(tds.info)Subdirectory searching

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2.1 Subdirectory searching
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Older TeX installations store large numbers of related files in single
directories, for example, all `TFM' files and/or all TeX input files.

   This monolithic arrangement hinders maintenance of a TeX system: it
is difficult to determine what files are used by what packages, what
files need to be updated when a new version is installed, or what files
should be deleted if a package is removed.  It is also a source of error
if two or more packages happen to have input files with the same name.

   Therefore, the TWG felt each package should be in a separate
directory. But we recognized that explicitly listing all directories to
be searched would be unbearable.  A site may wish to install dozens of
packages.  Aside from anything else, listing that many directories would
produce search paths many thousands of characters long, overflowing the
available space on some systems.

   Also, if all directories are explicitly listed, installing or
removing a new package would mean changing a path as well as installing
or removing the actual files.  This would be a time-consuming and
error-prone operation, even with implementations that provide some way
to specify the directories to search at runtime.  On systems without
runtime configuration, it would require recompiling software, an
intolerable burden.

   As a result, the TWG concluded that a comprehensive TDS requires
implementations to support some form of implicit subdirectory
searching.  More precisely, implementations must make it possible to
specify that TeX, Metafont, and their companion utilities search in both
a specified directory and recursively through all subdirectories of that
directory when looking for an input file.  Other forms of subdirectory
searching, for example recursive-to-one-level searches, may also be
provided.  We encourage implementors to provide subdirectory searching
at the option of the installer and user for all paths.

   The TDS does not specify a syntax for specifying recursive
searching, but we encourage implementors to provide interoperability
(see Section Note: More on subdirectory searching).