I have a simple question. How can I configure and show duplex mode i.e. full or half duplex of serial interface which is running PPP or HDLC? Could anyone please advise?
No, it is not possible to set it.
The only serial protocol that are half-duplex, are SDLC multipoint for IBM networking. Legacy thing not of interest to us.
You are confusing the data link layer with the physcial layer.
HDLC, which is at the data link layer, can indeed be run in half or full duplex.
However whether or not it does or doesn't largely depends on the capabilities of the underlying physical transmission layer. And as Peter says serial interfaces are point-to-point and so always run at full duplex.
The serial interfaces are always full-duplex because they are point-to-point and they do not run over a shared medium. If you check the connector pinout you will see that each serial interface has separate Tx and Rx pins and therefore can transmit and receive data simultaneously. Therefore, there is no duplex setting and I believe it would not make sense.
Check this page for pinout details: