a simple linux shell command to show you how much space a directory is using is helpful in maitaining a good disk managment. Lately i wanted to find out how much space i had left in my dedicated server, well to display or to show how much is being used, this is the command you can send:

NOTE: you must be logged in as root

cd /

du -hc --max-depth=1

this will show you something like this:
[root@hostname /]# du -hc --max-depth=1
0       ./net

1.6G    ./usr
16K     ./lost+found
515M    ./proc
8.0K    ./misc
648M    ./var
112K    ./dev
8.0K    ./mnt
66M     ./lib
8.0K    ./selinux
7.3M    ./bin
77M     ./root
4.2M    ./boot
28K     ./tmp
2.7M    ./backups
20M     ./sbin
8.0K    ./media
8.0K    ./srv
0       ./sys
43M     ./etc
42M     ./home
8.0K    ./opt
3.0G    .
3.0G    total

and addition, you could also use these commands to show you disk space being use by your sytem on the linux or unix shell command:

cd /
df -H
cd /
df -sh *

you will see something like this:
[root@hostname/]# df -H
Filesystem             Size   Used  Avail Use% Mounted on
                       387G   2.1G   365G   1% /
/dev/hda1              104M    11M    89M  11% /boot
tmpfs                  265M      0   265M   0% /dev/shm