postrm is in ifupdown2 1.0~git20170314-1.
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 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116  | #!/bin/sh
# postrm script for ifupdown2
#
# see: dh_installdeb(1)
set -e
# summary of how this script can be called:
#        * <postrm> `remove'
#        * <postrm> `purge'
#        * <old-postrm> `upgrade' <new-version>
#        * <new-postrm> `failed-upgrade' <old-version>
#        * <new-postrm> `abort-install'
#        * <new-postrm> `abort-install' <old-version>
#        * <new-postrm> `abort-upgrade' <old-version>
#        * <disappearer's-postrm> `disappear' <overwriter>
#          <overwriter-version>
# for details, see http://www.debian.org/doc/debian-policy/ or
# the debian-policy package
process_udev()
{
    if [ -e /etc/udev/rules.d/80-networking.rules ]; then
        udevlink=$(readlink /etc/udev/rules.d/80-networking.rules 2>/dev/null || true)
        [ -n "$udevlink" -a "$udevlink" = "/dev/null" ] && rm -f /etc/udev/rules.d/80-networking.rules
    fi
    if  [ -e /etc/udev/rules.d/60-bridge-network-interface.rules ]; then
        udevlink=$(readlink /etc/udev/rules.d/60-bridge-network-interface.rules 2>/dev/null || true)
        [ -n "$udevlink" -a "$udevlink" = "/dev/null" ] && rm -f /etc/udev/rules.d/60-bridge-network-interface.rules
    fi
}
postrm_remove()
{
    process_udev
}
# Note: We don't remove /etc/network/interfaces
postrm_purge()
{
	rm -f /run/network/ifstatenew
}
# restore file if we diverted it on install/upgrade
_postrm_divert()
{
    diversions=$(LC_ALL=C.UTF-8 dpkg-divert --list | grep "^diversion of $1 .* by ifupdown2$"  | wc -l 2> /dev/null)
    if [ $diversions -gt 0 ];
    then
        dpkg-divert --remove --package ifupdown2 --rename $1
    fi
}
postrm_divert()
{
    for filename in ifup ifdown ifquery ifreload
    do
        _postrm_divert "/sbin/$filename"
        _postrm_divert "/usr/share/bash-completion/completions/$filename"
        _postrm_divert "/etc/bash_completion.d/$filename"
        _postrm_divert "/usr/share/man/man8/$filename.8.gz"
    done
    _postrm_divert "/usr/share/man/man5/interfaces.5.gz"
}
case "$1" in
	purge)
		postrm_purge
	;;
	remove)
		postrm_remove
		postrm_divert
	;;
    abort-install)
        postrm_divert
    ;;
	upgrade|failed-upgrade|abort-upgrade|disappear)
	;;
	*)
		echo "postrm called with unknown argument \`$1'" >&2
		exit 1
	;;
esac
dpkg-maintscript-helper rm_conffile "/etc/bash_completion.d/ifup" 1.1 -- $@
# dh_installdeb will replace this with shell code automatically
# generated by other debhelper scripts.
# Automatically added by dh_systemd_start
if [ -d /run/systemd/system ]; then
	systemctl --system daemon-reload >/dev/null || true
fi
# End automatically added section
# Automatically added by dh_systemd_enable
if [ "$1" = "remove" ]; then
	if [ -x "/usr/bin/deb-systemd-helper" ]; then
		deb-systemd-helper mask networking.service >/dev/null
	fi
fi
if [ "$1" = "purge" ]; then
	if [ -x "/usr/bin/deb-systemd-helper" ]; then
		deb-systemd-helper purge networking.service >/dev/null
		deb-systemd-helper unmask networking.service >/dev/null
	fi
fi
# End automatically added section
exit 0
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