An equivalent of xkill is the windowkill function in xdotool. If you don't have xdotool installed, please run

sudo apt-get install xdotool

With this tool you can indeed kill a window by using the search option and by specifying its class:

xdotool search --class gedit windowkill

You can find the pid of the process with getwindowpid, but just specifying the name of the class of window (i.e --class gedit) is enough. The tool is non-interactive in the sense that you do not need to click on a window to kill it as you sometimes need to do with xkill.

More information on the options available with xdotool are available with man xdotool and at the Ubuntu manpages online. The manpages explain well how xdotool can be incorporated into a script much easier than xkill could be.

So I think probably xdotool's windowkill option is what you want and it may be more handy than that of xkill.

However, if you still want to use xkill and feed it the window's resource id, so that it closes the window without prompting you, you could parse the output of xwininfo, as in this example using the program Meld:

xkill -id $(xwininfo -name Meld | awk -F ':*"*' '{print $3}')

However, the name of the program sent to xwininfo must be in the proper case, so launch the program beforehand, and, in our example here, see if it is Meld or meld.

I prefer the xdotool command I gave above, as the case doesn't have to be correct: Meld or meld will work as the search --name switch instead uses regex to match a given string and ignores the case.