Re: Contributing to Emacs Development

[srivasta]

Hi,
>>"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. 

        manoj

UPDATE-ALTERNATIVES(8)   Debian GNU/Linux  UPDATE-ALTERNATIVES(8)
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/>
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