path: retroarch_1.22.2+dfsg-2/gfx/include/userland/host_applications/linux/apps/raspicam/README.md
	--signal			-s	Signal mode
The camera is run for the requested time (-t), and a captures can be initiated throughout that time by sending a USR1 signal to the camera process. If USR2 signal is received the application makes a capture and exits. This can be done using the kill command. You can find the camera process ID using the 'ps ax' command or using pidof command.
		kill -USR1 <process id of raspistill>