set_wm_class.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <X11/Xlib.h>
#include <X11/Xutil.h>

int main(int argc, char *argv[])
{
	unsigned long value;
	char *terminatedAt;
	XClassHint class;
	Status status;
	Display *display;
	Window window;
	
	if ( argc != 3 ) {
		printf( "Usage: %s <window id> <window class>\n", argv[0] );
		return 1;
	}
	window = strtoul( argv[1], &terminatedAt, 0 );
	if ( *terminatedAt != '\0' ) {
		printf( "Could not parse window id: %s\n", argv[1] );
		return 2;
	}
	
	display = XOpenDisplay( NULL );
	status = XGetClassHint( display, window, &class );
	if ( !status ) return 4;
	
	XFree( class.res_class );
	class.res_class = strdup( argv[2] );
	printf("Setting WM_CLASS of window %lu to \"%s\", \"%s\"\n", window, class.res_name, class.res_class );
	XSetClassHint( display, window, &class );
	
	XCloseDisplay( display );
	XFree( class.res_name );
	XFree( class.res_class );
	return 0;
}

Set the class part of WM_CLASS for a given window id in X. Apparently there is no command-line tool to do this: xprop can set the first half (the name) but not the second part (the class).

Hi, thanks for this neat little script. To really separate the windows, you need to set class.res_name too so I added this in a fork.

You could just use xdotool..

apt install -y xdotool
xdotool search --name "Title of App" set_window --class "New WM Class"

Thanks @Fmstrat! Copied that to the StackOverflow question that prompted this gist.