/usr/lib/curtin/helpers/install-grub is in curtin-common 18.1-5-g572ae5d6-0ubuntu1.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 | #!/bin/bash
# This file is part of curtin. See LICENSE file for copyright and license info.
[ "${0%/*}" = "$0" ] && . ./common || . "${0%/*}/common"
install_grub "$@"
# vi: ts=4 expandtab syntax=sh
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