Provavelmente algo como
#!/bin/sh
MatchLabel=Backup
CheckPath=/mnt/usb0
realDevice='readlink -f /dev/disk/by-label/$MatchLabel 2>/dev/null'
mountedDevice='awk "/ $CheckPath /{print \}" /etc/mtab 2>/dev/null'
test "$mountedDevice" && realMountedDevice='readlink -f $mountedDevice'
if test "$realDevice" = "$realMountedDevice"; then
echo $MatchLabel mounted to $CheckPath
else
echo $realMountedDevice was unexpectedly mounted to $CheckPath
fi