dpkg/man/dpkg-maintscript-helper.1

.\" dpkg manual page - dpkg-maintscript-helper(1)
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.\" Copyright © 2010-2012 Raphaël Hertzog <hertzog@debian.org>
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.TH dpkg\-maintscript\-helper 1 "2014-04-28" "Debian Project" "dpkg suite"
.SH NAME
dpkg\-maintscript\-helper \- works around known dpkg limitations in maintainer scripts
.
.SH SYNOPSIS
.B dpkg\-maintscript\-helper
.IR command " [" parameter "...] \fB\-\-\fP " maint-script-parameter ...
.
.SH COMMANDS AND PARAMETERS
.P
\fBsupports\fP \fIcommand\fP
.P
\fBrm_conffile\fP \fIconffile\fP [\fIprior-version\fP [\fIpackage\fP]]
.P
\fBmv_conffile\fP \fIold-conffile\fP \fInew-conffile\fP [\fIprior-version\fP [\fIpackage\fP]]
.P
\fBsymlink_to_dir\fP \fIpathname\fP \fIold-target\fP [\fIprior-version\fP [\fIpackage\fP]]
.P
\fBdir_to_symlink\fP \fIpathname\fP \fInew-target\fP [\fIprior-version\fP [\fIpackage\fP]]
.
.SH DESCRIPTION
.P
This program is designed to be run within maintainer scripts to achieve
some tasks that \fBdpkg\fP can't (yet) handle natively either because of
design decisions or due to current limitations.
.P
Many of those tasks require coordinated actions from several maintainer
scripts (\fBpreinst\fP, \fBpostinst\fP, \fBprerm\fP, \fBpostrm\fP). 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 \fBDPKG_MAINTSCRIPT_NAME\fP and on the maintainer scripts arguments
that you have to forward after a double hyphen.
.
.SH COMMON PARAMETERS
.TP
.I prior-version
Defines the latest version of the package whose upgrade should trigger the
operation. It is important to calculate \fIprior-version\fP correctly so
that the operations are correctly performed even if the user rebuilt the
package with a local version. If \fIprior-version\fP 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,
\fIprior-version\fP 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 \fB2.0\-1\fP of a package,
\fIprior-version\fP should be set to \fB2.0\-1~\fP. This will cause the
conffile to be removed even if the user rebuilt the previous version
\fB1.0\-1\fP as \fB1.0\-1local1\fP. Or a package switching a path from
a symlink (shipped in version \fB1.0\-1\fP) to a directory (shipped in
version \fB2.0\-1\fP), but only performing the actual switch in the
maintainer scripts in version \fB3.0\-1\fP, should set \fIprior-version\fP
to \fB3.0\-1~\fP.
.TP
.I package
The package name. When the package is "Multi-Arch: same", this parameter
must include the architecture qualifier. If empty or omitted, the
\fBDPKG_MAINTSCRIPT_PACKAGE\fP environment variable (as set by
\fBdpkg\fP) will be used.
.TP
.B \-\-
All the parameters of the maintainer scripts have to be forwarded to the
program after \fB\-\-\fP.
.SH CONFFILE RELATED TASKS
.P
When upgrading a package, \fBdpkg\fP will not automatically remove a conffile
(a configuration file for which \fBdpkg\fP 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.
.P
This means that if a package is intended to rename or remove a conffile,
it must explicitly do so and \fBdpkg\-maintscript\-helper\fP can be used
to implement graceful deletion and moving of conffiles within maintainer
scripts.
.
.SS Removing a conffile
.P
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.
.P
All of this is implemented by putting the following shell snippet in the
\fBpreinst\fP, \fBpostinst\fP and \fBpostrm\fP maintainer scripts:
.P
    dpkg\-maintscript\-helper rm_conffile \\
        \fIconffile\fP \fIprior-version\fP \fIpackage\fP \-\- "$@"
.P
\fIconffile\fP is the filename of the conffile to remove.
.P
Current implementation: in the \fBpreinst\fP, it checks if the conffile
was modified and renames it either to \fIconffile\fP\fB.dpkg\-remove\fP (if not
modified) or to \fIconffile\fP\fB.dpkg\-backup\fP (if modified). In the
\fBpostinst\fP, the latter file is renamed to \fIconffile\fP\fB.dpkg\-bak\fP
and kept for reference as it contains user modifications but the former will
be removed. If the package upgrade aborts, the \fBpostrm\fP reinstalls the
original conffile. During purge, the \fBpostrm\fP will also delete the
\fB.dpkg\-bak\fP file kept up to now.
.
.SS Renaming a conffile
.P
If a conffile is moved from one location to another, you need to make sure
you move across any changes the user has made. This may seem a simple
change to the \fBpreinst\fP script at first, however that will result in
the user being prompted by \fBdpkg\fP to approve the conffile edits even
though they are not responsible of them.
.P
Graceful renaming can be implemented by putting the following shell
snippet in the \fBpreinst\fP, \fBpostinst\fP and \fBpostrm\fP maintainer
scripts:
.P
    dpkg\-maintscript\-helper mv_conffile \\
        \fIold-conffile\fP \fInew-conffile\fP \fIprior-version\fP \fIpackage\fP \-\- "$@"
.P
\fIold-conffile\fP and \fInew-conffile\fP are the old and new name of the
conffile to rename.
.P
Current implementation: the \fBpreinst\fP checks if the conffile has been
modified, if yes it's left on place otherwise it's renamed to
\fIold-conffile\fP\fB.dpkg\-remove\fP. On configuration, the \fBpostinst\fP
removes \fIold-conffile\fP\fB.dpkg\-remove\fP and renames \fIold-conffile\fP
to \fInew-conffile\fP if \fIold-conffile\fP is still available. On
abort\-upgrade/abort\-install, the \fBpostrm\fP renames
\fIold-conffile\fP\fB.dpkg\-remove\fP back to \fIold-conffile\fP if required.
.
.SH SYMLINK AND DIRECTORY SWITCHES
.
When upgrading a package, \fBdpkg\fP will not automatically switch a symlink
to a directory or vice-versa. Downgrades are not supported and the path
will be left as is.
.
.SS 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 \fBpreinst\fP script at first, however that will result
in some problems in case of admin local customization of the symlink
or when downgrading the package.
.P
Graceful renaming can be implemented by putting the following shell
snippet in the \fBpreinst\fP, \fBpostinst\fP and \fBpostrm\fP maintainer
scripts:
.P
    dpkg\-maintscript\-helper symlink_to_dir \\
        \fIpathname\fP \fIold-target\fP \fIprior-version\fP \fIpackage\fP \-\- "$@"
.P
\fIpathname\fP is the name of the old symlink (the path will be a
directory at the end of the installation) and \fIold-target\fP the
target of the former symlink at \fIpathname\fP.
.P
Current implementation: the \fBpreinst\fP checks if the symlink exists
and points to \fIold-target\fP, if not then it's left in place, otherwise
it's renamed to \fIpathname\fP\fB.dpkg\-backup\fP. On configuration,
the \fBpostinst\fP removes \fIpathname\fP\fB.dpkg\-backup\fP if
\fIpathname\fP\fB.dpkg\-backup\fP is still a symlink. On
abort\-upgrade/abort\-install, the \fBpostrm\fP renames
\fIpathname\fP\fB.dpkg\-backup\fP back to \fIpathname\fP if required.
.
.SS Switching a directory to symlink
.
If a real directory is switched to a symlink, you need to make sure
before unpacking that the directory is removed. This may seem a simple
change to the \fBpreinst\fP script at first, however that will result
in some problems in case the directory contains conffiles, pathnames
owned by other packages, locally created pathnames, or when downgrading
the package.
.P
Graceful switching can be implemented by putting the following shell
snippet in the \fBpreinst\fP, \fBpostinst\fP and \fBpostrm\fP maintainer
scripts:
.P
    dpkg\-maintscript\-helper dir_to_symlink \\
        \fIpathname\fP \fInew-target\fP \fIprior-version\fP \fIpackage\fP \-\- "$@"
.P
\fIpathname\fP is the name of the of the old directory (the path will be a
symlink at the end of the installation) and \fInew-target\fP is the target
of the new symlink at \fIpathname\fP.
.P
Current implementation: the \fBpreinst\fP 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 \fIpathname\fP\fB.dpkg\-backup\fP, and an empty staging
directory named \fIpathname\fP is created, marked with a file so that
dpkg can track it. On configuration, the \fBpostinst\fP finishes the
switch if \fIpathname\fP\fB.dpkg\-backup\fP is still a directory and
\fIpathname\fP is the staging directory; it removes the staging directory
mark file, moves the newly created files inside the staging directory
to the symlink target \fInew-target\fP/, replaces the now empty staging
directory \fIpathname\fP with a symlink to \fInew-target\fP, and
removes \fIpathname\fP\fB.dpkg\-backup\fP. On
abort\-upgrade/abort\-install, the \fBpostrm\fP renames
\fIpathname\fP\fB.dpkg\-backup\fP back to \fIpathname\fP if required.
.
.SH INTEGRATION IN PACKAGES
.P
Given that \fBdpkg\-maintscript\-helper\fP is used in the \fBpreinst\fP,
using it unconditionally requires a pre-dependency to ensure that the
required version of \fBdpkg\fP has been unpacked before. The required version
depends on the command used, for \fBrm_conffile\fP and \fBmv_conffile\fP
it is 1.15.7.2, for \fBsymlink_to_dir\fP and \fBdir_to_symlink\fP
it is 1.17.5:
.P
    \fBPre\-Depends:\fP dpkg (>= 1.17.5)
.P
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 \fBdpkg\fP:
.P
    if dpkg\-maintscript\-helper supports \fIcommand\fP; then
        dpkg\-maintscript\-helper \fIcommand\fP ...
    fi
.P
The command \fBsupports\fP will return 0 on success, 1 otherwise. The
\fBsupports\fP 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.