Nitpick, sort of, but in unix, a | b executes a and b in parallel, not sequentially. This somewhat complicates the analogy with programming. They're more like functions operating on a data stream.

Edit: I might be misinterpreting your use of "execute _after_". You may not mean "after the completion of a" but instead "operating on the results of a, asynchronously" in which case, apologies.