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

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

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

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 lim‐
       itations.

       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 opera‐
              tion. 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).

              If  the conffile has not been shipped for several versions, and you are now modify‐
              ing 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 ver‐
              sion 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 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. 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,  otherwise it should not usually include the
              architecture qualifier (as it would disallow cross-grades, or switching from  being
              architecture  specific  to  architecture  all  or vice versa).  If the parameter is
              empty or omitted, the DPKG_MAINTSCRIPT_PACKAGE and  DPKG_MAINTSCRIPT_ARCH  environ‐
              ment  variables (as set by dpkg) will be used to generate an arch-qualified package
              name.

       --     All the parameters of the maintainer scripts have to be forwarded  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  ver‐
       sion.  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-main‐
       tained 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 explic‐
       itly do so and dpkg-maintscript-helper can be used to implement graceful deletion and mov‐
       ing 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 modi‐
       fied). In the postinst, the latter file is renamed to conffile.dpkg-bak and kept for  ref‐
       erence  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  pre‐
       inst, 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-conf‐
       file.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, how‐
       ever 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 pre‐
       inst, 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 config‐
       uration, the postinst removes pathname.dpkg-backup if pathname.dpkg-backup is still a sym‐
       link.  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,  path‐
       names 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 pre‐
       inst, 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 conf‐
       files,  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 direc‐
       tory  named pathname is created, marked with a file so that dpkg can track it. On configu‐
       ration, 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  path‐
       name.dpkg-backup back to pathname if required.

INTEGRATION IN PACKAGES
       When using a packaging helper, please check if it has native dpkg-maintscript-helper inte‐
       gration, 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  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

       The  command  supports  will  return  0 on success, 1 otherwise. The supports 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)

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