## 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.