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I often want to compare two files using the diff command.  But I also want to compare the outputs of multiple command outputs, e.g. two directory listings, grep outputs, or archive contents.  You can accomplish this by using “named pipes”.  Here is a basic example:

$ diff <(ls dir_a/ | grep -v .svn) <(ls dir_b/ | grep -v .svn)

(Source: How can you diff two pipelines in Bash? - Stack Overflow)