John Gill Commentary Joshua 1:18

"Whosoever he be that shall rebel against thy commandment, and shall not hearken unto thy words in all that thou commandest him, he shall be put to death: only be strong and of good courage." — Joshua 1:18 (ASV)

Whosoever [he be] that doth rebel against your commandment ,
&c.] Refuses to go over Jordan with his brethren the children of Israel, when commanded by Joshua so to do:

and will not hearken to your words in all that you command him ;
or, if he does go over, yet will not obey orders to take such a post, or go against such a city, or march into such a part of the country:

he shall be put to death ;
this part of military discipline they agree to, and hereby declare their entire submission to him as their general; some understand this as spoken by all Israel, and of their promise of obedience to Joshua, as their governor, in all things:

only be strong, and of a good courage ;
which also is not to be understood as a condition of their submission and obedience, but as a hearty wish and prayer for him, that he might have strength and courage necessary to the great work he was engaging in, and which to see would be no small encouragement to follow and obey him.