This file is indexed.

/usr/share/initramfs-tools/scripts/live-premount/readonly is in live-boot-initramfs-tools 3.0~a24-1ubuntu1.

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The actual contents of the file can be viewed below.

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#!/bin/sh

#set -e

# initramfs-tools header

PREREQ="udev"

prereqs()
{
	echo "${PREREQ}"
}

case "${1}" in
	prereqs)
		prereqs
		exit 0
		;;
esac

# live-boot script

# make sure all harddisk devices are read-only
# this is important for forensic investigations
if grep -qe forensic -qe readonly /proc/cmdline
then
	for device in /dev/hd* /dev/sd* /dev/vd*
	do
		if [ -b "$device" ]
		then
			printf " * Setting device %-9s to read-only mode: " $device >/dev/console
			blockdev --setro $device && printf "done [ execute \"blockdev --setrw %-9s\" to unlock]\n" $device >/dev/console || printf "failed\n" >/dev/console
		fi
	done
fi