John Gill Commentary 1 Samuel 20:13

"Jehovah do so to Jonathan, and more also, should it please my father to do thee evil, if I disclose it not unto thee, and send thee away, that thou mayest go in peace: and Jehovah be with thee, as he hath been with my father." — 1 Samuel 20:13 (ASV)

The Lord do so and much more to Jonathan
Recompense evil more than can be thought of and expressed, should he neglect to inform David of the good disposition of Saul unto him:

but if it please my father [to do] you evil ;
if he seems determined upon it to take away your life:

then I will show it you ;
not by a messenger, by whom it would not be safe to communicate it, lest by that means Saul would know where he was, and come and slay him; but Jonathan would come himself, and acquaint him with it:

and send you away, that you may go in peace ;
give him leave, and advise him to depart, and provide for his own safety, adding his blessing on him, and prayer for him:

and the Lord be with you, as he has been with my father ;
in the beginning of his reign, giving him counsel and advice in all things, and victory over his enemies, succeeding and prospering him in whatsoever he engaged in; the Targum is, "the Word of the Lord be for your help." Jonathan seemed to be fully apprized that David was to succeed in the kingdom.