How to find the program name of an X window?

Running xprop _NET_WM_PID from a terminal will put X into a special mode where you can click on a window to get that window's PID in the terminal.

Then run ls -l /proc/<pid>/exe and look at where the link is to to figure out what the program's file name is.

The following is a little python script take from a program I'm working on:

#! /usr/bin/env python

import os, subprocess

# Function based on code from apport
def get_window_pid():
    xprop = subprocess.Popen(['xprop', '_NET_WM_PID'],
            stdout=subprocess.PIPE, stderr=subprocess.PIPE)
    (out, err) = xprop.communicate()
    if xprop.returncode == 0:
        try:
            return int(out.split()[-1])
        except ValueError:
            return -1
    else:
        return -1

def get_window_exe():
    pid = get_window_pid()
    
    if pid == -1:
        return ''
        
    return os.path.realpath('/proc/' + str(pid) + '/exe')

if __name__=='__main__':
    exe = get_window_exe()
    if exe == '':
        print 'Unable to identify window\'s executable' 
    else:
        print exe

Save this into a file and run by typing python <filename>.py. Click on the window and it will print the program's name.

Based on iBelieve's answer:

alias fxname='basename $(ll /proc/$(xprop _NET_WM_PID | awk "{print \$NF}")/exe | awk "{print \$NF}")'

The $NF in the awk commands must be escaped so it's not interpreted as a shell variable. As soon as you run it a reticle will appear for you to click on the desired window.