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## Invoking `diff`

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 `-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.

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.

- diff Options: Summary of options to `diff`.