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gawk: Getline Notes

 
 4.10.9 Points to Remember About 'getline'
 -----------------------------------------
 
 Here are some miscellaneous points about 'getline' that you should bear
 in mind:
 
    * When 'getline' changes the value of '$0' and 'NF', 'awk' does _not_
      automatically jump to the start of the program and start testing
      the new record against every pattern.  However, the new record is
      tested against any subsequent rules.
 
    * Some very old 'awk' implementations limit the number of pipelines
      that an 'awk' program may have open to just one.  In 'gawk', there
      is no such limit.  You can open as many pipelines (and coprocesses)
      as the underlying operating system permits.
 
    * An interesting side effect occurs if you use 'getline' without a
      redirection inside a 'BEGIN' rule.  Because an unredirected
      'getline' reads from the command-line data files, the first
      'getline' command causes 'awk' to set the value of 'FILENAME'.
      Normally, 'FILENAME' does not have a value inside 'BEGIN' rules,
      because you have not yet started to process the command-line data
      files.  (d.c.)  (See ⇒BEGIN/END; also ⇒Auto-set.)
 
    * Using 'FILENAME' with 'getline' ('getline < FILENAME') is likely to
      be a source of confusion.  'awk' opens a separate input stream from
      the current input file.  However, by not using a variable, '$0' and
      'NF' are still updated.  If you're doing this, it's probably by
      accident, and you should reconsider what it is you're trying to
      accomplish.
 
    * ⇒Getline Summary, presents a table summarizing the 'getline'
      variants and which variables they can affect.  It is worth noting
      that those variants that do not use redirection can cause
      'FILENAME' to be updated if they cause 'awk' to start reading a new
      input file.
 
    * If the variable being assigned is an expression with side effects,
      different versions of 'awk' behave differently upon encountering
      end-of-file.  Some versions don't evaluate the expression; many
      versions (including 'gawk') do.  Here is an example, courtesy of
      Duncan Moore:
 
           BEGIN {
               system("echo 1 > f")
               while ((getline a[++c] < "f") > 0) { }
               print c
           }
 
      Here, the side effect is the '++c'.  Is 'c' incremented if
      end-of-file is encountered before the element in 'a' is assigned?
 
      'gawk' treats 'getline' like a function call, and evaluates the
      expression 'a[++c]' before attempting to read from 'f'.  However,
      some versions of 'awk' only evaluate the expression once they know
      that there is a string value to be assigned.
 
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