dpkg-maintscript-helper(1) dpkg suite dpkg-maintscript-helper(1)NAMEdpkg-maintscript-helper - works around known dpkg limitations in main‐
tainer scripts
SYNOPSISdpkg-maintscript-helper command [parameter...] -- maint-script-parame‐
ter...
COMMANDS AND PARAMETERS
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 auto‐
matically 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-ver‐
sion 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).
For example, for a conffile removed in version 2.0-1 of a pack‐
age, 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.
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.
package
The package name. When the package is "Multi-Arch: same", this
parameter must include the architecture qualifier. If empty or
omitted, the DPKG_MAINTSCRIPT_PACKAGE environment variable (as
set by dpkg) will be used.
-- All the parameters of the maintainer scripts have to be for‐
warded 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 acci‐
dent 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 pack‐
age's maintainer scripts, usually with a tool like debconf or ucf.
This means that if a package is intended to rename or remove a conf‐
file, it must explicitly do so and dpkg-maintscript-helper can be used
to implement graceful deletion and moving of conffiles within main‐
tainer 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 -- "$@"
conffile is the filename of the conffile to remove.
Current implementation: in the preinst, it checks if the conffile was
modified and renames it either to conffile.dpkg-remove (if not modi‐
fied) or to conffile.dpkg-backup (if modified). In the postinst, the
latter file is renamed to conffile.dpkg-bak and kept for reference as
it contains user modifications but the former will be removed. If the
package upgrade aborts, the postrm reinstalls the original conffile.
During purge, the postrm will also delete the .dpkg-bak file kept up to
now.
Renaming a conffile
If a conffile is moved from one location to another, you need to make
sure you move across any changes the user has made. This may seem a
simple change to the preinst script at first, however that will result
in the user being prompted by dpkg to approve the conffile edits even
though they are not responsible of them.
Graceful renaming can be implemented by putting the following shell
snippet in the preinst, postinst and postrm maintainer scripts:
dpkg-maintscript-helper mv_conffile \
old-conffile new-conffile prior-version package -- "$@"
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-conf‐
file.dpkg-remove. On configuration, the postinst removes old-conf‐
file.dpkg-remove and renames old-conffile to new-conffile if old-conf‐
file 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.
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 -- "$@"
pathname is the name of the old symlink (the path will be a directory
at the end of the installation) and old-target the target of the former
symlink at pathname.
Current implementation: the preinst checks if the symlink exists and
points to old-target, if not then it's left in place, otherwise it's
renamed to pathname.dpkg-backup. On configuration, the postinst removes
pathname.dpkg-backup if pathname.dpkg-backup is still a symlink. On
abort-upgrade/abort-install, the postrm renames pathname.dpkg-backup
back to pathname if required.
Switching a directory to symlink
If a real directory is switched to a symlink, you need to make sure
before unpacking that the directory is removed. This may seem a simple
change to the preinst script at first, however that will result in some
problems in case the directory contains conffiles, pathnames owned by
other packages, locally created pathnames, or when downgrading the
package.
Graceful switching can be implemented by putting the following shell
snippet in the preinst, postinst and postrm maintainer scripts:
dpkg-maintscript-helper dir_to_symlink \
pathname new-target prior-version package -- "$@"
pathname is the name of the 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.
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 path‐
name.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 sym‐
link 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
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 1.17.5:
Pre-Depends: dpkg (>= 1.17.5)
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 pro‐
gram only if we know that the required command is supported by the cur‐
rently installed dpkg:
if dpkg-maintscript-helper supports command; then
dpkg-maintscript-helper command ...
fi
Debian Project 2013-12-11 dpkg-maintscript-helper(1)
