With ADB, a lot of possibilities are open to administrate and tweak your Android device. A tool like QtADB is a good example of what can be done.
Some time back, I discovered that you can take some screenshots of your Android device thru ADB with a inbuilt utility called screencap. But it only works when you are running a normal android session, not when you are in recovery mode.
After some searches, I found that many devices, depending on their hardware, can provide some screen capture thru a frame buffer.
Both these approaches open some nice possibilities to take screenshot of your device while running any type of application (tutorials, games, …) or even running a recent version of recovery like TWRP where ADB is activated by default.
This article explains how to take a screenshot of an android device in ADB mode thru some simple console commands by using both screencap or frame buffer methods.
It also provides a GUI tool to take sreenshot of your android device with a single click from the comfort of your linux desktop.
It has been written and tested on Ubuntu 14.04 LTS, but it should be applicable to any modern Linux flavor. Frame buffer capture under TWRP recovery has been tested on Nexus S, Nexus 7 2012 and Nexus 5.