John Gill Commentary Exodus 29:9

"And thou shalt gird them with girdles, Aaron and his sons, and bind head-tires on them: and they shall have the priesthood by a perpetual statute: and thou shalt consecrate Aaron and his sons." — Exodus 29:9 (ASV)

And thou shalt gird them with girdles (Aaron and his sons) ,
&c.] Aaron with the girdle of the ephod, and with the girdle of needlework, and his sons with common girdles made for them; all which showed what strength, diligence, and expedition were necessary for the discharge of their office:

and put the bonnets on them ;
upon their heads, which differed only from the high priest's mitre in the manner of rolling or wrapping, as has been observed on (Exodus 28:39Exodus 28:40) :

and the priest's office shall be theirs for a perpetual statute ;
that is, shall descend from father to son in Aaron's family throughout all generations, until the Messiah should come; who would be a priest of another order, and put an end to the Aaronic priesthood, by fulfilling what that was a type of, and so abolishing it:

and thou shall consecrate Aaron and his sons ;
or "fill the hand of them"; that is, with sacrifices to offer for themselves and others, (See Gill on Exodus 28:41). The Targums of Onkelos and Jonathan are, ``shall offer the offering of Aaron, and the offering of his sons,'' of which there is an after account, and was one part of their consecration.