2.10.5. The Unix Filename

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 cp and 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 (/etc/shadow). We can freely run passwd but we can not edit /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.