Thank you for that, fsmithred.

I can confirm that if I remove the executable bit from dbus-launch and dbus-daemon, yad is still able to create the notification icon. However, if dbus-launch and dbus-daemon are executable, dbus is consistently launched each time yad creates an icon. This is very strange.

I'll ask yad's developer and will keep exploring. I'll take a look at some PPIDs to exclude the possibility that some other utility/daemon is starting dbus when yad is detected.

@fsmithred - On your no-dbus system, are you able to create a system tray icon using yad? What happens when you run $ yad --notification --image=dialog-ok --text="testing" &?

I redirected the script's stderr to a log to investigate what's going on when pgrep -f 'regex' && foo fails to work as expected.

Lo and behold, a bunch of "pgrep: write error" entries show up in the log.

So it seems the issue here is that when a shell script runs in the background, pgrep gets grumpy unless its stdout is redirected somewhere (even if only to /dev/null).

HevyDevy, wifi-monitor is a barebones shell script that assumes the user only uses wifi, never ethernet.

What you are suggesting would certainly be possible, but the more complex logic would require an overhaul of the script.

Feel free to adapt the script to suit your needs. If you run into issues, email me on here and I'll try to help.

@HevyDevy - I looked around and the only utility I could find to easily put an icon and tooltip in the system tray is yad. It's either yad or roll your own...

...so I did roll my own tiny alternative to yad using python:

#!/usr/bin/python3

import sys
from PyQt4 import QtGui, QtCore

def show_icon(icon, tooltip):
	class SystemTrayIcon(QtGui.QSystemTrayIcon):
		def __init__(self, icon, parent=None):
		   QtGui.QSystemTrayIcon.__init__(self, icon, parent)
		   self.setToolTip(tooltip)
		def exit(self):
		  QtCore.QCoreApplication.exit()

	app = QtGui.QApplication(sys.argv)
	w = QtGui.QWidget()
	trayIcon = SystemTrayIcon(QtGui.QIcon(icon), w)
	trayIcon.show()
	sys.exit(app.exec_())

icon=sys.argv[1]
tooltip=sys.argv[2]
show_icon(icon, tooltip)

If you'd like to try it:
1. Paste this code into a blank text file, name it show-icon, put it somewhere in your PATH, make it executable
2. Install the script's two dependencies: $ sudo apt install python3 python3-pyqt4
3. Test it: $ show-icon /path/to/icon.png "hello world" &

If the yad test I suggested in post #5 doesn't work, try this python script. If the python script ("show-icon") works, let me know and I can customize wifi-monitor for you (to use show-icon instead of yad).

Thanks, fsmithred. Good to know it works in Beowulf with lxpanel

It should work in Xfce as well. Note that, in the interest of simplicity, the icon only shows up when internet is connected. If there is no internet connectivity, then no icon is the expected behavior.

I found that xinput --test-xi2 --root and xinput --test-xi2 --root <keyboard_id> work.

I tinkered with ralph.ronnquist's solution in post #13 and figured out that this is what's actually going on when you decompress the logic:

$ master_id=$(LANG=C xinput list | grep -i 'master keyboard' | egrep -o 'id=[0-9]+' | egrep -o '[0-9]+')
$ keyboard_id=$(LANG=C xinput list $master_id | grep 'XIKeyClass' | egrep -o '[0-9]+')

This version is easier for my pea brain to grasp.

Yes, that works even on my wife's wonky hardware where multiple "slave keyboards" are listed.

eileen@vaio:~$ xinput list --id-only | xargs -n 1 -I+ sh -c "xinput list + | grep -wv + | grep XIKeyClass" | egrep -o '[0-9]+'
12

Wow, ralph.ronnquist. Very impressive. Thank you!

I understand the logic of the command, but don't understand how you figured out that it was the necessary logic. Will chew on it.

BTW, I had also found that nugget on stackexchange, but do not want to give users anything in addition to the shell script I've created. The script is here if you are curious:
https://github.com/bdantas/iksilo

Thank you both. I think we're getting warmer. Does this work on all your test systems?

LANG=C xinput --list | grep -i keyboard | egrep -iv 'virtual|video|button|bus' | egrep -o 'id=[0-9]+' | egrep -o '[0-9]+'