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(web2c.info)\input filenames

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`\input' filenames
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  TeX, Metafont, and MetaPost source programs can all read other source
files with the `\input' (TeX) and `input' (MF and MP) primitives:
     \input NAME % in TeX

  The file NAME can always be terminated with whitespace; for Metafont
and MetaPost, the statement terminator `;' also works.  (LaTeX and
other macro packages provide other interfaces to `\input' that allow
different notation; here we are concerned only with the primitive
operation.)  This means that `\input' filenames cannot directly contain
whitespace, even though Unix has no trouble.  Sorry.

  On the other hand, various C library routines and Unix itself use the
null byte (character code zero, ASCII NUL) to terminate strings.  So
filenames in Web2c cannot contain nulls, even though TeX itself does
not treat NUL specially.

  Furthermore, some older Unix variants do not allow eight-bit
characters (codes 128-255) in filenames.

  For maximal portability of your document across systems, use only the
characters `a'-`z', `0'-`9', and `.', and restrict your filenames to at
most eight characters (not including the extension), and at most a
three-character extension.  Do not use anything but simple filenames,
since directory separators vary among systems; instead, add the
necessary directories to the appropriate search path.

  Finally, the present Web2c implementation does `~' and `$' expansion
on NAME, unlike Knuth's original implementation and older versions of
Web2c.  Thus:
     \input ~jsmith/$foo.bar
  will dereference the environment variable or Kpathsea config file
value `foo' and read that file extended with `.bar' in user `jsmith''s
home directory.  (You can also use braces, as in `${foo}bar' if you
want to follow the variable name with a letter, numeral, or `_'.)

  (So you could define an environment variable value including
whitespace and get the program to read such a filename that way, if you
need to.)

  In all the common TeX formats (plain TeX, LaTeX, AMSTeX), the
characters `~' and `~' have special category codes, so to actually use
these in a document you have to change their catcodes or use `\string'.
(The result is unportable anyway, see the suggestions above.)  The
place where they are most likely to be useful is when typing
interactively.