uniq [-udc [+n] [-n]] [input [output]]

Find unique lines in a file.
Default mode = -ud.
Note - file must be sorted.

Switches -
u	- output only unique lines.
d	- output one copy of 'just the repeated lines'.
n	- skip first n chars/fields. +n = chars / -n = fields.
c	- output unique lines & count of duplicates.
