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gawk: While Statement

 7.4.2 The 'while' Statement
 In programming, a "loop" is a part of a program that can be executed two
 or more times in succession.  The 'while' statement is the simplest
 looping statement in 'awk'.  It repeatedly executes a statement as long
 as a condition is true.  For example:
      while (CONDITION)
 BODY is a statement called the "body" of the loop, and CONDITION is an
 expression that controls how long the loop keeps running.  The first
 thing the 'while' statement does is test the CONDITION.  If the
 CONDITION is true, it executes the statement BODY.  (The CONDITION is
 true when the value is not zero and not a null string.)  After BODY has
 been executed, CONDITION is tested again, and if it is still true, BODY
 executes again.  This process repeats until the CONDITION is no longer
 true.  If the CONDITION is initially false, the body of the loop never
 executes and 'awk' continues with the statement following the loop.
 This example prints the first three fields of each record, one per line:
      awk '
          i = 1
          while (i <= 3) {
              print $i
      }' inventory-shipped
 The body of this loop is a compound statement enclosed in braces,
 containing two statements.  The loop works in the following manner:
 first, the value of 'i' is set to one.  Then, the 'while' statement
 tests whether 'i' is less than or equal to three.  This is true when 'i'
 equals one, so the 'i'th field is printed.  Then the 'i++' increments
 the value of 'i' and the loop repeats.  The loop terminates when 'i'
 reaches four.
    A newline is not required between the condition and the body;
 however, using one makes the program clearer unless the body is a
 compound statement or else is very simple.  The newline after the open
 brace that begins the compound statement is not required either, but the
 program is harder to read without it.
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