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gawk: Nextfile Statement

 
 7.4.9 The 'nextfile' Statement
 ------------------------------
 
 The 'nextfile' statement is similar to the 'next' statement.  However,
 instead of abandoning processing of the current record, the 'nextfile'
 statement instructs 'awk' to stop processing the current data file.
 
    Upon execution of the 'nextfile' statement, 'FILENAME' is updated to
 the name of the next data file listed on the command line, 'FNR' is
 reset to one, and processing starts over with the first rule in the
 program.  If the 'nextfile' statement causes the end of the input to be
 reached, then the code in any 'END' rules is executed.  An exception to
 this is when 'nextfile' is invoked during execution of any statement in
 an 'END' rule; in this case, it causes the program to stop immediately.
 ⇒BEGIN/END.
 
    The 'nextfile' statement is useful when there are many data files to
 process but it isn't necessary to process every record in every file.
 Without 'nextfile', in order to move on to the next data file, a program
 would have to continue scanning the unwanted records.  The 'nextfile'
 statement accomplishes this much more efficiently.
 
    In 'gawk', execution of 'nextfile' causes additional things to
 happen: any 'ENDFILE' rules are executed if 'gawk' is not currently in
 an 'END' or 'BEGINFILE' rule, 'ARGIND' is incremented, and any
 'BEGINFILE' rules are executed.  ('ARGIND' hasn't been introduced yet.
 ⇒Built-in Variables.)
 
    With 'gawk', 'nextfile' is useful inside a 'BEGINFILE' rule to skip
 over a file that would otherwise cause 'gawk' to exit with a fatal
 error.  In this case, 'ENDFILE' rules are not executed.  ⇒
 BEGINFILE/ENDFILE.
 
    Although it might seem that 'close(FILENAME)' would accomplish the
 same as 'nextfile', this isn't true.  'close()' is reserved for closing
 files, pipes, and coprocesses that are opened with redirections.  It is
 not related to the main processing that 'awk' does with the files listed
 in 'ARGV'.
 
      NOTE: For many years, 'nextfile' was a common extension.  In
      September 2012, it was accepted for inclusion into the POSIX
      standard.  See the Austin Group website
      (http://austingroupbugs.net/view.php?id=607).
 
    The current version of BWK 'awk' and 'mawk' also support 'nextfile'.
 However, they don't allow the 'nextfile' statement inside function
 bodies (⇒User-defined).  'gawk' does; a 'nextfile' inside a
 function body reads the first record from the next file and starts
 processing it with the first rule in the program, just as any other
 'nextfile' statement.
 
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