postinst is in zsh 5.0.2-3ubuntu6.
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 | #!/bin/sh
set -e
case "$1" in
    (configure)
#    if test -z "$2"; then
	    add-shell /bin/zsh
	    add-shell /usr/bin/zsh
#    fi
    ;;
    (abort-upgrade|abort-remove|abort-deconfigure)
	exit 0
    ;;
    (*)
	echo "postinst called with unknown argument \`$1'" >&2
	exit 0
    ;;
esac
update-alternatives --remove zsh /usr/bin/zsh
update-alternatives --remove ksh /usr/bin/zsh
update-alternatives --remove ksh /bin/zsh4
update-alternatives --install /bin/zsh zsh /bin/zsh5 50 \
		--slave /usr/bin/zsh zsh-usrbin /bin/zsh5
update-alternatives --install /bin/rzsh rzsh /bin/zsh5 50 \
		--slave /usr/share/man/man1/rzsh.1.gz rzsh.1.gz /usr/share/man/man1/zsh.1.gz
# Replace documentation directory with symlink
docdir="/usr/share/doc/zsh"
if [ -d $docdir -a ! -L $docdir ]; then
    if rmdir $docdir 2>/dev/null; then
        ln -sf zsh-common $docdir
    fi
fi
exit 0
 |