Charles Ellicott Commentary 2 Samuel 3:31

"And David said to Joab, and to all the people that were with him, Rend your clothes, and gird you with sackcloth, and mourn before Abner. And king David followed the bier." — 2 Samuel 3:31 (ASV)

Rend your clothes. —David commands a public mourning with the usual signs of torn clothes and sackcloth, and lays this command especially upon Joab, who is thus required, as it were, to do public penance for his act. David himself followed the bier as chief mourner.