Used Space On Disk

df -h

To get the amount of free space on the disk as a single value in kB you can use this:

df -k /tmp | tail -1 | awk '{print $4}'

or in MB:

df -BM /tmp | tail -1 | awk '{print $4}'

(info about this here)

Used Space Of Current Directory

du -sh /*

Full Disk!

A few things you can do…

Remove old compressed log files from the system:

sudo rm -v /var/log/*.gz

Clean-Up Installation Packages  .To remove partial packages, clean the cache, remove unused dependencies use:

sudo apt-get autoclean
sudo apt-get clean
sudo apt-get autoremove