preinst is in jackd1 1:0.124.1+20140122git5013bed0-3build2.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31  | #!/bin/sh -e
# Remove a no-longer used conffile
rm_conffile() {
    local PKGNAME="$1"
    local CONFFILE="$2"
    [ -e "$CONFFILE" ] || return 0
    local md5sum="$(md5sum $CONFFILE | sed -e 's/ .*//')"
    local old_md5sum="$(dpkg-query -W -f='${Conffiles}' $PKGNAME | \
            sed -n -e "\' $CONFFILE ' { s/ obsolete$//; s/.* //; p }")"
    if [ "$md5sum" != "$old_md5sum" ] && \
       [ "$md5sum" != "c9d1f0caa7b49ce7f0d2bc7b458bdd3f" ]; then
        echo "Obsolete conffile $CONFFILE has been modified by you."
        echo "Saving as $CONFFILE.dpkg-bak ..."
        mv -f "$CONFFILE" "$CONFFILE".dpkg-bak
    else
        echo "Removing obsolete conffile $CONFFILE ..."
        rm -f "$CONFFILE"
    fi
}
LASTVERSION=0.116.2+svn3592-2
case "$1" in
install|upgrade)
    if dpkg --compare-versions "$2" le "$LASTVERSION"; then
        rm_conffile jackd "/etc/init.d/jackd"
    fi
esac
 |