Set intersection and difference at the command line

A few years ago I wrote about comm, a utility that lets you do set theory at the command line. It's a really useful little program, but it has two drawbacks: the syntax is hard to remember, and the input files must be sorted.

If A and B are two sorted lists,

 comm A B

prints A - B, B - A, and A B. You usually don't want all three, and so comm lets you filter the output. It's a little quirky in that you specify what youdon't want instead of what you do. And you have to remember that 1, 2, and 3 correspond to A - B, B - A, and A B respectively.

A couple little scripts can hide the quirks. I have a script intersect

 comm -12 <(sort "$1") <(sort "$2")

and another script setminus

 comm -23 <(sort "$1") <(sort "$2")

that sort the input files on the fly and eliminate the need to remember comms filtering syntax.

The setminus script computes A - B. To find B - A call the script with the arguments reversed.

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