postinst is in adduser 3.113ubuntu2.
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 | #!/bin/sh
set -e
# create an initial adduser configfile if it does not exist yet
if [ ! -e "/etc/adduser.conf" ]; then
	cp /usr/share/adduser/adduser.conf /etc/adduser.conf
fi
# modify adduser.conf
if . /usr/share/debconf/confmodule
then	
	db_get adduser/homedir-changed || RET="false"
	if [ "$RET" != "true" ]
	then
		db_get adduser/homedir-permission || RET="true"
		if [ "$RET" = "false" ]
		then
			NEW_PERMISSION="0751"
		else
			NEW_PERMISSION="0755"
		fi
		if grep -q '^DIR_MODE=' /etc/adduser.conf
		then
			OLD_PERMISSION=$(cat /etc/adduser.conf|sed -ne 's/^DIR_MODE=\([0-9]*\).*$/\1/p')
			if [ "$OLD_PERMISSION" != "$NEW_PERMISSION" ]; then
				mv /etc/adduser.conf /etc/adduser.conf.dpkg-save
				cat /etc/adduser.conf.dpkg-save | \
				sed -e "s/^DIR_MODE=.*$/DIR_MODE=$NEW_PERMISSION/" > \
				/etc/adduser.conf
			fi
		else
			cp /etc/adduser.conf /etc/adduser.conf.dpkg-save
			echo "DIR_MODE=$NEW_PERMISSION" >> /etc/adduser.conf
		fi
	fi
fi
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