The command df can be used to monitor disk usage. For instance:

root@hostname:directory# df
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/md0              10481980    602996   9878984   6% /
tmpfs                  1036104         0   1036104   0% /dev/shm
/dev/md1              58040976     32848  58008128   1% /home
tmpfs                    10240      2616      7624  26% /dev
root@hostname:directory# 

This presents the space used on each partiton of the disk. To show this in a human readable format, you may use the -h option.

root@hostname:directory# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/md0               10G  589M  9.5G   6% /
tmpfs                1012M     0 1012M   0% /dev/shm
/dev/md1               56G   33M   56G   1% /home
tmpfs                  10M  2.6M  7.5M  26% /dev
root@hostname:directory#

The listing now shows disk usage in megabytes(M) or gigabytes(G) used and available for each disk partition.