update-alternatives(8) dpkg utilities update-alternatives(8)
NAME
update-alternatives - maintain symbolic links determining default com
mands
SYNOPSIS
update-alternatives [options] --install genname symlink altern priority
[--slave genname symlink altern]...
update-alternatives [options] --remove name path
update-alternatives [options] --remove-all name
update-alternatives [options] --all
update-alternatives [options] --auto name
update-alternatives [options] --display name
update-alternatives [options] --list name
update-alternatives [options] --config name
update-alternatives [options] --set name path
DESCRIPTION
update-alternatives creates, removes, maintains and displays informa
tion about the symbolic links comprising the Debian alternatives sys
tem.
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 dif
ferent 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.
Debians 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 alterna
tive. Instead, it is a symbolic link to a name in the alternatives
directory, which in turn is a symbolic link to the actual file refer
enced. This is done so that the system administrators 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-alter
natives 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 system administrator.
Link groups are in automatic mode when they are first introduced to the
system. If the system administrator makes changes to the systems
automatic settings, this will be noticed the next time update-alterna
tives is run on the changed links group, and the group will automati
cally 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 regard
ing 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 automatic 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-alternatives mecha
nism.
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 sym
links.
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 configured.
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 prior
ity 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 administrators set
tings.
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 set
ting 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
OPTIONS
Exactly one action must be specified if update-alternatives is to per
form any meaningful task. Any number of the common options may be
specified together with any action.
COMMON OPTIONS
--verbose
Generate more comments about what update-alternatives is doing.
--quiet
Dont generate any comments unless errors occur. This option is
not yet implemented.
--test Dont actually do anything, just say what would be done. This
option is not yet implemented.
--help Give some usage information.
--version
Tell which version of update-alternatives this is.
--altdir directory
Specifies the alternatives directory, when this is to be differ
ent from the default.
--admindir directory
Specifies the administrative directory, when this is to be dif
ferent from the default.
ACTIONS
--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 alter
native being introduced for the master link. The arguments
after --slave are the generic name, symlink name in the alterna
tives 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 alterna
tives systems 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 informa
tion. If the group is in automatic mode, and the newly added
alternatives priority is higher than any other installed alter
natives 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 equiva
lent 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 alterna
tive 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.
--auto link
Switch the master symlink link to automatic mode. In the pro
cess, 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. Information displayed includes the groups mode
(auto or manual), which alternative the symlink currently points
to, what other alternatives are available (and their correspond
ing 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.
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 channel. If problems occur, update-alternatives out
puts 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.
BUGS
If you find a bug, please report it using the Debian bug-tracking sys
tem, or, if that is not possible, email the author directly.
If you find any discrepancy between the operation of update-alterna
tives and this manual page, it is a bug, either in the implementation
or the documentation; please report it.
AUTHOR
Debian update-alternatives is copyright 1995 Ian Jackson. It 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 1997/98 Charles Briscoe-Smith. This is
free documentation; see the GNU General Public Licence version 2 or
later for copying conditions. There is NO WARRANTY.
You can find the GNU GPL in /usr/share/common-licenses/GPL on any
Debian system.
SEE ALSO
ln(1), FHS, the Filesystem Hierarchy Standard.
Debian Project 2006-11-24 update-alternatives(8)
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