How do I find out disk usage under UNIX operating systems using GUI, CLI and Perl / shell programs?

You need use the command line utility called du to displays the file system block usage. In this example find out /tmp dir disk usage statistics (open the terminal and type the following command):
$ du /tmp
Sample outputs:

4	/tmp/vmware-root
8	/tmp/pulse-xc7xdoM9vB2K
4	/tmp/.X11-unix
4	/tmp/keyring-7qXGnQ
4	/tmp/.exchange-vivek
4	/tmp/.winbindd
8	/tmp/plugtmp
4	/tmp/virtual-vivek.C81Sd0
4	/tmp/VMwareDnD
4	/tmp/ssh-mhNeIv1961
4	/tmp/.ICE-unix
8	/tmp/orbit-vivek
4	/tmp/.esd-1000
31644	/tmp

The -h option provides “Human-readable” outpu i.e. you will see it in Byte, Kilobyte, Megabyte, Gigabyte, Terabyte and Petabyte:
$ du -h /tmp
Sample outputs:

4.0K	/tmp/vmware-root
8.0K	/tmp/pulse-xc7xdoM9vB2K
4.0K	/tmp/.X11-unix
4.0K	/tmp/keyring-7qXGnQ
4.0K	/tmp/.exchange-vivek
4.0K	/tmp/.winbindd
8.0K	/tmp/plugtmp
4.0K	/tmp/virtual-vivek.C81Sd0
4.0K	/tmp/VMwareDnD
4.0K	/tmp/ssh-mhNeIv1961
4.0K	/tmp/.ICE-unix
8.0K	/tmp/orbit-vivek
4.0K	/tmp/.esd-1000
33M	/tmp

df: Display Free Disk Space

To show statistics about the amount of free disk space on the specified file system or on the file system of which file is a part use the df command as follows:
$ df
$ df -h
Sample outputs:

Filesystem     Size    Used   Avail Capacity  Mounted on
/dev/wd0a      938M   43.0M    848M     5%    /
/dev/wd0e      817M    2.0K    776M     0%    /home
/dev/wd0d      2.9G    573M    2.2G    20%    /usr

GUI Tools: Disk Usage Analyzer (Gnome Version)

Disk Usage Analyzer is a graphical, menu-driven application to analyze disk usage in any UNIX / Linux / BSD Gnome desktop environment. Disk Usage Analyzer can easily scan either the whole filesystem tree, or a specific user-requested directory branch (local or remote). To start this tool visit Gnome menu click on > Applications > Accessories > Select the Disk Usage Analyzer
Alternatively, you can start Disk Usage Analyzer from a terminal window, just type:
baobab
baobab /path/to/dir
baobab /home/vivek/mp3/
Sample outputs:

• Start a full filesystem scan
• Select a specific local directory branch to scan
• Select a remote server and folder to scan etc

To start a full filesystem scan select Analyzer > Scan Filesystem from the menu, or press on the Scan Filesystem toolbar button. Disk Usage Analyzer will display sizes in the directory tree as allocated space. This means that the displayed sizes refer to the actual disk usage and not to the apparent directory size. If you want to view the apparent file size, uncheck View > Allocated Space .

NCurses Disk Usage

ncdu (NCurses Disk Usage) is a curses-based version of the well-know du, and provides a fast way to see what directories are using your disk space. You can install it as follows under Debian / Ubuntu Linux:
$ sudo apt-get install ncdu
To install the port under FreeBSD, enter::
# cd /usr/ports/sysutils/ncdu/ && make install clean
OR
# pkg_add -r ncdu
Simply type ncdu at shell prompt:
$ ncdu
Sample outputs:

Python Sample Code

You can also use python as follows:

#!/usr/bin/python
import os
size = os.statvfs('/')
output=(size.f_bavail * size.f_frsize) / 1024
print "Available disp space" ,output, "k"

Perl Sample Code

Perl can be also used to find the disk space:

#!/usr/bin/perl
use strict;
use warnings;
use Filesys::DiskSpace;
my $dir = "/home";
my ($fs_type, $fs_desc, $used, $avail, $fused, $favail) = df $dir;
my $df_free = (($avail) / ($avail+$used)) * 100.0;
my $out = sprintf("Disk space on $dir: %0.2f\n",$df_free);
print $out;

Shell Scripts

You can monitor UNIX / Linux disk space usage with shell script and send an email alert if the percentage of space is <= 95%.