The df (disk free) command displays the amount of free disk space. This is often quite handy when determining if there is enough space to store an image on a particular hard drive. Best used with -h (human readable) flag.
Example:
$ df -h
Example of what output “might” look like:
Filesystem Size Used Avail Use% Mounted on /dev/hda2 9.1G 2.1G 6.6G 25% / /dev/hda1 97M 9.2M 83M 10% /boot none 62M 0 62M 0% /dev/shm
I use this command a lot because I manage disk usage on our mail server. If you manage a mail server for an ISP you know what I mean…. 
