LFS201 -- Ch16 -- filesystems -- What is "cmd"?

It should read "Directly writing to disk from the command line (ignoring.." It was poorly phrased. This would mean doing something like "dd if=/dev/random of=/dev/sda1" or echo "Hello this is some message" > /dev/sda1" or "cp somefile /dev/sda1". All of these commands would wipe out whatever is in the beginning of the first partition on the disk; i.e., corrupting the superblock and making it unusable. In Linux this tends to be recoverable as there are multiple copies of the superblock on the disk, but believe me, you don't want to deal with this (and how hard it is to fix depends on the type of filesystem)