postinst is in dpkg 1.19.0.5ubuntu2.
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 32 33 34 35 36 37 38 39 40 41 42 43  | #!/bin/sh
# See deb-postinst(5).
set -e
# Create the database files if they don't already exist
create_database() {
    admindir=${DPKG_ADMINDIR:-/var/lib/dpkg}
    for file in diversions statoverride status; do
	if [ ! -f "$admindir/$file" ]; then
	    touch "$admindir/$file"
	fi
    done
}
# Create log file and set default permissions if possible
create_logfile() {
    logfile=$DPKG_ROOT/var/log/dpkg.log
    touch $logfile
    chmod 644 $logfile
    chown root:root $logfile 2>/dev/null || chown 0:0 $logfile
}
case "$1" in
    configure)
	create_database
	create_logfile
	;;
    abort-upgrade|abort-deconfigure|abort-remove)
	;;
    *)
	echo "$0 called with unknown argument '$1'" 1>&2
	exit 1
	;;
esac
exit 0
 |