update-alternatives(8) - phpMan

update-alternatives(8) dpkg utilities update-alternatives(8)

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 sym-
bolic 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 or prerm
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 not change the links; it will leave all the decisions to the sys-
tem administrator.

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 link. The current choice is marked with a '*'
and the choice with the highest priority with a '+'. You will then be prompted for your
choice regarding this link group. Once you make a change, the link group will no longer
be in auto mode. You will need to use the --auto option in order to return to the auto-
matic mode.

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. That is - 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-alterna-
tives mechanism.

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

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

symlink
Without any further qualification, this means a symbolic link in the alternatives
directory: one which the system administrator is expected to adjust.

alternative
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 link in a link group which determines how the other links in the group are con-
figured.

slave link
A 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 genname symlink altern priority [--slave genname symlink altern]...
Add a group of alternatives to the system. genname is the generic name for the
master link, symlink is the name of its symlink in the alternatives directory, and
altern 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 for a slave link. Zero or more --slave options, each followed by
three arguments, may be specified.

If the master symlink 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 sym-
links 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, or removed if there is no such alternative left. Associ-
ated 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.

--auto link
Switch the master symlink link to automatic mode. In the process, this symlink and
its slaves are updated to point to the highest priority installed alternatives.

--display link
Display information about the link group of which link is the master link. Infor-
mation displayed includes the group's mode (auto or manual), which alternative the
symlink currently points to, what other alternatives are available (and their cor-
responding slave alternatives), and the highest priority alternative currently
installed.

--list link
Display all targets of the link group.

--config link
Show available alternatives for a link group and allow the user to interactively
select which one to use. The link group is updated and taken out of auto mode.

--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.

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

--verbose
Generate more comments about what update-alternatives is doing.

--quiet
Don't generate any comments unless errors occur. This option is not yet imple-
mented.

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
0 The requested action was successfully performed.

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

DIAGNOSTICS
update-alternatives chatters incessantly about its activities on its standard output chan-
nel. 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 num-
ber 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, or, if that is
not possible, email the author directly.

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.

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

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.

Debian Project 2007-07-18 update-alternatives(8)

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