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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.	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)
	      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).

1.18.24				  2017-05-17	    dpkg-maintscript-helper(1)

NAME | SYNOPSIS | COMMANDS AND PARAMETERS | DESCRIPTION | COMMON PARAMETERS | CONFFILE RELATED TASKS | SYMLINK AND DIRECTORY SWITCHES | INTEGRATION IN PACKAGES | SEE ALSO

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