These commands deal with file input; they ensure that the non-existence of a requested file can be handled in a user-friendly way.
\IfFileExists
{<file-name>} {<true>} {<false>}
If the file exists then the code specified in <true> is executed.
If the file does not exist then the code specified in <false> is executed.
This command does not input the file.
\InputIfFileExists
{<file-name>} {<true>} {<false>}
This inputs the file <file-name> if it exists and, immediately before the input, the code specified in <true> is executed.
If the file does not exist then the code specified in <false> is executed.
It is implemented using \IfFileExists
.