postinst is in elvish 0.11+ds1-3.
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)
        add-shell /usr/bin/elvish
    ;;
esac
exit 0
 |