preinst is in hardening-wrapper 1.36ubuntu1.1.
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 | #!/bin/sh -e
dodiv()
{
	dpkg-divert --package hardening-wrapper --add --rename \
		--divert /usr/bin/"$1".real /usr/bin/"$1"
}
if [ "install" = "$1" ] || [ "upgrade" = "$1" ]; then
	# handle diversions (change debian/rules, debian/h-w.postrm too)
	for ver in 4.2 4.3 4.4 4.5 4.6
	do
		for i in gcc g++
		do
			dodiv "$i-$ver"
		done
	done
	dodiv ld.bfd
	dodiv ld.gold
fi
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