m4: Shell commands
13 Macros for running shell commands
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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.
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