Guide to LaTeX (4th Edition) (Tools and Techniques

D.2. L AT E X programming commands 447
and the processing continues. Warnings may be split into several lines
with the \MessageBreak command, just like error texts.
Two last commands of this type are
\ClassInfo{class name}{info text}
\PackageInfo{package name}{info text}
which write their texts only to the transcript file, and not to the monitor.
They are otherwise just like the corresponding NoLine warnings.
D.2.8 Inputting files
Files other than classes and packages may also be input, in which case it
is often desirable to make sure that they exist beforehand. Or, alternative
actions might be taken depending on the existence of a certain file. These
goals are met with
\IfFileExists{file name}{true}{false}
\InputIfFileExists{file name}{true}{false}
Both these commands test for the presence of the specified file name in
the area that L AT E X is looking for files, and execute true if it is found,
otherwise false. In addition, \InputIfFileExists reads in that file after
executing true.
These commands are not restricted to the preamble, nor to class or
package files. In fact, the regular \input command is defined in terms of
them.
Many special classes make use of these commands to read in a local
configuration file. For example, the class ltxdoc contains
\InputIfFileExists{ltxdoc.cfg}
{\typeout{Local config file ltxdoc.cfg used}}
{}
just before \ProcessOptions is called. This allows one to have a local
configuration that might specify
\PassOptionsToClass{a4paper}{article}
for a European installation, without altering the files that are processed
with the ltxdoc class.
D.2.9 Checking files
Although not really part of programming, two L AT E X features to keep track
of input files are described here. The first of these, already mentioned in
Section 9.1.1, is the command
\listfiles