We can identify a number of composite Command design patterns. In the previous example, we designed a composite object that implemented a sequence of operations. 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 semantic equivalent of the shell sequence operator, ;, in the Command_Sequence class definition. This concept of a sequence is so ubiquitous that many programming languages (such as the shell and Python) don't require an explicit operator; the shell's 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...
