with-bind-mount

#!/bin/bash

usage_content=(
	"Usage: ${0##*/} [--if-exists] src dest [--] cmd arg..."
)

usage() {
	printf '%s\n' "${usage_content[@]}"
}

ignore_missing=0

(( $# > 2 )) || { usage; exit 1; }

while [[ $1 = -* ]]; do
	case $1 in
		--) shift; break;;
		--if-exists)
			ignore_missing=1;;
		-h|--help) usage; exit;;
		*) usage; exit 1;;
	esac
	shift
done

[[ $EUID = 0 ]] || {
	echo "ERROR: ${0##*/} requires root" >&2
	exit 1
}
source read-mount-table.bash || exit

source=$1; shift
dest=$1; shift
[[ $1 = -- ]] && shift
(( $# )) || { usage; exit 1; }

if [[ -e $source && -e $dest ]]; then
	dest_mountpoint=$(stat -c %m "$dest")
	dest_mid=${relroot_to_mid[$dest_mountpoint]}
	if [[ ${mid_to_options_str[$dest_mid]} = *" shared:"* ]]; then
		echo "ERROR: $dest_mountpoint has not been unshared" >&2
		exit 1
	fi
	mount -o bind -- "$source" "$dest" || exit
else
	if (( ignore_missing )); then
		echo "WARNING: Either $source or $dest does not exist" >&2
	else
		echo "ERROR: Either $source or $dest does not exist" >&2
		exit 1
	fi
fi

exec -- "$@"