Or HaChaim on Deuteronomy 3:28/אור החיים על דברים ג׳:כ״ח

1 וצו את יהושע, "and command Joshua, etc." What precisely was Moses to command Joshua? Yalkut Shimoni item 823 quoting Sifri on Pinchas claims that Moses was to command Joshua not to be taken in by the Gibeonites. Another opinion cited claims that Moses warned Joshua concerning possible strife amongst the people. There is no hint in the text of this and unless it is based on tradition we do not have to accept this explanation. I believe that the plain meaning of the verse may be understood in light of Samuel I 13,14: ויצוהו ה׳ לנגיד על עמו, "the Lord appointed him as ruler over His people." The word means "he appointed." Please refer to what I have written on Exodus 6,13 ויצום אל בני ישראל. Moses simply told Joshua to exert authority over Israel already while he was still alive. He personally would support him in this before the eyes of the whole nation.