How do I find out what program 'owns' a hotkey?

Question

I'm running XFCE 4.12 on top of Gentoo with a 4.2.0 kernel. My PlayPause button on my keyboard used to work as a global hotkey for VLC. Now VLC won't even recognize the key. It does see "Alt + Media Play Pause" but not the key alone.

Is there a way to see if and what program might be capturing that key?

When I run xdotool key "XF86LogGrabInfo" the tail /var/log/Xorg.0.log file reads

[ 10138.690] (II) Printing all currently active device grabs:
[ 10138.690] (II) End list of active device grabs

Answer 1

To find out which app/program grabbed your key use the debug keysym XF86LogGrabInfo. Use xdotool to press keys + XF86LogGrabInfo at the same time e.g. in a terminal run

KEY=XF86AudioPlay
xdotool keydown ${KEY}; xdotool key XF86LogGrabInfo; xdotool keyup ${KEY} 

Then check for output with tail /var/log/Xorg.0.log
Note that with gnome 3/gdm and systemd this is no longer logged to Xorg.0.log (it's instead logged to the journal). In that case you could run

journalctl -f

and then in another terminal run the xdotool commands. Switch to the first terminal and you'll see something like

/usr/lib/gdm/gdm-x-session[629]: Active grab 0x40c0a58e (xi2) on device 'Virtual core keyboard' (3):
/usr/lib/gdm/gdm-x-session[629]: client pid 708 /usr/bin/gnome-shell
/usr/lib/gdm/gdm-x-session[629]: at 32595124 (from passive grab) (device frozen, state 6)
/usr/lib/gdm/gdm-x-session[629]: xi2 event mask for device 3: 0xc000
/usr/lib/gdm/gdm-x-session[629]: passive grab type 2, detail 0xac, activating key 172

In the above example the program (the client) that grabbed the key is gnome-shell.

How do I figure out what the keys are called?

Check out the manpage of xdotool using man xdotool or an online version, as it lists a number of the special keys. For instance, "alt+r", "Control_L+J", "ctrl+alt+n", "BackSpace". The LinuxQuestions wiki also has a list of X Keysyms one could use. To make things a bit easier, xdotool also has aliases for some of these, such that pressing Shift-Alt-Tab would for instance just be shift+alt+Tab. To verify that this does indeed click that key combination, you could send the input to xev, which is a program that will print whatever key or mouse events it gets to the console. Just do sleep 2; xdotool keydown ${KEY} and switch to the xev window before two seconds has passed to see the keys being clicked on that window. It should then output a series of events, such as these:

PropertyNotify event, serial 168, synthetic NO, window 0x1e00001,
    atom 0x13e (_GTK_EDGE_CONSTRAINTS), time 4390512, state PropertyNewValue

MappingNotify event, serial 168, synthetic NO, window 0x0,
    request MappingKeyboard, first_keycode 8, count 248

KeyPress event, serial 168, synthetic NO, window 0x1e00001,
    root 0x163, subw 0x0, time 4390719, (882,657), root:(1000,771),
    state 0x0, keycode 64 (keysym 0xffe9, Alt_L), same_screen YES,
    XLookupString gives 0 bytes: 
    XmbLookupString gives 0 bytes: 
    XFilterEvent returns: False

KeyPress event, serial 169, synthetic NO, window 0x1e00001,
    root 0x163, subw 0x0, time 4390738, (882,657), root:(1000,771),
    state 0x8, keycode 23 (keysym 0xff09, Tab), same_screen YES,
    XLookupString gives 1 bytes: (09) " "
    XmbLookupString gives 1 bytes: (09) "   "
    XFilterEvent returns: False

Answer 2

the following commands helped me to identify all programs grabbing keyboard shortcuts:

$ lsof -c Xorg | grep ".log" # find out the path for the Xorg log file
Xorg    16025   sr    4w      REG              254,1    225839 3933229 /home/sr/.local/share/xorg/Xorg.0.log
$ xdotool key XF86LogGrabInfo # prints all Xorg registered grabs
$ cat /home/sr/.local/share/xorg/Xorg.0.log | grep grabs
[ 39303.019] (II) Printing all currently active device grabs:
[ 39303.019] (II) End list of active device grabs
[ 39303.019] Printing all currently registered grabs
[ 39303.019]   Printing all registered grabs of client pid 16024 xinit /home/sr/.xinitrc -- /etc/X11/xinit/xserverrc :0 vt1 -keeptty -auth /tmp/serverauth.6AO52I3zhe
[ 39303.019]   Printing all registered grabs of client pid 16045 unclutter 
[ 39303.019]   Printing all registered grabs of client pid 16047 xscreensaver 
[ 39303.019]   Printing all registered grabs of client pid 16109 /usr/lib/firefox/firefox 
[ 39303.019]   Printing all registered grabs of client pid 16055 xscreensaver-systemd 
[ 39303.019]   Printing all registered grabs of client pid 16057 xbindkeys 
[ 39303.027]   Printing all registered grabs of client pid 16039 /usr/bin/openbox --startup /usr/lib/openbox/openbox-autostart OPENBOX
[ 39303.039]   Printing all registered grabs of client pid 16048 copyq 
[ 39303.039]   Printing all registered grabs of client pid 16049 konsole 
[ 39303.039]   Printing all registered grabs of client pid 16095 /usr/bin/copyq --clipboard-access monitorClipboard
[ 39303.039]   Printing all registered grabs of client pid 16109 /usr/lib/firefox/firefox 
[ 39303.039]   Printing all registered grabs of client pid 16456 subl --detached
[ 39303.039]   Printing all registered grabs of client pid 16465 /usr/lib/at-spi2-registryd --use-gnome-session
[ 39303.039]   Printing all registered grabs of client pid 17178 xdotool key XF86LogGrabInfo
[ 39303.039] End list of registered passive grabs

so in my case the following apps were grabbing key bindings: xinit, unclutter, xscreensaver, firefox, xbindkeys, openbox, copyq, konsole, subl, at-spi2-registryd