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Path: utzoo!watmath!clyde!burl!ulysses!allegra!alice!rob
From: rob@alice.UUCP
Newsgroups: net.unix-wizards
Subject: awk
Message-ID: <2647@alice.UUCP>
Date: Fri, 9-Mar-84 11:28:28 EST
Article-I.D.: alice.2647
Posted: Fri Mar  9 11:28:28 1984
Date-Received: Sat, 10-Mar-84 12:34:42 EST
Organization: AT&T Bell Laboratories, Murray Hill
Lines: 7

From the (admittedly most recent) awk reference manual:

Any given file [from > >> or |] is opened only once per execution of
.IT awk
and remains open until 
.IT awk
terminates.