trusty (1) dpkg-maintscript-helper.1.gz

Provided by: dpkg_1.17.5ubuntu5.8_amd64 bug

NAME
       dpkg-maintscript-helper - works around known dpkg limitations in maintainer scripts


SYNOPSIS
       dpkg-maintscript-helper command [parameter...] -- maint-script-parameter...


COMMANDS AND PARAMETERS
       rm_conffile conffile [prior-version [package]]

       mv_conffile old-conffile new-conffile [prior-version [package]]

       symlink_to_dir pathname old-target [prior-version [package]]

       dir_to_symlink pathname new-target [prior-version [package]]


DESCRIPTION
       This  program is designed to be run within maintainer scripts to achieve some tasks that dpkg can't (yet)
       handle natively either because of design decisions or due to current limitations.

       Many of those tasks require coordinated actions  from  several  maintainer  scripts  (preinst,  postinst,
       prerm,  postrm).  To  avoid  mistakes the same call simply needs to be put in all scripts and the program
       will automatically adapt its behaviour based on the environment variable DPKG_MAINTSCRIPT_NAME and on the
       maintainer scripts arguments that you have to forward after a double hyphen.


COMMON PARAMETERS
       prior-version
              Defines  the  latest  version  of  the  package  whose upgrade should trigger the operation. It is
              important to calculate prior-version correctly so that the operations are correctly performed even
              if  the  user rebuilt the package with a local version. If prior-version is empty or omitted, then
              the operation is tried on every upgrade (note: it's  safer  to  give  the  version  and  have  the
              operation tried only once).

              For  example, for a conffile removed in version 2.0-1 of a package, prior-version should be set to
              2.0-1~. This will cause the conffile to be removed even if the user rebuilt the  previous  version
              1.0-1 as 1.0-1local1.

              If  the  conffile  has  not  been  shipped  for  several  versions,  and you are now modifying the
              maintainer scripts to clean up the obsolete file, prior-version should be based on the version  of
              the  package  that  you  are  now  preparing, not the first version of the package that lacked the
              conffile.

       package
              The package name. When the  package  is  "Multi-Arch:  same",  this  parameter  must  include  the
              architecture qualifier. If empty or omitted, the DPKG_MAINTSCRIPT_PACKAGE environment variable (as
              set by dpkg) will be used.

       --     All the parameters of the maintainer scripts have to be forwarded to the program after --.


       When upgrading a package, dpkg will not automatically remove a conffile (a configuration file  for  which
       dpkg  should  preserve  user  changes) if it is not present in the newer version. There are two principal
       reasons for this; the first is that the conffile could've been dropped by accident and the  next  version
       could  restore  it,  users  wouldn't  want  their changes thrown away. The second is to allow packages to
       transition files from a dpkg-maintained conffile  to  a  file  maintained  by  the  package's  maintainer
       scripts, usually with a tool like debconf or ucf.

       This  means  that  if  a package is intended to rename or remove a conffile, it must explicitly do so and
       dpkg-maintscript-helper can be used to  implement  graceful  deletion  and  moving  of  conffiles  within
       maintainer scripts.

   Removing a conffile
       If  a conffile is completely removed, it should be removed from disk, unless the user has modified it. If
       there are local modifications, they should be preserved.  If  the  package  upgrades  aborts,  the  newly
       obsolete conffile should not disappear.

       All  of  this  is  implemented by putting the following shell snippet in the preinst, postinst and postrm
       maintainer scripts:

           dpkg-maintscript-helper rm_conffile \
               conffile prior-version package -- "$@"

       conffile is the filename of the conffile to remove.

       Current implementation: in the preinst, it checks if the conffile was modified and renames it  either  to
       conffile.dpkg-remove  (if  not  modified)  or to conffile.dpkg-backup (if modified). In the postinst, the
       latter file is renamed to conffile.dpkg-bak and kept for reference as it contains user modifications  but
       the  former  will be removed. If the package upgrade aborts, the postrm reinstalls the original conffile.
       During purge, the postrm will also delete the .dpkg-bak file kept up to now.

   Renaming a conffile
       If a conffile is moved from one location to another, you need to make sure you move  across  any  changes
       the user has made. This may seem a simple change to the preinst script at first, however that will result
       in the user being prompted by dpkg to approve the conffile edits even though they are not responsible  of
       them.

       Graceful  renaming can be implemented by putting the following shell snippet in the preinst, postinst and
       postrm maintainer scripts:

           dpkg-maintscript-helper mv_conffile \
               old-conffile new-conffile prior-version package -- "$@"

       old-conffile and new-conffile are the old and new name of the conffile to rename.

       Current implementation: the preinst checks if the conffile has been modified, if yes it's left  on  place
       otherwise  it's  renamed  to  old-conffile.dpkg-remove.  On  configuration,  the  postinst  removes  old-
       conffile.dpkg-remove and renames old-conffile to new-conffile if  old-conffile  is  still  available.  On
       abort-upgrade/abort-install,   the  postrm  renames  old-conffile.dpkg-remove  back  to  old-conffile  if
       required.


       When upgrading a package, dpkg will not automatically switch a symlink to a directory or vice-versa.

   Switching a symlink to directory
       If a symlink is switched to a real directory, you need to make sure before unpacking that the symlink  is
       removed.  This  may seem a simple change to the preinst script at first, however that will result in some
       problems in case of admin local customization of the symlink or when downgrading the package.

       Graceful renaming can be implemented by putting the following shell snippet in the preinst, postinst  and
       postrm maintainer scripts:

           dpkg-maintscript-helper symlink_to_dir \
               pathname old-target prior-version package -- "$@"

       pathname is the name of the old symlink (the path will be a directory at the end of the installation) and
       old-target the target of the former symlink at pathname.

       Current implementation: the preinst checks if the symlink exists and points to old-target,  if  not  then
       it's  left  in  place,  otherwise  it's  renamed  to pathname.dpkg-backup. On configuration, the postinst
       removes pathname.dpkg-backup if pathname.dpkg-backup is still a symlink. On  abort-upgrade/abort-install,
       the postrm renames pathname.dpkg-backup back to pathname if required.

   Switching a directory to symlink
       If  a  real directory is switched to a symlink, you need to make sure before unpacking that the directory
       is removed. This may seem a simple change to the preinst script at first, however  that  will  result  in
       some  problems  in  case  the  directory  contains  conffiles, pathnames owned by other packages, locally
       created pathnames, or when downgrading the package.

       Graceful switching can be implemented by putting the following shell snippet in the preinst, postinst and
       postrm maintainer scripts:

           dpkg-maintscript-helper dir_to_symlink \
               pathname new-target prior-version package -- "$@"

       pathname  is  the  name  of  the  of  the  old  directory  (the  path will be a symlink at the end of the
       installation) and new-target is the target of the new symlink at pathname.

       Current implementation: the preinst checks if the directory exists, does not contain conffiles, pathnames
       owned  by  other  packages,  or locally created pathnames, if not then it's left in place, otherwise it's
       renamed to pathname.dpkg-backup, and an empty staging directory named pathname is created, marked with  a
       file   so   that   dpkg   can   track   it.  On  configuration,  the  postinst  finishes  the  switch  if
       pathname.dpkg-backup is still a directory and pathname is the staging directory; it removes  the  staging
       directory  mark  file,  moves  the newly created files inside the staging directory to the symlink target
       new-target/, replaces the now empty staging directory pathname with a symlink to new-target, and  removes
       pathname.dpkg-backup.  On  abort-upgrade/abort-install,  the  postrm renames pathname.dpkg-backup back to
       pathname if required.


INTEGRATION IN PACKAGES
       Given that dpkg-maintscript-helper is used in the preinst,  using  it  unconditionally  requires  a  pre-
       dependency  to  ensure  that  the required version of dpkg has been unpacked before. The required version
       depends on the command used, for rm_conffile and mv_conffile  it  is  1.15.7.2,  for  symlink_to_dir  and
       dir_to_symlink it is 1.17.5:

           Pre-Depends: dpkg (>= 1.17.5)

       But in many cases the operation done by the program is not critical for the package, and instead of using
       a pre-dependency we can call the program only if we know that the required command is  supported  by  the
       currently installed dpkg:

           if dpkg-maintscript-helper supports command; then
               dpkg-maintscript-helper command ...
           fi