7. Diversions - overriding a package’s version of a file (from old Packaging Manual)

It is possible to have dpkg not overwrite a file when it reinstalls the package it belongs to, and to have it put the file from the package somewhere else instead.

This can be used locally to override a package’s version of a file, or by one package to override another’s version (or provide a wrapper for it).

Before deciding to use a diversion, read Alternative versions of an interface - update-alternatives (from old Packaging Manual) to see if you really want a diversion rather than several alternative versions of a program.

There is a diversion list, which is read by dpkg, and updated by a special program dpkg-divert. Please see dpkg-divert(8) for full details of its operation.

When a package wishes to divert a file from another, it should call dpkg-divert in its preinst to add the diversion and rename the existing file. For example, supposing that a smailwrapper package wishes to install a wrapper around /usr/sbin/smail:

dpkg-divert --package smailwrapper --add --rename \
    --divert /usr/sbin/smail.real /usr/sbin/smail

The --package smailwrapper ensures that smailwrapper’s copy of /usr/sbin/smail can bypass the diversion and get installed as the true version. It’s safe to add the diversion unconditionally on upgrades since it will be left unchanged if it already exists, but dpkg-divert will display a message. To suppress that message, make the command conditional on the version from which the package is being upgraded:

if [ upgrade != "$1 ] || dpkg --compare-versions "$2" lt 1.0-2; then
    dpkg-divert --package smailwrapper --add --rename \
        --divert /usr/sbin/smail.real /usr/sbin/smail
fi

where 1.0-2 is the version at which the diversion was first added to the package. Running the command during abort-upgrade is pointless but harmless.

The postrm has to do the reverse:

if [ remove = "$1" -o abort-install = "$1" -o disappear = "$1 ]; then
    dpkg-divert --package smailwrapper --remove --rename \
        --divert /usr/sbin/smail.real /usr/sbin/smail
fi

If the diversion was added at a particular version, the postrm should also handle the failure case of upgrading from an older version (unless the older version is so old that direct upgrades are no longer supported):

if [ abort-upgrade = "$1 ] && dpkg --compare-versions "$2" lt 1.0-2; then
    dpkg-divert --package smailwrapper --remove --rename \
        --divert /usr/sbin/smail.real /usr/sbin/smail
fi

where 1.0-2 is the version at which the diversion was first added to the package. The postrm should not remove the diversion on upgrades both because there’s no reason to remove the diversion only to immediately re-add it and since the postrm of the old package is run after unpacking so the removal of the diversion will fail.

Do not attempt to divert a file which is vitally important for the system’s operation - when using dpkg-divert there is a time, after it has been diverted but before dpkg has installed the new version, when the file does not exist.

Do not attempt to divert a conffile, as dpkg does not handle it well.