Introduction to UNIX files

In UNIX systems any element is represented in the form of a file. All files are architectured around a single tree structure where the base, called the root, is written "/".

File types

UNIX systems define different file types:

• Physical files, entered on the hard disk. This is a file in the generally understood sense of the word;
• Directories are files (nodes) of the tree structure able to contain files or other directories. A directory contains at least a parent directory (written ..), relating to the directory at the higher level, and a current directory (written .), i.e. itself;
• Links are special files enabling several names (links) to be associated to a single and same file. This system makes it possible to have several instances of the same file in several places in the tree structure without needing to copy it, which in particular helps to ensure maximum coherence and save on disk space. There are two types of links:
  • Symbolic links representing virtual pointers (shortcuts) to the real files. In the event that a symbolic link is deleted, the file which it points to is not deleted. Symbolic links are created using the In -s command according to the following syntax:

    ln -s name-of-real-file name-of-symbolic-link

  • Physical links (also called hardlinks) represent an alternative name for a file. So, when a file has two physical links, the deletion of one or other of these links does not lead to the deletion of the file. More specifically, while there is at least one physical link remaining, the file is not deleted. On the other hand, when all physical links for the same file are deleted the file is too. Please note however, that it is only possible to create physical links within the single and same file system. Physical links are created using the In (without the option -n) command according to the following syntax:

    ln name-of-real-file name-of-physical-link

• Virtual files do not really exist because they only exist in the memory. These files, located in particular in the /proc directory contain information about the system (processor, memory, hard disks, processes, etc.);
• The Device files located in the /dev/ directory relate to the system devices. This concept may be disconcerting for a new user for the first time.

The concept of mount points

Files in a UNIX system are organised into a single tree structure. It is however possible to have several partitions using a mechanism called mounting which makes it possible to connect a partition to a directory in the main tree structure. So, the fact of mounting a partition in the directory /mnt/partition makes all the files in the partition accessible from this directory, called the "mount point".

File hierarchy under Unix

To ensure compatibility and portability, UNIX systems comply with the FHS (File Hierarchy Standard) standard. The basic hierarchy of a Unix system is as follows: