The uniq command is used to remove duplicate lines from a sorted file. It requires the duplicate entries to be in the adjacent lines and, hence, it is mostly used in combination with the sort command, which is used to sort the file contents first. The syntax of the uniq command is as follows:

$ sort <filename> | uniq
 or
$ sort -u <filename>

To count duplicate lines in the file, execute the command line, shown as follows :

$ sort <filename> | uniq -c

To display only the entries that are duplicates, execute the command line, shown as follows:

$ sort <filename> | uniq -cd