I'm puzzled.

Please see these simple commands:

This one works as expected ... dbus-monitor never terminates and whenever it outputs new lines, they are appended to "/data/eject.txt".

This one does work too. Any line containing the string "Eject" appears immediately in my shell window.

This one does not work as expected. To be more precisely, nothing happens. Nothing gets written to "/data/eject.txt", nothing gets written on the screen.

So I tried
Same result, nothing happens ... neither in "/data/eject.txt" nor on my screen.

As I said ... I am puzzled.

I suspect, I missed something elementary here about pipes and redirected output ...

hmm,
looks OK.

first of all
check permissions, check disk usage.

Try to add the option --line-buffered to grep: it forces the output to flush one line at a time. Otherwise the streamed output is flushed after a long time/size and you can't see it in real time.

Yeah, this did do the trick!

Thanks for the exceptional fast help, especially as I would never have suspected good old grep to be the culprit.

And my trust in pipes and redirection is restored :-)