Geektool is a program for adding customizable widgets to your Mac's desktop. Geektool runs almost entirely on shell scripts, which update every few seconds to display useful information on the desktop. Customizing Geektool is made easy by packaged scripts called Geeklets, which can be installed quickly and do not require knowledge of shell scripts to use.
Installing Geektool
Installing Geektool is simple; it doesn't need any files to install, just download the app from Tynsoe Projects and run it. You should be greeted by Geektool's main window.
Make sure you've enabled "Automatically launch at login" if you want Geektool to run after a reboot. It's also a good idea to put it in your Applications folder so that you don't accidentally delete it if you ever wipe your Downloads folder.
If you close this window, Geektool will continue to run in the background. If you want to stop it, you will have to launch the app again and uncheck "Enable", or click "Quit Geektool" from the menubar. You can also get to Geektool's settings from this menubar.
Finding and Adding Geeklets to Your Desktop
Many Geeklets can be found on the official repository. Another great source is the Geektool subreddit. Geeklets come either as .glet files or as individual scripts. The .glet files can be installed simply by opening them and adding them to Geektool.
Scripts can be installed by dragging a new "Shell" Geeklet to the desktop and pasting the script into the "Command:" box.
Tweaking Geektool
Shell Geeklets output text, and you can change the look and style of each one. From Geektool's settings, click a Geeklet to open the Properties window. At the bottom of the window is the style options, from which you can set the font to anything OS X supports, including custom fonts.
If you want to tweak the scripts that make Geektool function, you can do so. Click the "..." button beside the "Command:" box, which will bring up the fullscreen editor. From here, you can edit the scripts for any Geeklet. It's easiest to learn first by tweaking other's Geeklet scripts and then move up to writing your own.
This is not recommended for anyone without prior experience with shell scripts, as these are actual shell commands and can modify your file system. While it's hard to accidentally delete all of your files with Geektool, it is possible, so be careful.