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gawk: Assignment Options Assigning Variables on the Command Line
 Any 'awk' variable can be set by including a "variable assignment" among
 the arguments on the command line when 'awk' is invoked (⇒Other
 Arguments).  Such an assignment has the following form:
 With it, a variable is set either at the beginning of the 'awk' run or
 in between input files.  When the assignment is preceded with the '-v'
 option, as in the following:
 the variable is set at the very beginning, even before the 'BEGIN' rules
 execute.  The '-v' option and its assignment must precede all the file
 name arguments, as well as the program text.  (⇒Options for more
 information about the '-v' option.)  Otherwise, the variable assignment
 is performed at a time determined by its position among the input file
 arguments--after the processing of the preceding input file argument.
 For example:
      awk '{ print $n }' n=4 inventory-shipped n=2 mail-list
 prints the value of field number 'n' for all input records.  Before the
 first file is read, the command line sets the variable 'n' equal to
 four.  This causes the fourth field to be printed in lines from
 'inventory-shipped'.  After the first file has finished, but before the
 second file is started, 'n' is set to two, so that the second field is
 printed in lines from 'mail-list':
      $ awk '{ print $n }' n=4 inventory-shipped n=2 mail-list
      -| 15
      -| 24
      -| 555-5553
      -| 555-3412
    Command-line arguments are made available for explicit examination by
 the 'awk' program in the 'ARGV' array (⇒ARGC and ARGV).  'awk'
 processes the values of command-line assignments for escape sequences
 (⇒Escape Sequences).  (d.c.)
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