Why bspwm ignores first WM_CLASS when matching rules

[tealsnow]

Given qemu-system-x86_64, which reports WM_CLASS(STRING) = "qemu", "Qemu-system-x86_64".
Only the rule bspc rule -a Qemu-system-x86_64 state=floating gets applied, but bspc rule -a qemu state=floating does not.

[emanuele6]

bspc rule -a qemu state=floating tells bspwm to add a rule that applies state=floating to windows that have a "qemu" as their class name (the class name is the last element of WM_CLASS).

That window does not have "qemu" as class name; its class name is "Qemu-system-x86_64", so the rule -a qemu state=floating rule does not affect it.
"qemu" is the instance name of that window.

If you want to add a rule that matches that any window that has "qemu" as instance name regardless of the class name, you must tell bspwm that: any class name (*) and "qemu" instance name.

#!/bin/sh -
bspc rule -a '*:qemu' state=floating

This is not generally something that makes sense to do.
The purpose of the class name is to distinguish windows that belong to same program, and the purpose of the instance name is to distinguish kinds of subwindows of the same program; e.g.:

#!/bin/sh -
# firefox's windows have "Firefox" as class name:
#  * the browser windows have "Navigator" as instance name
#  * the history/downloads/bookmarks management windows have "Library" as instance name
#  * the "about" windows have "Browser" as instance name
#  * generic windows (e.g. file pickers) have "firefox" as instance name

# rules are applied in order:
# this set of rules sends all Firefox windows to the seventh desktop;
#  if they are Navigator or Library windows, they will be tiled
#  otherwise they will be floating

# send all firefox windows to the 7th desktop as floating
bspc rule -a 'Firefox' 'desktop=^7' state=floating

# rules that only match Navigator and Library instance names for Firefox windows
#  and overwrite the state=floating rule with state=tiled
bspc rule -a 'Firefox:Navigator' state=tiled
bspc rule -a 'Firefox:Library' state=tiled

There is no convention on the names of instance names; if the program uses custom ones instead of the standard (Classname with the first letter lowercased) is up to the developer of the program and names such are "Library", etc. are aribitrarily chosen by the developer; *:Library could match any kind of window from different programs ("Library" could mean anything).

So, if you want to use "qemu" in your rules, you would genearally use Qemu-system-x86_64:qemu, not *:qemu.

---

If you want to use *:qemu because you have multiple versions of qemu and they have different class names e.g. "Qemu-system-x86_64", "Qemu-system-x86", "Qemu-system-MIPS", etc. (I don't know if that is how it works.), that could be a decent use case for *:qemu keeping in mind that it could match urxvt -name qemu -e ~/bin/run_qemu.sh or any other window that has "qemu" as instance name, even if it's not actually a qemu window.

But, personally, I would prefer to use an external rule that checks the class name with a glob instead of only checking the instance name with *:qemu:

setup (run these commands in a terminal that is running any POSIX shell):

: >> ~/.config/bspwm/external_rules
chmod u+x -- ~/.config/bspwm/external_rules

bspwmrc:

bspc config external_rules_command ~/.config/bspwm/external_rules

~/.config/bspwm/external_rules:

#!/bin/sh -

class=$2 instance=$3

# if the class name is "Qemu-system" or "Qemu-system-something", and the
#  instance name is "qemu", apply `state=floating`
case $class in
  Qemu-system|Qemu-system-?*)
    [ "$instance" = qemu ] && printf 'state=floating\n'
esac

[tealsnow]

Wow, thank you for such a comprehensive answer. I guess it's just a misunderstanding of what xprop reports; it was my understanding that a window could have multiple class names, and that is what xprop was reporting.

If you want to use *:qemu because you have multiple versions of qemu and they have different class names e.g. "Qemu-system-x86_64", "Qemu-system-x86", "Qemu-system-MIPS", etc. (I don't know if that is how it works.).

That is exactly how it works. Different versions of qemu have different executables which all have different class names that express that, but they all share the same 'instance name' "qemu"; what I was trying to target with my rule. But as you say any window with the title 'qemu' would match the rule *:qemu.

The script you provided seems like the perfect solution to my problem, thank you, I really appreciate your help.