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gawk: History Sorting

 11.3.6 Removing Duplicates from Unsorted Text
 The 'uniq' program (⇒Uniq Program) removes duplicate lines from
 _sorted_ data.
    Suppose, however, you need to remove duplicate lines from a data file
 but that you want to preserve the order the lines are in.  A good
 example of this might be a shell history file.  The history file keeps a
 copy of all the commands you have entered, and it is not unusual to
 repeat a command several times in a row.  Occasionally you might want to
 compact the history by removing duplicate entries.  Yet it is desirable
 to maintain the order of the original commands.
    This simple program does the job.  It uses two arrays.  The 'data'
 array is indexed by the text of each line.  For each line, 'data[$0]' is
 incremented.  If a particular line has not been seen before, then
 'data[$0]' is zero.  In this case, the text of the line is stored in
 'lines[count]'.  Each element of 'lines' is a unique command, and the
 indices of 'lines' indicate the order in which those lines are
 encountered.  The 'END' rule simply prints out the lines, in order:
      # histsort.awk --- compact a shell history file
      # Thanks to Byron Rakitzis for the general idea
          if (data[$0]++ == 0)
              lines[++count] = $0
      END {
          for (i = 1; i <= count; i++)
              print lines[i]
    This program also provides a foundation for generating other useful
 information.  For example, using the following 'print' statement in the
 'END' rule indicates how often a particular command is used:
      print data[lines[i]], lines[i]
 This works because 'data[$0]' is incremented each time a line is seen.
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