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gawk: Tee Program

 11.2.5 Duplicating Output into Multiple Files
 The 'tee' program is known as a "pipe fitting."  'tee' copies its
 standard input to its standard output and also duplicates it to the
 files named on the command line.  Its usage is as follows:
      'tee' ['-a'] FILE ...
    The '-a' option tells 'tee' to append to the named files, instead of
 truncating them and starting over.
    The 'BEGIN' rule first makes a copy of all the command-line arguments
 into an array named 'copy'.  'ARGV[0]' is not needed, so it is not
 copied.  'tee' cannot use 'ARGV' directly, because 'awk' attempts to
 process each file name in 'ARGV' as input data.
    If the first argument is '-a', then the flag variable 'append' is set
 to true, and both 'ARGV[1]' and 'copy[1]' are deleted.  If 'ARGC' is
 less than two, then no file names were supplied and 'tee' prints a usage
 message and exits.  Finally, 'awk' is forced to read the standard input
 by setting 'ARGV[1]' to '"-"' and 'ARGC' to two:
      # tee.awk --- tee in awk
      # Copy standard input to all named output files.
      # Append content if -a option is supplied.
      BEGIN {
          for (i = 1; i < ARGC; i++)
              copy[i] = ARGV[i]
          if (ARGV[1] == "-a") {
              append = 1
              delete ARGV[1]
              delete copy[1]
          if (ARGC < 2) {
              print "usage: tee [-a] file ..." > "/dev/stderr"
              exit 1
          ARGV[1] = "-"
          ARGC = 2
    The following single rule does all the work.  Because there is no
 pattern, it is executed for each line of input.  The body of the rule
 simply prints the line into each file on the command line, and then to
 the standard output:
          # moving the if outside the loop makes it run faster
          if (append)
              for (i in copy)
                  print >> copy[i]
              for (i in copy)
                  print > copy[i]
 It is also possible to write the loop this way:
      for (i in copy)
          if (append)
              print >> copy[i]
              print > copy[i]
 This is more concise, but it is also less efficient.  The 'if' is tested
 for each record and for each output file.  By duplicating the loop body,
 the 'if' is only tested once for each input record.  If there are N
 input records and M output files, the first method only executes N 'if'
 statements, while the second executes N'*'M 'if' statements.
    Finally, the 'END' rule cleans up by closing all the output files:
      END {
          for (i in copy)
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