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 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).

	      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 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 owning the pathname(s).	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  environment  variables  (as set by dpkg when running the maintainer scripts) 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 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.

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
       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
       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
       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).

1.19.0.5							    2018-04-16						dpkg-maintscript-helper(1)