>
as alternative for
I<name>.
This is equivalent to
B<--config>
but is non-interactive and thus scriptable.

=item B<--remove> I<name> I<path>

Remove an alternative and all of its associated slave links.
I<name>
is a name in the alternatives directory, and
I<path>
is an absolute filename to which
I<name>
could be linked.
If
I<name>
is indeed linked to
I<path>,
I<name>
will be updated to point to another appropriate alternative
(and the group is put back in automatic mode), or
removed if there is no such alternative left.
Associated slave links will be updated or removed, correspondingly.
If the link is not currently pointing to
I<path>,
no links are changed;
only the information about the alternative is removed.

=item B<--remove-all> I<name>

Remove all alternatives and all of their associated slave links.
I<name>
is a name in the alternatives directory.

=item B<--all>

Call B<--config> on all alternatives.
It can be usefully combined with
B<--skip-auto> to review and configure all alternatives which are
not configured in automatic mode.
Broken alternatives are also displayed.
Thus a simple way to fix all broken alternatives is to call
B<yes '' | update-alternatives --force --all>.

=item B<--auto> I<name>

Switch the link group behind the alternative for
I<name>
to automatic mode.
In the process, the master symlink and its slaves are updated
to point to the highest priority installed alternatives.

=item B<--display> I<name>

Display information about the link group.
Information displayed includes the group's mode
(auto or manual),
the master and slave links,
which alternative the master link currently points to,
what other alternatives are available
(and their corresponding slave alternatives),
and the highest priority alternative currently installed.

=item B<--get-selections>

List all master alternative names (those controlling a link group)
and their status (since version 1.15.0).
Each line contains up to 3 fields (separated by
one or more spaces).
The first field is the alternative name, the second
one is the status (either B<auto> or B<manual>), and the last one contains
the current choice in the alternative (beware: it's a filename and thus
might contain spaces).

=item B<--set-selections>

Read configuration of alternatives on standard input in the format
generated by B<--get-selections> and reconfigure
them accordingly (since version 1.15.0).

=item B<--query> I<name>

Display information about the link group
like B<--display> does, but in a machine parseable way
(since version 1.15.0, see section L</QUERY FORMAT> below).

=item B<--list> I<name>

Display all targets of the link group.

=item B<--config> I<name>

Show available alternatives for a link group and allow the user to
interactively select which one to use.
The link group is updated.

=item B<--help>

Show the usage message and exit.

=item B<--version>

Show the version and exit.

=back

=head1 OPTIONS

=over

=item B<--altdir> I<directory>

Specifies the alternatives directory, when this is to be
different from the default.
Defaults to «I<%CONFDIR%/alternatives>».

=item B<--admindir> I<directory>

Specifies the administrative directory, when this is to be
different from the default.
Defaults to «I<%ADMINDIR%/alternatives>» if B<%ADMINDIR_ENVVAR%> has not
been set.

=item B<--instdir> I<directory>

Specifies the installation directory where alternatives links will be created
(since version 1.20.1).
Defaults to «I</>» if B<%INSTDIR_ENVVAR%> has not been set.

=item B<--root> I<directory>

Specifies the root directory (since version 1.20.1).
This also sets the alternatives, installation and administrative
directories to match.
Defaults to «I</>» if B<%INSTDIR_ENVVAR%> has not been set.

=item B<--log> I<file>

Specifies the log file (since version 1.15.0), when this is to be different
from the default (%LOGDIR%/alternatives.log).

=item B<--force>

Allow replacing or dropping any real file that is installed
where an alternative link has to be installed or removed.

=item B<--skip-auto>

Skip configuration prompt for alternatives which are properly configured
in automatic mode.
This option is only relevant with B<--config> or
B<--all>.

=item B<--quiet>

Do not generate any comments unless errors occur.

=item B<--verbose>

Generate more comments about what is being done.

=item B<--debug>

Generate even more comments, helpful for debugging, about what is being done
(since version 1.19.3).

=back

=head1 EXIT STATUS

=over

=item B<0>

The requested action was successfully performed.

=item B<2>

Problems were encountered whilst parsing the command line
or performing the action.

=back

=head1 ENVIRONMENT

=over

=item B<%INSTDIR_ENVVAR%>

If set and the B<--instdir> or B<--root> options have not been
specified, it will be used as the filesystem root directory.

=item B<%ADMINDIR_ENVVAR%>

If set and the B<--admindir> option has not been specified, it will
be used as the base administrative directory.

=back

=head1 FILES

=over

=item I<%CONFDIR%/alternatives/>

The default alternatives directory.
Can be overridden by the
B<--altdir>
option.

=item I<%ADMINDIR%/alternatives/>

The default administration directory.
Can be overridden by the
B<--admindir>
option.

=back

=head1 QUERY FORMAT

The B<--query> format is using an
RFC822-like flat format.
It's made of I<n> + 1 stanzas where I<n> is the number of alternatives
available in the queried link group.
The first
stanza contains the following fields:

=over

=item B<Name:> I<name>

The alternative name in the alternative directory.

=item B<Link:> I<link>

The generic name of the alternative.

=item B<Slaves:> I<list-of-slaves>

When this field is present, the B<next> lines hold all slave links
associated to the master link of the alternative.
There is one slave per line.
Each line contains one space, the generic name of the slave
alternative, another space, and the path to the slave link.

=item B<Status:> I<status>

The status of the alternative (B<auto> or B<manual>).

=item B<Best:> I<best-choice>

The path of the best alternative for this link group.
Not present if
there is no alternatives available.

=item B<Value:> I<currently-selected-alternative>

The path of the currently selected alternative.
It can also take the magic value B<none>.
It is used if the link doesn't exist.

=back

The other stanzas describe the available alternatives in the queried
link group:

=over

=item B<Alternative:> I<path-of-this-alternative>

Path to this stanza's alternative.

=item B<Priority:> I<priority-value>

Value of the priority of this alternative.

=item B<Slaves:> I<list-of-slaves>

When this field is present, the B<next> lines hold all slave alternatives
associated to the master link of the alternative.
There is one slave per line.
Each line contains one space, the generic name of the slave
alternative, another space, and the path to the slave alternative.

=back

=head2 Example

 $ update-alternatives --query editor
 Name: editor
 Link: /usr/bin/editor
 Slaves:
  editor.1.gz /usr/share/man/man1/editor.1.gz
  editor.fr.1.gz /usr/share/man/fr/man1/editor.1.gz
  editor.it.1.gz /usr/share/man/it/man1/editor.1.gz
  editor.pl.1.gz /usr/share/man/pl/man1/editor.1.gz
  editor.ru.1.gz /usr/share/man/ru/man1/editor.1.gz
 Status: auto
 Best: /usr/bin/vim.basic
 Value: /usr/bin/vim.basic

 Alternative: /bin/ed
 Priority: -100
 Slaves:
  editor.1.gz /usr/share/man/man1/ed.1.gz

 Alternative: /usr/bin/vim.basic
 Priority: 50
 Slaves:
  editor.1.gz /usr/share/man/man1/vim.1.gz
  editor.fr.1.gz /usr/share/man/fr/man1/vim.1.gz
  editor.it.1.gz /usr/share/man/it/man1/vim.1.gz
  editor.pl.1.gz /usr/share/man/pl/man1/vim.1.gz
  editor.ru.1.gz /usr/share/man/ru/man1/vim.1.gz

=head1 DIAGNOSTICS

With B<--verbose>
B<update-alternatives>
chatters incessantly about its activities on its standard output channel.
If problems occur,
B<update-alternatives>
outputs error messages on its standard error channel and
returns an exit status of 2.
These diagnostics should be self-explanatory;
if you do not find them so, please report this as a bug.

=head1 EXAMPLES

There are several packages which provide a text editor compatible
with B<vi>, for example B<nvi> and B<vim>.
Which one is used
is controlled by the link group B<vi>, which includes links for the
program itself and the associated manual page.

To display the available packages which provide B<vi> and the current
setting for it, use the B<--display> action:

=over

 update-alternatives --display vi

=back

To choose a particular B<vi> implementation, use this command as root
and then select a number from the list:

=over

 update-alternatives --config vi

=back

To go back to having the B<vi> implementation chosen automatically, do
this as root:

=over

 update-alternatives --auto vi

=back

=head1 SEE ALSO

L<ln(1)>,
FHS (the Filesystem Hierarchy Standard).