Using the terminology of ptys, the slave end connects to the CLI program and the master end connects to the user / keyboard / terminal emulator.
I have an FTDI USB UART connecting my laptop's USB port to an embedded computer's serial port. It appears on my laptop as /dev/ttyUSB0. My laptop is supposed to be the master end, and the embedded computer is supposed to be the slave end. But when I look at how /dev/ttyUSB0 behaves, it clearly thinks I'm the slave end. For instance, when the embedded computer sends a character to the laptop, the laptop send the character back again.
Is there a switch somewhere that tells the kernel driver that the laptop is the master, and not the slave?