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There are a few builtin macros in m4 that allow you to run shell
commands from within m4.
Note that the definition of a valid shell command is system dependent.
On UNIX systems, this is the typical /bin/sh. But on other
systems, such as native Windows, the shell has a different syntax of
commands that it understands. Some examples in this chapter assume
/bin/sh, and also demonstrate how to quit early with a known
exit value if this is not the case.