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