dpkg-maintscript-helper command [parameter...] -- maint-script-parameter...
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]]
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.
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~.
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.
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 -- "$@"
conffile is the filename of the conffile to remove.
Current implementation: in the preinst, it checks if the conffile was modified and renames it either to conffile.dpkg-remove (if not modified) or to conffile.dpkg-backup (if modified). In the postinst, the latter file is renamed to conffile.dpkg-bak and kept for reference as it contains user modifications but the former will be removed. If the package upgrade aborts, the postrm reinstalls the original conffile. During purge, the postrm will also delete the .dpkg-bak file kept up to now.
If a conffile is moved from one location to another, you need to make sure you move across any changes the user has made. This may seem a simple change to the preinst script at first, however that will result in the user being prompted by dpkg to approve the conffile edits even though they are not responsible of them.
Graceful renaming can be implemented by putting the following shell snippet in the preinst, postinst and postrm maintainer scripts:
dpkg-maintscript-helper mv_conffile \ old-conffile new-conffile prior-version package -- "$@"
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. Downgrades are not supported and the path will be left as is.
dpkg-maintscript-helper symlink_to_dir \ pathname old-target prior-version package -- "$@"
pathname is the absolute name of the old symlink (the path will be a directory at the end of the installation) and old-target is the target name of the former symlink at pathname. It can either be absolute or relative to the directory containing pathname.
Current implementation: the preinst checks if the symlink exists and points to old-target, if not then it's left in place, otherwise it's renamed to pathname.dpkg-backup. On configuration, the postinst removes pathname.dpkg-backup if pathname.dpkg-backup is still a symlink. On abort-upgrade/abort-install, the postrm renames pathname.dpkg-backup back to pathname if required.
Graceful switching can be implemented by putting the following shell snippet in the preinst, postinst and postrm maintainer scripts:
dpkg-maintscript-helper dir_to_symlink \ pathname new-target prior-version package -- "$@"
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.
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 184.108.40.206, 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.