postinst is in rxvt 1:2.7.10-6.
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 | #!/bin/sh
set -e
if [ "$1" = "configure" ]; then
  update-alternatives --install /usr/bin/x-terminal-emulator \
    x-terminal-emulator /usr/bin/rxvt-xterm 10 \
    --slave /usr/share/man/man1/x-terminal-emulator.1.gz \
    x-terminal-emulator.1.gz /usr/share/man/man1/rxvt-xterm.1.gz
  update-alternatives --install /usr/bin/x-terminal-emulator \
    x-terminal-emulator /usr/bin/rxvt-xpm 8 \
    --slave /usr/share/man/man1/x-terminal-emulator.1.gz \
    x-terminal-emulator.1.gz /usr/share/man/man1/rxvt-xpm.1.gz
  update-alternatives --install /usr/bin/rxvt \
    rxvt /usr/bin/rxvt-xterm 30 \
    --slave /usr/share/man/man1/rxvt.1.gz \
    rxvt.1.gz /usr/share/man/man1/rxvt-xterm.1.gz
  update-alternatives --install /usr/bin/rxvt \
    rxvt /usr/bin/rxvt-xpm 20 \
    --slave /usr/share/man/man1/rxvt.1.gz \
    rxvt.1.gz /usr/share/man/man1/rxvt-xpm.1.gz
fi
# Automatically added by dh_installmenu
inst=/etc/menu-methods/rxvt
if [ -f $inst ]; then
	chmod a+x $inst
	if [ -x "`which update-menus 2>/dev/null`" ]; then
		update-menus
	fi
fi
# End automatically added section
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