Estimated reading time: 2 minutes
Finding a window’s owner¶
Rach Holmes asks on our Linux list:
How do I find out what app keeps opening that annoying window in gnome?
Jack McIntyre made a couple of outstanding correct guesses, but predictably Rach still wanted to know a generic solution and I offer this tip in response.
The easy way¶
Many windows define a WM_COMMAND property that is set to the command name of
the application, if that exists all we have to do is read it:
$ xprop | grep WM_COMMAND
WM_COMMAND(STRING) = { "urxvt", "-cd", "/home/jay/Desktop" }
However, not all applications set this property. Examples of applications that don’t set this property include firefox and gimp. In fact, many of the modern applications you find on desktop systems don’t set this property at all.
The other way¶
EWMH, the Extended Window Manager Hints standard, defines a property called
_NET_WM_PID that, if defined, is the process id of the application that
opened the window. We have a couple of options to access that property, my
favourite would be wmctrl and another would be xprop which is often installed
by default.
$ wmctrl -lp
0x01200003 2 13398 kate 2009-10-06-TaD-Finding_a_windows_owner.textile [+] - gvim
0x01000006 3 9992 kate Irssi: [JNR___@freenode] [#github]
0x00c00005 1 10134 kate SpotMe - Opera
0x00600019 0 9992 kate jay@kate:~/Desktop/jnrowe.github.io
$ xprop | awk '/_NET_WM_PID/ {print $NF}'
9992
$ tr '\000' ' ' < /proc/9992/cmdline
/usr/bin/urxvtd -q -f -o
The wmctrl output has five space aligned columns. The first is the window id, the second the desktop is visible on, the third is the process id, the fourth is the client machine and the fifth is the window’s title string. And with the xprop example we must select a window with the mouse when calling xprop, and then we pluck the process id from the output.
The cmdline value for the process from procfs is finally read. The
tr command is needed because options are separated by null
terminators.
The problems¶
Back to the original question and Rach was looking for a way to stop the window
from showing up, the immediate response to the annoying window problem is to
find its process id and send it a SIGTERM. It seems like a reasonable idea,
but I’ve been choosing the urxvtd examples on purpose to prove
a point.
urxvtd is the terminal daemon provided by rxvt-unicode, all
terminals that are spawned by it will have the same _NET_WM_PID value and
WM_COMMAND string. This isn’t just an issue with urxvtd either,
it happens with all applications that operate in this way. Sending
a SIGTERM to every terminal opened by urxvtd probably isn’t what
we would want to do, so you should always think very carefully before sending
terminate signals.
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