postinst is in casacore-data-lines 0+git2016.11.26-2.
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 | #!/bin/sh
set -e
case "$1" in
    configure)
	if [ ! -d /var/lib/casacore/data/ephemerides/Lines/ ] ; then
	    mkdir -p /var/lib/casacore/data/ephemerides
            ln -sf /usr/share/casacore/data/ephemerides/Lines/ \
	       /var/lib/casacore/data/ephemerides/Lines
	fi
	;;
    abort-upgrade|abort-remove|abort-deconfigure)
	;;
    *)
        echo "postinst called with unknown argument \`$1'" >&2
        exit 1
	;;
esac
 |