A script to simplify entering a "chroot"-style container in the Gentoo install process

enter-container.sh

#!/bin/sh

HOSTNAME=
BIND=
SHELL=bash

while getopts "b:h:s:" opt; do
	case $opt in
	b)
		BIND="${BIND} ${OPTARG}"			
		;;
	h)
		HOSTNAME="${OPTARG}"
		;;
	s)
		SHELL="${OPTARG}"
		;;

	\?)
		echo "Invalid option: -$OPTARG" >&2
		;;
	:)
		echo "Option -$OPTARG requires an argument." >&2
		exit 1
		;;
	esac
done

shift $(($OPTIND - 1))
ROOT="$*"

# Use a mount namespace (replaces chroot method)
exec unshare -ipmuf /bin/sh -c "
pivot_root \"${ROOT}\" \"${ROOT}/tmp\"
mount --rbind /tmp/dev /dev
mount --rbind /tmp/sys /sys
mount -t proc none /proc
if [ -n \"${BIND}\" ]
then
echo \"${BIND}\" | while IFS=: read src dest opts; do
	mkdir -p \"\${dest}\"
	mount --bind \"/tmp\${src# }\" \"\${dest}\"
	[ -n \"\${opts}\" ] && mount -o \"remount,\${opts}\" \"\${dest}\"
done
fi
umount -l /tmp
hostname ${HOSTNAME} > /dev/null
cd
exec ${SHELL}
"