update-alternatives
maintain symbolic links determining default commands
see also :
ln
Synopsis
update-alternatives
[option...] command
add an example, a script, a trick and tips
examples
source
sudo update-alternatives --config java
sudo update-alternatives --config javac
sudo update-alternatives --config javaws
sudo update-alternatives --config javaws
sudo update-alternatives --config java-home
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
source
update-alternatives --config x-window-manager
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 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.
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 or drop 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.
--verbose
Generate more comments about
what update-alternatives is doing.
--quiet
Don’t generate any
comments unless errors occur.
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), 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.
--help
Show the usage message and exit.
--version
Show the version and exit.
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.
environment
DPKG_ADMINDIR
If set and the --admindir option has not been specified,
it will be used as the base administrative directory.
exit status
0
The requested action was successfully performed.
2
Problems were encountered whilst parsing the command line or
performing the action.
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 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:
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 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
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
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.
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.
see also
ln ,
FHS, the Filesystem Hierarchy Standard.