How To: Run Selenium Tests with Hudson on a Headless Linux Server, Part Two--xwd and xwud

In Part One, I explained how to start Xvfb to give you a virtual display to run graphical applications on. In this post, we'll verify that it's working properly by starting xclock and taking a screenshot of it. Being able to take a screenshot of a virtual display is a pretty useful skill all by itself. To take and view the screenshot, you'll use two different programs: xwd and xwud. The former takes the screenshot; the latter shows it. These should have been installed with your X server (I think).

First, start xclock on the system where Xvfb is running with xclock -display :5.0 so that it will run on the virtual display. Next, also on the system with Xfvb, take the screenshot with

xwd -root -display :5.0 -out xwdout

The breakdown:

-root

Tells xwd to capture the "root" window. This means it captures the entire screen. You can tell it to only capture a certain window if you want. See the -id and -name parameters in the man page if you want to try that.

-display :5.0

Tells xwd to look at screen 0 of display 5. This is why you have to know that info from when you started Xvfb!

-out xwdout

Gives the name of the output file to write the screenshot to. Xwd uses a custom binary format for its files. They can be read by xwud (next), and I understand that they can be converted to other formats by ImageMagick (free!), though I haven't tried it yet.

After taking the screenshot, you'll need to get the output file to a system that has a real X server running, so you can see the image. Some options for this are ftp or sftp, scp, or good old Sneakernet. Finally, view the screenshot you took: xwud -in xwdout. This shows the image stored in the file named xwdout. It should open a window where you can see a happy little xclock showing the time it was whenever you took the screenshot. Click anywhere in the window to close it.

That's it for this post. In Part 3: how to make Hudson, Selenium, and your build play with all this other stuff you've learned.