postinst is in nfs-common 1:1.2.5-3ubuntu3.
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.
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set -e
finish_rm_conffile() {
	local CONFFILE="$1"
	if [ -e "$CONFFILE.dpkg-backup" ]; then
		mv -f "$CONFFILE.dpkg-backup" "$CONFFILE.dpkg-bak"
	fi
	if [ -e "$CONFFILE.dpkg-remove" ]; then
		echo "Removing obsolete conffile $CONFFILE ..."
		rm -f "$CONFFILE.dpkg-remove"
	fi
}
case "$1" in
    configure)
        ucf --three-way /usr/share/nfs-common/conffiles/idmapd.conf /etc/idmapd.conf
        ucf --three-way /usr/share/nfs-common/conffiles/nfs-common.default /etc/default/nfs-common
        if ! getent passwd statd >/dev/null; then
            adduser --system --home /var/lib/nfs --no-create-home statd
        fi
        if dpkg --compare-versions "$2" ge 1:1.0.7-10 && dpkg --compare-versions "$2" lt 1:1.0.7-13; then
            usermod --home /var/lib/nfs statd || true
            if [ -d /home/statd ]; then
                rmdir --ignore-fail-on-non-empty /home/statd
            fi
        fi
        chown statd /var/lib/nfs/sm \
            /var/lib/nfs/sm.bak \
            /var/lib/nfs
        if [ -f /var/lib/nfs/state ]; then
            chown statd /var/lib/nfs/state
        fi
        
        if [ "$2" != "" ] || dpkg --compare-versions "$2" lt 1:1.1.0-14; then
            if dpkg-statoverride --list /sbin/mount.nfs >/dev/null 2>&1; then
                dpkg-statoverride --remove /sbin/mount.nfs
            fi
        fi
	if dpkg --compare-versions "$2" lt-nl 1:1.2.0-2ubuntu1
	then
		if [ -e /etc/init.d/nfs-common.dpkg-remove ]
		then
			invoke-rc.d nfs-common.dpkg-remove stop
		elif [ -e /etc/init.d/nfs-common.dpkg-backup ]
		then
			invoke-rc.d nfs-common.dpkg-backup stop
		fi
		update-rc.d nfs-common remove
	elif dpkg --compare-versions "$2" lt-nl 1:1.2.4-1ubuntu3
	then
		# handle the move of rpc_pipefs from /var/lib to /run
		invoke-rc.d idmapd stop
		invoke-rc.d gssd stop
		if [ -d /var/lib/nfs/rpc_pipefs ]; then
			rmdir --ignore-fail-on-non-empty /var/lib/nfs/rpc_pipefs
		fi
	fi
	# Migrate /lib/init/rw/sendsigs.omit.statd to /run.
	if [ -f /lib/init/rw/sendsigs.omit.d/statd ]; then 
	    mv /lib/init/rw/sendsigs.omit.d/statd /run/sendsigs.omit.d/statd
	fi
    ;;
esac
finish_rm_conffile /etc/init/rpc_pipefs.conf
# Automatically added by dh_installinit
if [ -x "/etc/init.d/statd" ]; then
	if [ -n "$2" ]; then
		_dh_action=restart
	else
		_dh_action=start
	fi
	invoke-rc.d statd $_dh_action || exit $?
fi
# End automatically added section
# Automatically added by dh_installinit
if [ -x "/etc/init.d/gssd" ]; then
	if [ -n "$2" ]; then
		_dh_action=restart
	else
		_dh_action=start
	fi
	invoke-rc.d gssd $_dh_action || exit $?
fi
# End automatically added section
# Automatically added by dh_installinit
if [ -x "/etc/init.d/idmapd" ]; then
	if [ -n "$2" ]; then
		_dh_action=restart
	else
		_dh_action=start
	fi
	invoke-rc.d idmapd $_dh_action || exit $?
fi
# End automatically added section
finish_rm_conffile /etc/init.d/nfs-common
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