Linux commands to check for Disk Usage and Free Space on disk

http://arunonfire.blogspot.com/2007/11/linux-commands-to-check-for-disk-usage.html

About du
Tells you how much space a file occupies.

Syntaxdu [-a] [-k] [-s] [-d] [-L] [-o] [-r] [-x] directories

Example :

[root@HACK1 Directory]# du 4 ./certs4 ./keys4 ./trust-certs100128 .
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About df

Tells you how much free disk space is available for each mount you have.

Syntax - Man on the details for more info

df [ -F FSType ] [-a [-b] [-e] [-g] [-k] [-l] [-n] [-t] [-V ] [ -o FSTypespecific_options ] [block_device directory file resource ... ] [-P]

Examples
df

In the below example when performing just the df command with no additional switches or specification of the file or directory you would get a listing of all file systems and their used and available space. Below is an example of what this command may display.

[root@HACK1 Directory]# df
Filesystem 1K-blocks Used Available Use% Mounted on
/dev/cciss/c0d0p1 12357692 11324400 907748 93% /
/dev/cciss/c0d0p6 43009872 16444384 24380652 41% /common
/dev/cciss/c0d0p3 256665 8413 234999 4% /grub
/dev/cciss/c0d0p2 12357724 32828 11697160 1% /partB
none 512700 16752 495948 4% /dev/shm

Thanks to Kavitha for the details