UPDATE-ALTERNATIVES(8) System Manager's Manual UPDATE-ALTERNATIVES(8)
NAME
alternatives - maintain symbolic links determining default commands
SYNOPSIS
alternatives [options] --install link name path priority [--slave link
name path]... [--initscript service] [--family name]
alternatives [options] --remove name path
alternatives [options] --set name path
alternatives [options] --auto name
alternatives [options] --display name
alternatives [options] --config name
alternatives [options] --list name
DESCRIPTION
alternatives creates, removes, maintains and displays information about
the symbolic links comprising the alternatives system. The alternatives
system is a reimplementation of the Debian alternatives system. It was
rewritten primarily to remove the dependence on perl; it is intended to
be a drop in replacement for Debian's update-dependencies script. This
man page is a slightly modified version of the man page from the Debian
project.
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 of editor to invoke if the user has not specified a par-
ticular preference.
The alternatives system aims to solve this problem. A generic name in
the filesystem is shared by all files providing interchangeable func-
tionality. 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 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, alternatives is called to update infor-
mation about that file in the alternatives system. alternatives is
usually called from the %post or %pre scripts in RPM 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. 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 system's
automatic settings, this will be noticed the next time 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, alternatives will list all of the
choices for the link group of which given name is the master link. You
will then be prompted for which of the choices to use for the 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 state.
TERMINOLOGY
Since the activities of 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/alternatives, containing alter-
natives' 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 alternatives 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 set-
tings.
OPTIONS
Exactly one action must be specified if alternatives is to perform any
meaningful task. Any number of the common options may be specified
together with any action.
COMMON OPTIONS
--verbose
Generate more comments about what alternatives is doing.
--quiet
Don't generate any comments unless errors occur. This option is
not yet implemented.
--test Don't actually do anything, just say what would be done. This
option is not yet implemented.
--help Give some usage information (and say which version of alterna-
tives this is).
--version
Tell which version of alternatives this is (and give some usage
information).
--keep-missing
When switching between alternatives, if the new variant does not
provide some files, keep the links pointed to the previous
implementation. It prevents issues with missing files due to
switching between versions.
--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 link name path priority [--slave slink sname spath]
[--initscript service]...
Add a group of alternatives to the system. name is the generic
name for the master link, link is the name of its symlink, path
is the alternative being introduced for the master link, and
priority is the priority of the alternatives group. Higher pri-
orities take precendence if no alternative is manually selected.
sname, slink and spath are the generic name, symlink name and
alternative for a slave link, and service is the name of any
associated initscript for the alternative. NOTE: --initscript
and --family are a Red Hat Linux specific options. Zero or more
--slave options, each followed by three arguments, may be speci-
fied.
If the master symlink specified exists already in the alterna-
tives 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 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.
If --initscript is used, the alternatives system will manage the
initscript associated with the alternative via chkconfig, regis-
tering and unregistering the init script depending on which
alternative is active.
NOTE: --initscript is a Red Hat Linux specific option.
--family can be used to group similar alternatives. If the group
is in manual mode and the alternative currently used is removed,
alternatives will try to change links to different one with same
family and highest priority.
NOTE: --family is a Red Hat Linux specific option.
--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.
--set name path
The symbolic link and slaves for link group name set to those
configured for path, and the link group is set to manual mode.
This option is not in the original Debian implementation.
--config name
Present the user with a configuration menu for choosing the mas-
ter link and slaves for link group name. Once chosen, the link
group is set to manual mode.
--auto name
Switch the master symlink name to automatic mode. In the
process, this symlink and its slaves are updated to point to the
highest priority installed alternatives.
--display name
Display information about the link group of which name is the
master link. Information displayed includes the group's 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 Display information about all link groups.
FILES
/etc/alternatives/
The default alternatives directory. Can be overridden by the
--altdir option.
/var/lib/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
alternatives chatters incessantly about its activities on its standard
output channel. If problems occur, 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.
BUGS
If you find a bug, please report it using the Red Hat bug tracking sys-
tem at http://bugzilla.redhat.com.
If you find any discrepancy between the operation of alternatives and
this manual page, it is a bug, either in the implementation or the doc-
umentation; please report it. Any significant differences between this
implementation and Debian's is also a bug and should be reported,
unless otherwise noted in this man page.
AUTHOR
alternatives is copyright 2002 Red Hat, Inc.. It is free software; see
the GNU General Public Licence version 2 or later for copying condi-
tions. There is NO warranty.
This manual page is copyright 1997/98 Charles Briscoe-Smith and 2002
Red Hat, Inc. This is free documentation; see the GNU General Public
Licence version 2 or later for copying conditions. There is NO WAR-
RANTY.
SEE ALSO
ln(1), FHS, the Filesystem Hierarchy Standard. alternatives.c chkcon-
fig.c COPYING leveldb.c leveldb.h Makefile ntsysv.c ook
27 January 2001 UPDATE-ALTERNATIVES(8)
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