.\" update\-alternatives.8
.\" This man page is copyright 1997 Charles Briscoe-Smith
.\" This is free documentation; you can redistribute it and/or modify
.\" it under the terms of the GNU General Public License as published
.\" by the Free Software Foundation; either version 2 of the License, or
.\" (at your option) any later version.  There is NO WARRANTY.  You can
.\" find the GNU GPL in /usr/share/common-licenses/GPL on any Debian system.
.TH update\-alternatives 8 "2007-07-18" "Debian Project" "dpkg utilities"
.SH NAME
update\-alternatives \- maintain symbolic links determining default commands
.
.SH SYNOPSIS
.B update\-alternatives
.RI [ options ]
.I command
.
.SH DESCRIPTION
.B update\-alternatives
creates, removes, maintains and displays information about the symbolic
links comprising the Debian alternatives system.
.PP
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.
.PP
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
.BR ed (1)
and
.BR nvi (1)
are both installed on the system, the alternatives system will cause
the generic name
.I /usr/bin/editor
to refer to
.I /usr/bin/nvi
by default. The system administrator can override this and cause
it
to refer to
.I /usr/bin/ed
instead,
and the alternatives system will not alter this setting until explicitly
requested to do so.
.PP
The generic name is not a direct symbolic link to the selected alternative.
Instead, it is a symbolic link to a name in the
.I alternatives
.IR 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
.I /etc
directory: the FHS (q.v.) gives reasons why this is a Good Thing.
.PP
When each package
providing a file with a particular functionality is
installed, changed or removed,
.B update\-alternatives
is called to update information about that file in the alternatives system.
.B update\-alternatives
is usually called from the
.B postinst
or
.B prerm
scripts in Debian packages.
.PP
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
.BR vi (1)
editor are installed, the man page referenced by
.I /usr/share/man/man1/vi.1
should correspond to the executable referenced by
.IR /usr/bin/vi .
.B update\-alternatives
handles this by means of
.I master
and
.I slave
links; when the master is changed, any associated slaves are changed
too.
A master link and its associated slaves make up a
.I link
.IR group .
.PP
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.
.PP
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
.B update\-alternatives
is run on the changed link's group,
and the group will automatically be switched to manual mode.
.PP
Each alternative has a
.I 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.
.PP
When using the
.I \-\-config
option,
.B update\-alternatives
will list all of the choices for the link group
of which given
.I name
is the master link. The current choice is marked with a '*' and the
choice with the highest priority with a '+'.
You will then be prompted for your choice regarding this link group.
Once you make a change, the link group will no longer be in
.I auto
mode. You will need to use the
.I \-\-auto
option in order to return to the automatic mode.
.PP
If you want to configure non-interactively you can use the
.I \-\-set
option instead (see below).
.PP
Different packages providing the same file need to do so
.BR cooperatively .
That is - the usage of
.B update\-alternatives
is
.B mandatory
for all involved packages in such case - it is not possible to
override some file in a package that does not employ the
.B update\-alternatives
mechanism.
.
.SH TERMINOLOGY
Since the activities of
.B update\-alternatives
are quite involved, some specific terms will help to explain its
operation.
.TP
generic name
A name, like
.IR /usr/bin/editor ,
which refers, via the alternatives system, to one of a number of
files of similar function.
.TP
symlink
Without any further qualification, this means a symbolic link in the
alternatives directory: one which the system administrator is expected
to adjust.
.TP
alternative
The name of a specific file in the filesystem, which may be made
accessible via a generic name using the alternatives system.
.TP
alternatives directory
A directory, by default
.IR /etc/alternatives ,
containing the symlinks.
.TP
administrative directory
A directory, by default
.IR /var/lib/dpkg/alternatives ,
containing
.BR update\-alternatives '
state information.
.TP
link group
A set of related symlinks, intended to be updated as a group.
.TP
master link
The link in a link group which determines how the other links in the
group are configured.
.TP
slave link
A link in a link group which is controlled by the setting of
the master link.
.TP
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.
.TP
manual mode
When a link group is in manual mode,
the alternatives system will not make any changes
to the system administrator's settings.
.
.SH COMMANDS
.TP
\fB\-\-install\fR \fIgenname symlink altern priority\fR [\fB\-\-slave\fR \fIgenname symlink altern\fR]...
Add a group of alternatives to the system.
.I genname
is the generic name for the master link,
.I symlink
is the name of its symlink in the alternatives directory, and
.I altern
is the alternative being introduced for the master link.
The arguments after \fB\-\-slave\fR are the generic name, symlink name in the
alternatives directory and the alternative for a slave link.
Zero or more
.B \-\-slave
options, each followed by three arguments,
may be specified.
.IP
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.
.TP
\fB\-\-set\fR \fIname path\fR
Set the program
.I path
as alternative for
.I name.
This is equivalent to
.IB \-\-config
but is non-interactive and thus scriptable.
.TP
\fB\-\-remove\fR \fIname path\fR
Remove an alternative and all of its associated slave links.
.I name
is a name in the alternatives directory, and
.I path
is an absolute filename to which
.I name
could be linked. If
.I name
is indeed linked to
.IR path ,
.I 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
.IR path ,
no links are changed;
only the information about the alternative is removed.
.TP
\fB\-\-remove\-all\fR \fIname\fR
Remove all alternatives and all of their associated slave links.
.I name
is a name in the alternatives directory.
.TP
.B \-\-all
Call \fB\-\-config\fP on all alternatives.
.TP
\fB\-\-auto\fR \fIlink\fR
Switch the master symlink
.I link
to automatic mode.
In the process, this symlink and its slaves are updated
to point to the highest priority installed alternatives.
.TP
\fB\-\-display\fR \fIlink\fR
Display information about the link group of which
.I link
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.
.TP
\fB\-\-list\fR \fIlink\fR
Display all targets of the link group.
.TP
\fB\-\-config\fR \fIlink\fR
Show available alternatives for a link group and allow the user to
interactively select which one to use. The link group is updated
and taken out of
.I auto
mode.
.TP
.B \-\-help
Show the usage message and exit.
.TP
.B \-\-version
Show the version and exit.
.
.SH OPTIONS
.TP
.BI \-\-altdir " directory"
Specifies the alternatives directory, when this is to be
different from the default.
.TP
.BI \-\-admindir " directory"
Specifies the administrative directory, when this is to be
different from the default.
.TP
.B \-\-verbose
Generate more comments about what
.B update\-alternatives
is doing.
.TP
.B \-\-quiet
Don't generate any comments unless errors occur.
This option is not yet implemented.
.
.SH FILES
.TP
.I /etc/alternatives/
The default alternatives directory.
Can be overridden by the
.B \-\-altdir
option.
.TP
.I /var/lib/dpkg/alternatives/
The default administration directory.
Can be overridden by the
.B \-\-admindir
option.
.
.SH "EXIT STATUS"
.IP 0
The requested action was successfully performed.
.IP 2
Problems were encountered whilst parsing the command line
or performing the action.
.
.SH DIAGNOSTICS
.B update\-alternatives
chatters incessantly about its activities on its standard output channel.
If problems occur,
.B 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.
.
.SH EXAMPLES
There are several packages which provide a text editor compatible
with \fBvi\fP, for example \fBnvi\fP and \fBvim\fP. Which one is used
is controlled by the link group \fBvi\fP, which includes links for the
program itself and the associated manpage.
.PP
To display the available packages which provide \fBvi\fP and the current
setting for it, use the \fI\-\-display\fP action:
.RS
.PP
.B update\-alternatives \-\-display vi
.RE
.PP
To choose a particular \fBvi\fP implementation, use this command as root
and then select a number from the list:
.RS
.PP
.B update\-alternatives \-\-config vi
.RE
.PP
To go back to having the \fBvi\fP implementation chosen automatically, do
this as root:
.RS
.PP
.B update\-alternatives \-\-auto vi
.RE
.
.SH BUGS
If you find a bug, please report it using the Debian bug-tracking system,
or, if that is not possible, email the author directly.
.PP
If you find any discrepancy between the operation of
.B update\-alternatives
and this manual page, it is a bug,
either in the implementation or the documentation; please report it.
.
.SH AUTHOR
Copyright (C) 1995 Ian Jackson
.sp
This is free software; see the GNU General Public Licence version 2 or
later for copying conditions. There is NO WARRANTY.
.PP
This manual page is copyright 1997,1998 Charles Briscoe-Smith.
.sp
This is free documentation; see the GNU General Public Licence version 2 or
later for copying conditions. There is NO WARRANTY.
.
.SH "SEE ALSO"
.BR ln (1),
FHS, the Filesystem Hierarchy Standard.