We can identify a number of composite design patterns. In the previous example, we designed a sequence composite. For inspiration, we can look at the bash shell composite operators: ;, &, |, as well as () for grouping. Beyond these, we have if, for, and while loops within the shell.

We looked at the sequence operator (;) in the Command_Sequence class definition. This concept of sequence is so ubiquitous that many programming languages (such as the shell and Python) don't require an explicit operator; the syntax simply uses end-of-line as an implied sequence operator.

The shell's & operator creates two commands that run concurrently instead of sequentially. We can create a Command_Concurrent class definition with a run() method that uses multiprocessing to create two subprocesses and waits for both to finish.

The | operator in the shell creates a pipeline: one command's output buffer is another command's input buffer; the commands run concurrently...