You have an installed package and you would like to see the if any of its installed files have been changed. If there are changes, you would like to see the differences.

Verify Package

The solution to finding modified package files is the dpkg -V *PACKAGE_NAME* command. It lists any files whose md5sums do not match those in the package metadata.

Example:

    $ dpkg -V minidlna
    ??5?????? c /etc/minidlna.conf

Show Differences in Modified Package Files

get the package locally

    apt-get download minidlna
    PKG=minidlna.*deb

look at the list of files in the package

    dpkg-deb --contents ${PKG}

Get the tarfile of the package and extract the file of interest to stdout:

    dpkg-deb --fsys-tarfile ${PKG} \
      | tar xfO - ./etc/minidlna.conf

Now put it all together. Given a debian package, DEB, and a file of interest, FILE, produce a difference listing using the following bash script:

#!/bin/bash

DEB="$1"
shift
FILES=("${@}")


error() {
    echo "$@" 1>&2
    exit 1
}


WORKDIR=$(mktemp -d)
trap "rm -rf ${WORKDIR}" 0 1 2 15

(cd ${WORKDIR}; apt-get download ${DEB})
DEB_FILE=$(eval echo ${WORKDIR}/${DEB}*.deb)

[ -f "${DEB_FILE}" ] || error "${DEB}: ${DEB_FILE} not found."

for FILE in "${FILES[@]}"; do
    MEMBER=$(dpkg-deb --contents ${DEB_FILE} |grep "${FILE}" |awk '{print $NF}')
    if [ -n "${MEMBER}" ]; then
        diff -u <(dpkg-deb --fsys-tarfile ${DEB_FILE} \
                  |tar xfO - "${MEMBER}") "${FILE}"
    fi
done

Here is a sample run:

    $ debdiff minidlna /etc/minidlna.conf
    Get:1 http://ports.ubuntu.com/ubuntu-ports bionic/universe arm64 minidlna arm64 1.2.1+dfsg-1 [128 kB]
    Fetched 128 kB in 2s (61.7 kB/s)
    --- /dev/fd/63  2020-04-22 10:51:12.718656103 -0600
    +++ /etc/minidlna.conf  2019-03-01 22:34:52.672223536 -0700
    @@ -78,11 +78,11 @@

     # Name that the DLNA server presents to clients.
     # Defaults to "hostname: username".
    -#friendly_name=
    +friendly_name=My Media Server

     # Serial number the server reports to clients.
     # Defaults to the MAC address of nework interface.
    -#serial=
    +serial=104857665536

     # Model name the server reports to clients.
     #model_name=Windows Media Connect compatible (MiniDLNA)