The Goal to have specific applications—like Firefox, VLC, or others—to always start in fullscreen when launched in Openbox. I don’t want to press keys or click buttons every time. I want it automated, clean, declarative.

Step 1: Find the Window Class

Before Openbox can apply rules to an app, it needs to know how to identify the app’s window.

To discover this, I use xprop:

xprop | grep WM_CLASS

I click the app window. This command returns something like:

WM_CLASS(STRING) = "urxvt", "URxvt"

In this case, “urxvt” and “URxvt” is the window’s class name. That’s the identifier Openbox needs. Important: It’s usually the first string (here, “urxvt”) that Openbox uses in rc.xml. Write this name down.

Step 2: Configure Openbox

I edit the Openbox configuration file:

vim ~/.config/openbox/rc.xml

Inside the applications tag, I add this:

<application name="urxvt">   
  <fullscreen>yes</fullscreen> 
</application>

Replace “urxvt” with the name you got from xprop.

Step 3: Reload Openbox

After saving the file, I apply the new config:

openbox --reconfigure

Now, every time I launch urxvt (or whatever app I configured), it will automatically open in fullscreen. No more manual toggling.