postinst is in tar 1.27.1-1.
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 | #!/bin/sh
set -e
case "$1" in
    configure|abort-deconfigure|abort-remove)
	update-alternatives --install /usr/sbin/rmt rmt /usr/sbin/rmt-tar 50 \
		--slave /usr/share/man/man8/rmt.8.gz rmt.8.gz \
			/usr/share/man/man8/rmt-tar.8.gz 
    ;;
    upgrade)
    ;;
esac
 |