After installing Kali Linux on hard drive or on virtual machine, we may have come across with a warning message of "Low Disk usage". It happens when we allocate insufficient disk space during partition for kali linux or else when we do a regular "update" and "upgrade" of patches or downloading softwares. Kali Linux comes with a graphical built-in tool, i.e. Disk Usage Analyzer. This tool helps you to check the disk space usage. There are other options too, to check usage as per file system.
If you need to check from terminal(for non-gui users), then there are two classic commands available for every linux distros, i.e.
(a) df: Report file system disk space usage
(b) du: Estimate file space usage
df command is used to check free disk space. You can type df -h or df -k to list free disk space. It displays statistics about the amount of free disk space on the specified file system or on the file system of which file is a part. Values are displayed in 512-byte per block counts.
du command is used to show how much space one ore more files or directories is using. Type du -sh where -s option summarize the space a directory is using and -h option provides "Human-readable" output.