.. index:: filenames
.. _filenames:

The Unix Filename
------------------

:t:`Linux for Programmers and Users`, Section 3.26

* Filename limited to 255 characters. Can not contain / or NULL character.

* Filenames are case-sensitive; chap and Chap are two different filenames.

* Group of filenames held together in a directory.

* Directory contains name of the file.

* Both files and directories are subject to access control.

.. note:: 

    Because a directory contains the filenames, creating or deleting files
    requires manipulation of the directory. But a directory can be modified
    only by the kernel. We can't edit a directory file with an editor but
    only by running commands (like :command:`cp` and :command:`rm`) that
    make the kernel change the directory entry.

    A directory contains filenames not files.

    It's a notable feature of the system that we can access critical resources
    not directly, but only by running commands that access them. For instance,
    we add an entry to a directory by running the cp command. The passwd
    command updates a sensitive file (:file:`/etc/shadow`). We can freely run
    passwd but we can not edit :file:`/etc/shadow` directly.

    A file has a unique number as well as other attributes, and only the number
    and filename are stored in the directory. The other file attributes are
    stored, not in the file, but in a separate area of the disk not directly
    accessible to users.