The following operands shall be supported:
file1, file2
A pathname of a file to be compared. If either the
file1 or file2 operand is '-', the standard input shall
be used in its place.
If both file1 and file2 are directories, diff shall not compare
block special files, character special files, or FIFO special
files to any files and shall not compare regular files to
directories. Further details are as specified in Diff Directory
Comparison Format. The behavior of diff on other file types is
implementation-defined when found in directories.
If only one of file1 and file2 is a directory, diff shall be
applied to the non-directory file and the file contained in the
directory file with a filename that is the same as the last
component of the non-directory file.
