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uniq: print unique records in a file & histogram of records
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$ uniq -u -d -c sorted_file

uniq ONLY works on sorted files. It identifies lines which are unique, equally 
duplicated lines and make a histogram!

-c .. produce a histogram
-d .. print only duplicated lines
-f n .. ignore the first fields
-s n .. ignore the first n characters
-u .. print only lines that are not repeated in the input
-i .. case insensitive

Less useful options
-f NFIELDS (avoid comparing the first NFIELDS fields)
-i ignore case
-s NCHARS (avoid comparing the first NCHARS characters)
-w NCHARS (compare no more than NCHARS in lines)

$ cat a 
6
4
2
6
2
4
2
1

$ sort -n a | uniq   #print each distinct member
1
2
4
6

$ sort -n a | uniq -u    #print lines which are not duplicated
1

$ sort a | uniq -d    #print only lines which are repeated
2
4
6

$ sort a | uniq -c    #histogram 
   1 1
   3 2
   2 4
   2 6
   ^
   #