Manpages

NAME

update−alternatives − maintain symbolic links determining default commands

SYNOPSIS

update−alternatives [options] command

DESCRIPTION

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 postinst (configure) or prerm (install) scripts in Debian packages.

It is often useful for a number of alternatives to be synchronised, 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.

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.

−−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), 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. 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 update-alternatives −−get−selections and reconfigure them accordingly.

−−query name

Display information about the link group like −−display does, but in a machine parseable way (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.

−−version

Show the version and exit.

OPTIONS

−−altdir directory

Specifies the alternatives directory, when this is to be different from the default.

−−admindir directory

Specifies the administrative directory, when this is to be different from the default.

−−log file

Specifies the log file, when this is to be different from the default (/var/log/alternatives.log).

−−force

Let update-alternatives replace any real file that is installed where an alternative link has to be installed.

−−skip−auto

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

−−verbose

Generate more comments about what update−alternatives is doing.

−−quiet

Don’t generate any comments unless errors occur.

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.

EXIT STATUS

QUERY FORMAT

The update−alternatives −−query format is using an RFC822-like flat format. It’s made of n + 1 blocks where n is the number of alternatives available in the queried link group. The first block contains the following fields:
Link: <link>

The generic name of the alternative.

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 blocks describe the available alternatives in the
queried link group:
Alternative: <path of this alternative>

Path to this block’s alternative.

Priority: <priority value>

Value of the priority of this alternative.

Slaves: <list of slaves>

When this header 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
Link: editor
Status: auto
Best: /usr/bin/vim.gtk
Value: /usr/bin/vim.gtk

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

Alternative: /usr/bin/vim.gtk
Priority: 50
Slaves:
editor.1.gz /usr/share/man/man1/vim.1.gz
editor.ru.1.gz /usr/share/man/ru/man1/vim.1.gz
editor.pl.ISO8859-2.1.gz /usr/share/man/pl.ISO8859-2/man1/vim.1.gz
editor.it.ISO8859-1.1.gz /usr/share/man/it.ISO8859-1/man1/vim.1.gz
editor.pl.UTF-8.1.gz /usr/share/man/pl.UTF-8/man1/vim.1.gz
editor.it.1.gz /usr/share/man/it/man1/vim.1.gz
editor.fr.UTF-8.1.gz /usr/share/man/fr.UTF-8/man1/vim.1.gz
editor.fr.1.gz /usr/share/man/fr/man1/vim.1.gz
editor.it.UTF-8.1.gz /usr/share/man/it.UTF-8/man1/vim.1.gz
editor.pl.1.gz /usr/share/man/pl/man1/vim.1.gz
editor.fr.ISO8859-1.1.gz /usr/share/man/fr.ISO8859-1/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 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

BUGS

If you find a bug, please report it using the Debian bug-tracking system.

If you find any discrepancy between the operation of update−alternatives and this manual page, it is a bug, either in the implementation or the documentation; please report it.

AUTHORS

Copyright © 1995 Ian Jackson
Copyright © 2009 Raphaël Hertzog

This is free software; see the GNU General Public Licence version 2 or later for copying conditions. There is NO WARRANTY.

This manual page is copyright 1997,1998 Charles Briscoe-Smith and others.

This is free documentation; see the GNU General Public Licence version 2 or later for copying conditions. There is NO WARRANTY.

SEE ALSO

ln(1), FHS, the Filesystem Hierarchy Standard.