diff
The format for running the diff command is:
diff options... from-file to-file
In the simplest case, diff compares the contents of the two files
from-file and to-file. A file name of `-' stands for
text read from the standard input. As a special case, `diff - -'
compares a copy of standard input to itself.
If from-file is a directory and to-file is not, diff
compares the file in from-file whose file name is that of to-file,
and vice versa. The non-directory file must not be `-'.
If both from-file and to-file are directories,
diff compares corresponding files in both directories, in
alphabetical order; this comparison is not recursive unless the
`-r' or `--recursive' option is given. diff never
compares the actual contents of a directory as if it were a file. The
file that is fully specified may not be standard input, because standard
input is nameless and the notion of "file with the same name" does not
apply.
diff options begin with `-', so normally from-file and
to-file may not begin with `-'. However, `--' as an
argument by itself treats the remaining arguments as file names even if
they begin with `-'.
An exit status of 0 means no differences were found, 1 means some differences were found, and 2 means trouble.
diff.