Acting US Atty's are appointed by the AG. Smith was in acting capacity from March 2017 until September 2017. He left when Donald Cochran was nominated for US Atty.Jack Smith was appointed as the acting US Attorney for Tennessee by Donald Trump.
In response to a request by the Attorney General that its
office be vested with authority to appoint interim United
States Attorneys, Congress enacted former section 546(d) of
title 28 of the United States Code in 1986.\19\ Pursuant to
this authority, the Attorney General was authorized to appoint
an interim United States Attorney for 120 days and, if the
Senate did not confirm a new United States Attorney within such
period, the district court was then authorized to appoint an
interim United States Attorney to serve until a permanent
replacement was confirmed.\20\ By retaining a role for the
district court in the selection of an interim United States
Attorney, former section 546(d) allowed the Judicial Branch to
act as a check on Executive power. In practice, if a vacancy
was expected, the Attorney General would typically solicit the
opinion of the chief judge of the relevant district regarding
possible temporary appointments.\21\
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Twenty years after section 546(d) was enacted, the USA
PATRIOT Improvement and Reauthorization Act of 2005 removed the
court's role entirely on March 9, 2006.\22\ As amended, section
546(c) now provides that ``[a] person appointed as United
States attorney under this section may serve until the
qualification of a United States Attorney for such district
appointed by the President under section 541 of this title.''
\23\
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The 2005 Act amended section 546 in two critical respects.
It not only removed district court judges from the interim
appointment process, vesting the Attorney General with the sole
authority; \24\ it also eliminated the 120-day limit on how
long an interim United States Attorney appointed by the
Attorney General could serve.\25\ As a result of the Act,
judicial input in the interim appointment process was
eliminated and, perhaps more importantly, it created a possible
loophole that could permit United States Attorneys appointed on
an interim basis to serve indefinitely without Senate
confirmation.
https://www.congress.gov/congressional-report/110th-congress/house-report/58/1