应该是

diff <(man uniq) <(man guniq)

要从评论中回答saua的问题:

To answer saua's question from the comments:

Bash将<(...)变成一个命名管道，diff程序将其视为文件.据diff所知，它正在比较两个文件.

Bash turns <(...) into a named pipe, which the diff program sees as a file. So as far as diff knows, it's comparing two files.