Ubuntu Manpage: update-alternatives - maintain symbolic links determining default commands

NAME

       update-alternatives - maintain symbolic links determining default commands

SYNOPSIS

       update-alternatives [option...] command

DESCRIPTION

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 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 manual 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.

TERMINOLOGY

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.

COMMANDS

--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.

OPTIONS

--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).

EXIT STATUS

       0   The requested action was successfully performed.

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

ENVIRONMENT

       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.

FILES

       /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.

QUERY FORMAT

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.

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

DIAGNOSTICS

       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.

EXAMPLES

       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 manual page.

       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