‘And Joseph said to them, “It is as I said to you, you are spies. You will prove yourselves in this way. By the life of Pharaoh you shall not go forth from here unless your youngest brother comes here. Send one of your number and let him fetch your brother, and you shall be put in custody so that your words may be proved, whether there is truth in you. Or else by the life of Pharaoh you are surely spies.” '
Joseph intends that just as he went into slavery and then into custody they too will experience the same. He wants them to taste something of what he had known to see how they will come out of it. Thus he proposes that one should go and return with this supposed youngest son while the remainder are held in custody.
“You shall be put in custody.” Literally, ‘you shall be bound'. But in Genesis 40 ‘being bound' is mentioned frequently where men clearly had some freedom (Genesis 42:3-5 compare Genesis 39:20). Therefore it may or may not include being restricted with ropes.
To swear by the life of Pharaoh was a solemn oath for the life of Pharaoh was the life of a god. Perhaps he is ensuring that they realise he is a true Egyptian.