Adam Clarke on 2 Samuel 20:23

Joab was over all the host - He had murdered Amasa, and seized on the supreme command: and such was his power at present, and the service which he had rendered to the state by quelling the rebellion of Sheba, that David was obliged to continue him; and dared not to call him to account for his murders without endangering the safety of the state by a civil war.

Benaiah - over the Cherethites - Benaiah was over the archers and slingers.

See the notes on 2Sam 8:18.