dpkg-maintscript-helper(1)        dpkg suite        dpkg-maintscript-helper(1)



NAME
       dpkg-maintscript-helper  - works around known dpkg limitations in main-
       tainer scripts

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

COMMANDS AND PARAMETERS
       supports command

       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 auto-
       matically  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-ver-
              sion 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).

              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. This applies to all  other
              actions in the same way.

              For  example, for a conffile removed in version 2.0-1 of a pack-
              age, 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. Or a package switching a path from
              a  symlink (shipped in version 1.0-1) to a directory (shipped in
              version 2.0-1), but only performing the  actual  switch  in  the
              maintainer scripts in version 3.0-1, should set prior-version to
              3.0-1~.

       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 for-
              warded to the program after --.

CONFFILE RELATED TASKS
       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 acci-
       dent 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 pack-
       age's maintainer scripts, usually with a tool like debconf or ucf.

       This  means  that  if a package is intended to rename or remove a conf-
       file, it must explicitly do so and dpkg-maintscript-helper can be  used
       to  implement  graceful  deletion  and moving of conffiles within main-
       tainer 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 modi-
       fied) 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-conf-
       file.dpkg-remove.  On  configuration,  the  postinst  removes old-conf-
       file.dpkg-remove and renames old-conffile to new-conffile if  old-conf-
       file  is  still  available.  On abort-upgrade/abort-install, the postrm
       renames old-conffile.dpkg-remove back to old-conffile if required.

SYMLINK AND DIRECTORY SWITCHES
       When upgrading a package, dpkg will not automatically switch a  symlink
       to a directory or vice-versa. Downgrades are not supported and the path
       will be left as is.

   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  absolute name of the old symlink (the path will be a
       directory at the end of the installation) and old-target is the  target
       name  of  the  former symlink at pathname. It can either be absolute or
       relative to the directory containing 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 absolute name 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. It can either be absolute or relative to
       the directory containing 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 path-
       name.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 sym-
       link   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
       When using a packaging helper, please check  if  it  has  native  dpkg-
       maintscript-helper  integration, which might make your life easier. See
       for example dh_installdeb(1).

       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 1.17.14:

           Pre-Depends: dpkg (>= 1.17.14)

       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 pro-
       gram only if we know that the required command is supported by the cur-
       rently installed dpkg:

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

       The  command  supports  will return 0 on success, 1 otherwise. The sup-
       ports command will check if the environment variables as  set  by  dpkg
       and  required by the script are present, and will consider it a failure
       in case the environment is not sufficient.

SEE ALSO
       dh_installdeb(1).



Debian Project                    2014-09-01        dpkg-maintscript-helper(1)
