uniq

Report or filter out repeated lines in a file.
Reads standard input comparing adjacent lines, and writes a copy of each unique input line to the standard output.
The second and succeeding copies of identical adjacent input lines are not written.

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

Options
     -c      Precede each output line with the count of the number of times
             the line occurred in the input, followed by a single space.

     -d      Distinct - Don't output lines that are not repeated in the input.

     -f fields
             Ignore the first fields in each input line when doing compar-
             isons.  A field is a string of non-blank characters separated
             from adjacent fields by blanks.  Field numbers are one based,
             i.e. the first field is field one.

     -s chars
             Ignore the first chars characters in each input line when doing
             comparisons.  If specified in conjunction with the -f option, the
             first chars characters after the first fields fields will be ignored.
             Character numbers are one based, i.e. the first character
             is character one.

     -u      Don't output lines that are repeated in the input.
             Print only lines that are unique in the INPUT.

By default, uniq prints the unique lines in a sorted file, it discards all but one of identical successive input lines. so that the OUTPUT contains unique lines.

uniq will only compare lines that appear successively in the input.

Repeated lines in the input will not be detected if they are not adjacent, so it may be necessary to sort the files first.

If an InputFile of - (or nothing) is given, then uniq will read from standard input.

If no OutputFile file is specified, uniq writes to standard output.

The historic +number and -number options have been deprecated but are still supported in this implementation.

Examples

Count the frequency of some words:

echo "one two three one three" | tr -cs "A-Za-z" "\n" | sort | uniq -c | sort -n -r

“Do you know what you are? You are a marvel. You are unique” ~ Pablo Picasso

Related macOS commands:

sort - Sort text files
tr - Translate, squeeze, and/or delete characters.