update-alternatives - maintain symbolic links determining default commands
update-alternatives [
option...]
command
update-alternatives creates, removes, maintains and displays information
about the symbolic links comprising the Debian 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.
Debian's 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
ed(1) and
nvi(1) are both installed on the system, the
alternatives system will cause the generic name
/usr/bin/editor to
refer to
/usr/bin/nvi by default. The system administrator can override
this and cause it to refer to
/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
alternatives
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
/etc 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,
update-alternatives is called to update information
about that file in the alternatives system.
update-alternatives is
usually called from the following Debian package maintainer scripts,
postinst (configure) to install the alternative and from
prerm
and
postrm (remove) to remove the alternative.
Note: in most (if
not all) cases no other maintainer script actions should call
update-alternatives, in particular neither of
upgrade nor
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
vi(1)
editor are installed, the man page referenced by
/usr/share/man/man1/vi.1 should correspond to the executable referenced
by
/usr/bin/vi.
update-alternatives handles this by means of
master and
slave links; when the master is changed, any
associated slaves are changed too. A master link and its associated slaves
make up a
link 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
update-alternatives is run on the
changed link's group, and the group will automatically be switched to manual
mode.
Each alternative has a
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
--config option,
update-alternatives will list all
of the choices for the link group of which given
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
auto mode. You
will need to use the
--auto option in order to return to the automatic
mode (or you can rerun
--config and select the entry marked as
automatic).
If you want to configure non-interactively you can use the
--set option
instead (see below).
Different packages providing the same file need to do so
cooperatively.
In other words, the usage of
update-alternatives is
mandatory
for all involved packages in such case. It is not possible to override some
file in a package that does not employ the
update-alternatives
mechanism.
Since the activities of
update-alternatives are quite involved, some
specific terms will help to explain its operation.
- generic name (or alternative link)
- A name, like /usr/bin/editor, which refers, via the
alternatives system, to one of a number of files of similar function.
- alternative name
- The name of a symbolic link in the alternatives
directory.
- 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.
- alternatives directory
- A directory, by default /etc/alternatives,
containing the symlinks.
- administrative directory
- A directory, by default /var/lib/dpkg/alternatives,
containing update-alternatives' state information.
- link group
- A set of related symlinks, intended to be updated as a
group.
- master link
- The alternative link in a link group which determines how
the other links in the group are configured.
- slave link
- An alternative link in a link group which is controlled by
the setting of the master link.
- 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.
- manual mode
- When a link group is in manual mode, the alternatives
system will not make any changes to the system administrator's
settings.
-
--install link name path priority
[--slave link name path]...
- Add a group of alternatives to the system. link is
the generic name for the master link, name is the name of its
symlink in the alternatives directory, and path is the alternative
being introduced for the master link. The arguments after --slave
are the generic name, symlink name in the alternatives directory and the
alternative path for a slave link. Zero or more --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 --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.
-
--set name path
- Set the program path as alternative for name.
This is equivalent to --config but is non-interactive and thus
scriptable.
-
--remove name path
- Remove an alternative and all of its associated slave
links. name is a name in the alternatives directory, and
path is an absolute filename to which name could be linked.
If name is indeed linked to path, 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 path, no links are changed; only
the information about the alternative is removed.
-
--remove-all name
- Remove all alternatives and all of their associated slave
links. name is a name in the alternatives directory.
- --all
- Call --config on all alternatives. It can be
usefully combined with --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 yes '' | update-alternatives --force
--all.
-
--auto name
- Switch the link group behind the alternative for
name to automatic mode. In the process, the master symlink and its
slaves are updated to point to the highest priority installed
alternatives.
-
--display 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.
- --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 auto or
manual), and the last one contains the current choice in the
alternative (beware: it's a filename and thus might contain spaces).
- --set-selections
- Read configuration of alternatives on standard input in the
format generated by --get-selections and reconfigure them
accordingly (since version 1.15.0).
-
--query name
- Display information about the link group like
--display does, but in a machine parseable way (since version
1.15.0, see section QUERY FORMAT below).
-
--list name
- Display all targets of the link group.
-
--config name
- Show available alternatives for a link group and allow the
user to interactively select which one to use. The link group is
updated.
- --help
- Show the usage message and exit.
- --version
- Show the version and exit.
-
--altdir directory
- Specifies the alternatives directory, when this is to be
different from the default. Defaults to «
/etc/alternatives».
-
--admindir directory
- Specifies the administrative directory, when this is to be
different from the default. Defaults to «
/var/lib/dpkg/alternatives» if DPKG_ADMINDIR has not
been set.
-
--instdir directory
- Specifies the installation directory where alternatives
links will be created (since version 1.20.1). Defaults to «
/» if DPKG_ROOT has not been set.
-
--root directory
- Specifies the root directory (since version 1.20.1). This
also sets the alternatives, installation and administrative directories to
match. Defaults to « /» if DPKG_ROOT has not
been set.
-
--log file
- Specifies the log file (since version 1.15.0), when this is
to be different from the default (/var/log/alternatives.log).
- --force
- Allow replacing or dropping any real file that is installed
where an alternative link has to be installed or removed.
- --skip-auto
- Skip configuration prompt for alternatives which are
properly configured in automatic mode. This option is only relevant with
--config or --all.
- --quiet
- Do not generate any comments unless errors occur.
- --verbose
- Generate more comments about what is being done.
- --debug
- Generate even more comments, helpful for debugging, about
what is being done (since version 1.19.3).
- 0
- The requested action was successfully performed.
- 2
- Problems were encountered whilst parsing the command line
or performing the action.
- DPKG_ROOT
- If set and the --instdir or --root options
have not been specified, it will be used as the filesystem root
directory.
- DPKG_ADMINDIR
- If set and the --admindir option has not been
specified, it will be used as the base administrative directory.
- /etc/alternatives/
- The default alternatives directory. Can be overridden by
the --altdir option.
- /var/lib/dpkg/alternatives/
- The default administration directory. Can be overridden by
the --admindir option.
The
--query format is using an RFC822-like flat format. It's made of
n + 1 stanzas where
n is the number of alternatives available in
the queried link group. The first stanza contains the following fields:
-
Name: name
- The alternative name in the alternative directory.
-
Link: link
- The generic name of the alternative.
-
Slaves: list-of-slaves
- When this field is present, the 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.
-
Status: status
- The status of the alternative (auto or
manual).
-
Best: best-choice
- The path of the best alternative for this link group. Not
present if there is no alternatives available.
-
Value: currently-selected-alternative
- The path of the currently selected alternative. It can also
take the magic value none. It is used if the link doesn't
exist.
The other stanzas describe the available alternatives in the queried link group:
-
Alternative: path-of-this-alternative
- Path to this stanza's alternative.
-
Priority: priority-value
- Value of the priority of this alternative.
-
Slaves: list-of-slaves
- When this field is present, the 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.
$ 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
With
--verbose update-alternatives chatters incessantly about its
activities on its standard output channel. If problems occur,
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.
There are several packages which provide a text editor compatible with
vi, for example
nvi and
vim. Which one is used is
controlled by the link group
vi, which includes links for the program
itself and the associated manpage.
To display the available packages which provide
vi and the current
setting for it, use the
--display action:
update-alternatives --display vi
To choose a particular
vi implementation, use this command as root and
then select a number from the list:
update-alternatives --config vi
To go back to having the
vi implementation chosen automatically, do this
as root:
update-alternatives --auto vi
ln(1), FHS (the Filesystem Hierarchy Standard).