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 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.

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), 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.

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.

EXIT STATUS

       0      The requested action was successfully performed.

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

ENVIRONMENT

       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 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 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 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