The format for running the diff command is:
diff options... files...
In the simplest case, two file names from-file and to-file are given, and diff compares the contents of 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 one file is a directory and the other is not, diff compares the file in the directory whose name is that of the non-directory. The non-directory file must not be -.
If two file names are given and both are directories, diff compares corresponding files in both directories, in alphabetical order; this comparison is not recursive unless the --recursive (-r) 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.
If the --from-file=file option is given, the number of file names is arbitrary, and file is compared to each named file. Similarly, if the --to-file=file option is given, each named file is compared to file.
diff options begin with ‘-’, so normally file names 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. Normally, differing binary files count as trouble, but this can be altered by using the --text (-a) option, or the -q or --brief option.