(m4.info)Command line files

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2.6 Specifying input files on the command line
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The remaining arguments on the command line are taken to be input file
names.  If no names are present, standard input is read.  A file name of
'-' is taken to mean standard input.  It is conventional, but not
required, for input files to end in '.m4'.

   The input files are read in the sequence given.  Standard input can
be read more than once, so the file name '-' may appear multiple times
on the command line; this makes a difference when input is from a
terminal or other special file type.  It is an error if an input file
ends in the middle of argument collection, a comment, or a quoted
string.

   The options '--define' ('-D'), '--undefine' ('-U'), '--synclines'
('-s'), and '--trace' ('-t') only take effect after processing input
from any file names that occur earlier on the command line.  For
example, assume the file 'foo' contains:

     $ cat foo
     bar

   The text 'bar' can then be redefined over multiple uses of 'foo':

     $ m4 -Dbar=hello foo -Dbar=world foo
     =>hello
     =>world

   If none of the input files invoked 'm4exit' (Note: M4exit), the
exit status of 'm4' will be 0 for success, 1 for general failure (such
as problems with reading an input file), and 63 for version mismatch
(Note: Using frozen files).

   If you need to read a file whose name starts with a '-', you can
specify it as './-file', or use '--' to mark the end of options.