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Re: Contributing to Emacs Development

From: srivasta
Subject: Re: Contributing to Emacs Development
Date: Sat, 11 Nov 2000 10:39:46 -0600

>>"Eli" == Eli Zaretskii <address@hidden> writes:

 >> Date: Sat, 11 Nov 2000 01:40:46 -0600
 >> From: address@hidden
 >> And using update alternatives, they can.

 Eli> I'm not sure I understand what ``update alternatives'' are.  Could you
 Eli> elaborate?

        I am attaching a manual page. Also, I thikn there may be a
 misunderstanding developing here; my solution is specific to Debian
 packaging, and not a generic solution for Emacs (shameless plug for
 Debian), In general, your pessimism may be warranted; In Debian, the
 solution may be trivial. 


NAME update-alternatives - maintain symbolic links determining default commands
SYNOPSIS update-alternatives [options] --install link name path priority [--slave link name path]...
update-alternatives [options] --remove name path
update-alternatives [options] --auto name
update-alternatives [options] --display name
update-alternatives [options] --config name
DESCRIPTION update-alternatives creates, removes, maintains and dis- plays 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 edi- tor, 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.
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 FSSTND (q.v.) gives reasons why this is a Good Thing.
When each package providing a file with a particular func- tionality is installed, changed or removed, update-alter- natives is called to update information about that file in the alternatives system. update-alternatives is usually called from the postinst or prerm scripts in Debian pack- ages.
It is often useful for a number of alternatives to be syn- chronised, 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/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 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 update-alternatives is run on the changed link's group, and the group will automati- cally 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 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 update-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, contain- ing 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 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 alter- natives system ensures that the links in the group point to the highest priority alternatives appro- priate for the group.
manual mode When a link group is in manual mode, the alterna- tives system will not make any changes to the sys- tem administrator's settings.
OPTIONS Exactly one action must be specified if update-alterna- tives 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 update-alterna- tives 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 update-alternatives this is).
--version Tell which version of update-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 gen path pri [--slave slink sgen spath] ... Add a group of alternatives to the system. gen is the generic name for the master link, link is the name of its symlink, and path is the alternative being introduced for the master link. sgen, slink and spath are the generic name, symlink name and alternative for a slave link. 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 informa- tion. 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.
--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, correspond- ingly. If the link is not currently pointing to path, no links are changed; only the information about the alternative is removed.
--auto link Switch the master symlink link to automatic mode. In the process, this symlink and its slaves are updated to point to the highest priority installed alternatives.
--display link Display information about the link group of which 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 corre- sponding slave alternatives), and the highest pri- ority alternative currently installed.
FILES /etc/alternatives/ The default alternatives directory. Can be over- ridden by the --altdir option.
/var/lib/dpkg/alternatives/ The default administration directory. Can be over- ridden by the --admindir option.
EXIT STATUS 0 The requested action was successfully performed.
2 Problems were encountered whilst parsing the com- mand line or performing the action.
DIAGNOSTICS update-alternatives chatters incessantly about its activi- ties 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.
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.
If you find any discrepancy between the operation of update-alternatives and this manual page, it is a bug, either in the implementation or the documentation; please report it.
AUTHOR Debian GNU/Linux update-alternatives is copyright 1995 Ian Jackson. 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. This is free documentation; see the GNU General Public Licence version 2 or later for copying conditions. There is NO WARRANTY.
You can find the GNU GPL in /usr/share/common-licenses/GPL on any Debian system.
SEE ALSO ln(1), FSSTND, the linux Filesystem Standard.
DEBIAN 19 January 1998 1
 A mighty creature is the germ, Though smaller than the pachyderm. His
 customary dwelling place Is deep within the human race. His childish
 pride he often pleases By giving people strange diseases. Do you, my
 poppet, feel infirm? You probably contain a germ. Ogden Nash
Manoj Srivastava     <address@hidden>    <http://www.golden-gryphon.com/>
1024R/C7261095 print CB D9 F4 12 68 07 E4 05  CC 2D 27 12 1D F5 E8 6E
1024D/BF24424C print 4966 F272 D093 B493 410B  924B 21BA DABB BF24 424C

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