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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 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
       Note: This can be replaced in most cases by the "remove-on-upgrade"
       flag in DEBIAN/conffiles (since dpkg 1.20.6), see deb-conffiles(5).

       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.

       Note: The symlinks and directories created during these switches need
       to be shipped in the new packages, or dpkg will not be able to remove
       them on purge.

   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.

ENVIRONMENT
       DPKG_ROOT
           If set, it will be used as the filesystem root directory.

       DPKG_ADMINDIR
           If set, it will be used as the dpkg data directory.

       DPKG_COLORS
           Sets the color mode (since dpkg 1.19.1).  The currently accepted
           values are: auto (default), always and never.

SEE ALSO
       dh_installdeb(1).

1.21.22                           2023-05-11        dpkg-maintscript-helper(1)

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