「update-alternatives」

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update-alternatives,维护用于确定默认命令的符号链接(算是默认软件吧)。

命令行语法格式(SYNOPSIS)

update-alternatives [option…] command

命令简述(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 (remove and deconfigure) 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.

术语(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.

支持的命令(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), the master and slave links, 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 (since version
1.15.0). 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 –get-selections and
reconfigure them accordingly (since version 1.15.0).

–query name
Display information about the link group like –display does, but in a machine parseable way (since
version 1.15.0, see section QUERY FORMAT below).

–list name
显示一个链接组的所有目标。

–config name
为一个链接组显示所有可用的选项,并允许用户交互地选择要使用的选项。链接组是已更新的。

–help
显示帮助信息并退出。

–version
显示版本信息并退出。

命令选项(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 (since version 1.15.0), when this is to be different from the default
(/var/log/alternatives.log).

–force
Allow replacing or dropping 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 is being done.

–quiet
Don’t generate any comments unless errors occur.

退出码(EXIT STATUS)

0 The requested action was successfully performed.

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

相关环境变量(ENVIRONMENT)

DPKG_ADMINDIR
If set and the –admindir option has not been specified, it will be used as the base administrative
directory.

相关文件(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.

查询格式(QUERY FORMAT)

The –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

问题诊断(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.

简单示例(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

相关文档(SEE ALSO)

ln(1), FHS, the Filesystem Hierarchy Standard.

参考文献

  • man 1 update-alternatives, version 1.18.24