File (command) - Specification

Specification

The Single Unix Specification (SUS) specifies that a series of tests are performed on the file specified on the command line:

1. if the file cannot be read, its status undetermined, or its type undetermined, file will indicate that the file was processed and its type was undetermined.
2. file must be able to determine the types directory, FIFO, socket, block special file, and character special file
3. zero-length files are identified as such
4. an initial part of file is considered and file is to use position-sensitive tests
5. the entire file is considered and file is to use context-sensitive tests
6. the file is identified as a data file

file's position-sensitive tests are normally implemented by matching various locations within the file against a textual database of magic numbers (see the Usage section). This differs from other simpler methods such as file extensions and schemes like MIME.

In most implementations, the file command uses a database to drive the probing of the lead bytes. That database is implemented in a file called magic, whose location is usually in /etc/magic, /usr/share/file/magic or a similar location.