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I'm using Arch Linux, and have both Gnome and the i3 window manager installed. When running i3, I'm trying to initiate the composite manager Compton. But trying to do so results in the following error:
$ compton
[ 04/11/2019 22:32:36.443 register_cm FATAL ERROR ] Another composite manager is already running
I think this means that Compton is already running, or Mutter is running, but I'm not sure which.
Is there a command I can use to determine which composite manager is currently running?
You can use inxi.
inxi -Gxx | grep compositor
The output looks like this
alternate: ati,fbdev compositor: compton resolution: <xdpyinfo missing>
and you can see that Compton is currently being used as a compositor. With no compositor, there is no grep match.
Switches:
-G Show graphics info (card(s), driver, display protocol (if available), display server, resolution, renderer, OpenGL version).-xx Show extra, extra data. (with -G, show chip vendor:product ID for each video card; OpenGL compatibility version; compositor (experimental); alternate Xorg drivers. 
inxi -Sxx should do it instead (even though, at least without wmctrl installed, it is simply a lookup table checking running processes AFAICT)
-S only shows system info, I don't think that will show any additional compositors. I think you're right that inxi does not heuristically/canonically detect compositors, it can only detect compositors that it knows about. I noticed that picom is missing since the rename of the fork, so I added it. Let me know if you noticed any other missing compositors.
Feb 24, 2020 at 18:46
I don't have either Compton or Mutter installed, but the xcompmgr sample X composition manager doesn't bother to determine if another composition manager is running, but goes on and just calls XCompositeRedirectSubwindows() and handles the error that may be generated by it in the handler installed with XSetErrorHandler:
static int
error (Display *dpy, XErrorEvent *ev)
{
...
if (ev->request_code == composite_opcode &&
ev->minor_code == X_CompositeRedirectSubwindows)
{
fprintf (stderr, "Another composite manager is already running\n");
exit (1);
This seems to be the only possible way to do it: XCompositeGetOverlayWindow() will always succeed and map the overlay window, whether it's already in use or not:
CompositeGetOverlayWindowThis request indicates that the client wishes to use the Composite Overlay Window of this screen. If this Composite Overlay Window has not yet been mapped, it is mapped by this request.
Note:
Mutter is implemented as a dynamic library (usually loaded by gnome-shell), not as a separate process.
grep -sl mutter /proc/*/maps
on Linux will find all the processes that are using it.
There is no command to know if any compositor is currently running, as far as I know. But you can use pgrep to know if a given compositor is currently running:
$ pgrep compton
18169
pgrep will give you the PID so you can terminate the process using kill:
$ kill 18169
Alternatively, pkill can terminate the process by feeding it the process name instead of the PID:
$ pkill compton
Some compositors (like Xfce's) may need a different approach to be deactivated:
$ xfwm4 --compositor=off --replace
pgrep compton seems to work, but when using Gnome pgrep mutter doesn't return anything
pgrep mutter doesn't return anything because mutter is implemented as a dynamic lib loaded by gnome-shell, not as a separate process. grep -sl mutter /proc/*/maps to find the pid of any process that may using mutter (or something using lsof, but I was never able to learn how to use lsof)