The df command is used to view the amount of free diskspace on a server. The most common usage of this command will include the -h flag. This flag will make the output be more human readable.
Here is an example of this command from one of our shared servers:
# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/sda7 1012M 573M 388M 60% /
/dev/sda1 99M 31M 64M 33% /boot
none 2.0G 0 2.0G 0% /dev/shm
/dev/sda8 51G 37G 13G 75% /home
/dev/sda6 1012M 35M 926M 4% /tmp
/dev/sda2 6.6G 4.4G 1.9G 71% /usr
/dev/sda5 6.0G 2.1G 3.7G 37% /var
/dev/sdb1 68G 35G 30G 54% /home2
/tmp 1012M 35M 926M 4% /var/tmp
This command provides some useful information about current free space on your system. This would be a good thing to check before adding any new large clients to a server. Checking the free disk space can assure that you have sufficient space for the new account.