While xwininfo and xprop will provide the raw data needed to write the criteria, manually copy/pasting all the relevant pieces of output into a criterion list can be tedious, so I've written a small script to automate this.
Running the script and clicking on a window will output all the criteria for it, ready to be copy/pasted after an i3 config command (remove any criteria that aren't needed).
Tip: to catch a splashscreen or transient popup "as it happens", run the script like this (using The GIMP as an example):
$ gimp-2.8 & ./i3-get-window-criteria
and click on the splashscreen as soon as it appears.
Example output for the above command:
[class="Gimp-2.8" id=81788931 instance="gimp-2.8" title="GIMP Startup" window_role="gimp-startup"]
This output can be copy/pasted where needed, then edited to remove any criteria which would make the filter too specific for a particular use case.
Here's the script:
#!/bin/sh
# i3-get-window-criteria - Get criteria for use with i3 config commands
# To use, run this script, then click on a window.
# Output is in the format: [<name>=<value> <name>=<value> ...]
# Known problem: when WM_NAME is used as fallback for the 'title="<string>"' criterion,
# quotes in "<string>" are not escaped properly. This is a problem with the output of `xprop`,
# reported upstream: https://bugs.freedesktop.org/show_bug.cgi?id=66807
PROGNAME=`basename "$0"`
# Check for xwininfo and xprop
for cmd in xwininfo xprop; do
if ! which $cmd > /dev/null 2>&1; then
echo "$PROGNAME: $cmd: command not found" >&2
exit 1
fi
done
match_int='[0-9][0-9]*'
match_string='".*"'
match_qstring='"[^"\\]*(\\.[^"\\]*)*"' # NOTE: Adds 1 backreference
{
# Run xwininfo, get window id
window_id=`xwininfo -int | sed -nre "s/^xwininfo: Window id: ($match_int) .*$/\1/p"`
echo "id=$window_id"
# Run xprop, transform its output into i3 criteria. Handle fallback to
# WM_NAME when _NET_WM_NAME isn't set
xprop -id $window_id |
sed -nr \
-e "s/^WM_CLASS\(STRING\) = ($match_qstring), ($match_qstring)$/instance=\1\nclass=\3/p" \
-e "s/^WM_WINDOW_ROLE\(STRING\) = ($match_qstring)$/window_role=\1/p" \
-e "/^WM_NAME\(STRING\) = ($match_string)$/{s//title=\1/; h}" \
-e "/^_NET_WM_NAME\(UTF8_STRING\) = ($match_qstring)$/{s//title=\1/; h}" \
-e '${g; p}'
} | sort | tr "\n" " " | sed -r 's/^(.*) $/[\1]\n/'
