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UPDATE-ALTERNATIVES


Section: Maintenance Commands (8)
Updated: 27 January 2001
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NAME

alternatives – maintain symbolic links determining default commands
 

SYNOPSIS

alternatives

[options]

–install

link name path priority

[--slave

link name

path]…

[--initscript

service]

alternatives

[options]

–remove

name path

alternatives

[options]

–set

name path

alternatives

[options]

–auto

name

alternatives

[options]

–display

name

alternatives

[options]

–config

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
different 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 particular preference.

The 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,
alternatives

is called to update information 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 symlinks.

administrative directory

A directory, by default
/var/lib/alternatives,

containing
alternatives

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 priority 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 settings.

 

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
alternatives

this is).

–version


Tell which version of
alternatives

this is (and give some usage information).

–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.

 

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 priorities
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

is a Red Hat Linux specific option.
Zero or more
–slave

options, each followed by three arguments,
may be specified.


If the master symlink 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.


If
–initscript

is used, the alternatives system will manage the initscript
associated with the alternative via
chkconfig,

registering and unregistering the init script depending on
which alternative is active.


NOTE:

–initscript

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 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.

–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 master 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 corresponding slave alternatives),
and the highest priority alternative currently installed.


 

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 system
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 documentation; 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 conditions. 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 WARRANTY.
 

SEE ALSO

ln(1),

FHS, the Filesystem Hierarchy Standard.
alternatives.c
chkconfig.c
COPYING
leveldb.c
leveldb.h
Makefile
ntsysv.c
ook



 

Index



NAME

SYNOPSIS

DESCRIPTION

TERMINOLOGY

OPTIONS


COMMON OPTIONS

ACTIONS


FILES

EXIT STATUS

DIAGNOSTICS

BUGS

AUTHOR

SEE ALSO



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