uniq is a Unix utility which, when fed a text file, outputs the file with adjacent identical lines collapsed to one.
First appearing in Version 3 Unix, it is a kind of filter program. Typically it is used after sort. It can also output only the duplicate lines (with the -d option), or add the number of occurrences of each line (with the -c option).
An example: To see the list of lines in a file, sorted by the number of times each occurs:
- sort file | uniq -c | sort -n
- McIlroy, M. D. (1987). A Research Unix reader: annotated excerpts from the Programmer's Manual, 1971–1986 (PDF) (Technical report). CSTR. Bell Labs. 139.