13. Macros for running shell commands

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.