-u As the guilty party for most of sort's comparison options, I can attest that efficiency was not an objective of -u. It was invented precisely because uniq had proved useful, but not when one was interested in uniqueness only of some key aspect of the data. -u differs from uniq in that -u selects samples based on equality of keys, not equality of lines. In the default case of whole-line keys, sort -u of course does exactly what sort|uniq does. For many applications of -u with keys, the non-key fields are not of interest. Then sed s/nonkeys//|sort|uniq may suffice. But sed did not exist when -u was invented. And not all sort key specs are easily imitated in sed.