path: plymouth_24.004.60-5.1/docs/development.txt
Then, attaching with gdb from another ssh session:
    # gdb attach $(/sbin/pidof plymouthd)
Then controlling plymouth from the third ssh session:


path: plymouth_24.004.60-5.1/docs/development.txt
often simplest to use gdb's built-in text mode interface:
    # gdb attach $(/sbin/pidof plymouthd)
You can also start plymouthd in the debugger directly: