`uniq`

Report or filter out repeated lines in a file (POSIX)

Syntax:

uniq [-c] [-d|-u] [-f fields] [-s chars]
     [input_file [output_file]]

Deprecated syntax:

uniq [-c] [-d|-u] [-n] [+m]
     [input_file [output_file]]

Options:

-n: (deprecated, replaced by -f) Ignore the first n fields when doing comparisons, where n is a number.
+m: (deprecated, replaced by -s) Ignore the first m characters when doing comparisons, where m is a number.
-c: Precede each output line with the number of times the line occurred in the input.
-d: Suppress the writing of lines that aren't repeated in the input.
-f fields: Ignore the first fields on each input line when doing comparisons, where fields is a positive decimal integer. A field is a string of nonblank characters separated from adjacent fields by blanks.
-s chars: Ignore the first chars characters when doing comparisons, where chars is a positive decimal integer. If specified in conjunction with the -f option, the first chars characters after the first fields fields are ignored.
-u: Suppress the writing of lines that are repeated in the input.
input_file: The pathname of the input file. If you don't specify any input files, the standard input is used.
output_file: The pathname of the output file. This name must differ from the name of the input file. If you don't specify an output file, the standard output is used.

The uniq utility reads an input text file, comparing adjacent lines, and writes one copy of each input line to the output. The second and succeeding copies of repeated adjacent input lines aren't written.

To obtain a report of unique lines in a file, the input file must be sorted prior to running uniq.