The Linux (Virtual) File System

Probably the most important difference (from the user's perspective) between the DOS/Windows world and the Linux world is the organization and operation of the file system. Some ideas will seem very familiar, and others will seem completely alien, but with a good understanding of the Linux file system, you have the skill to avoid some of the most common problems experienced by new Linux users.

What is a file system?

Ever go looking for a book in the library? In order to find what you're looking for, you must understand the library's file system, or method of organization. Fiction books are easy, they use a simple type of file system commonly known as alphabetical by author. Nonfiction books can be tougher because they use a different type of file system, the Dewey Decimal System. What if you don't understand the Dewey Decimal System? What if you only know the title of a book and you don't know whether it's fiction or non-fiction? How can we find our book now? Luckily there is a librarian to help us, and she has created a card catalog (or in most libraries now, a computerized catalog) listing all the books available and where to find them.

Now, picture your hard drive as the library. Like the library, your hard drive can have different file systems. The operating system is your librarian, who has provided the directory tree (sometimes also referred to as "the file system", with a slightly different meaning) as a catalog of the files available.

Unfortunately, the world of computers is not as standardized as our libraries. In computers, every librarian or operating system has its own type of file system, and other operating systems often don't know how to read it. Lucky for us, Linux is a versatile operating system and it does understand the file system used by Windows 95 & Windows 98.

How The Linux File System Is Different

Typically one device will have one file system. (There are many exceptions to this rule, however.) For example, each floppy disk or CD-ROM contains a file system. Your operating system can give you access to these file systems. DOS/Windows and Linux, like all modern operating systems, use a hierarchical directory structure. The hierarchy of the file system begins at the root directory, called "/" (slash) on Linux and "\" (backslash) on DOS/Windows. Beneath or inside the root directory there may be files and other directories. Each directory may contain files and other directories, and so on.

Under DOS/Windows, each file system/device (the division between the device and the file system that resides on it is not clear in DOS/Windows) is assigned a device name, a drive letter. There is very little flexibility for assigning drive letters. The first floppy drive is always drive A:, the first hard drive partition is always drive C:, etc. These file systems are all separate from each other, each having its own root directory and hierarchy. The only way to access the files on one of these devices is by specifying the drive letter.

Linux also assigns a device name to each device, but this is not how the files on that device are accessed. There are no drive letters in Linux. Instead, Linux creates a Virtual File System for us, which makes all the files on all the devices appear to exist on one global device. In Linux, there is one root directory, and every file you have access to is located under it somewhere. Furthermore, the Linux root directory does not have to be in any particular place. It might not be on your first hard drive. It might not even be on your computer. Linux can use a network shared resource as its root directory.

Adding File Systems to the Tree

(For instructions on how to do this, see Getting to CD's and Floppies.)
To gain access to files on another device, you must first tell Linux where in the directory tree you would like those files to appear. This process is called mounting a file system. For example, you will frequently need to access files from CD-ROM. In order to do this, you must tell Linux, "Take the file system from this CD-ROM and make it appear under the directory /mnt." The directory given to Linux is called the mount point. In this case it is /mnt. The /mnt directory exists on all Linux systems, and it is intended specifically for use as a mount point for temporary media like floppy disks or CDROMs. It may be empty, or it may contain subdirectories for mounting individual devices.

Linux does not require you to use /mnt as the mount point for other file systems. You may mount file systems anywhere in the directory tree. However, it is good practice to create empty directories that are reserved as mount points. Linux will allow you to mount a file system on a directory that is not empty. Any files in that directory will become invisible in the virtual file system. They will still exist on disk, but you will be unable to access them until the file system that obscures them is unmounted. There are rare cases where this might be desirable, but in general it can create unexpected behavior and is not advisable. For these reasons, only the administrator may authorize the mounting of file systems.

At least one and perhaps many file systems are automatically mounted by Linux at boot time. The system administrator can control which file systems are mounted at boot time, and can pre-determine the mount points for specific file systems. The sysadmin can also designate some file systems that may be mounted by normal users, and can specify when mounted file systems are checked for errors and backed up. All this information is stored in the file /etc/fstab, which anyone can read to discover what file systems are available and mountable by users.

Quick Example

Let's take an example to show how this virtual file system is useful. Say that you are in charge of maintaining a store of sales data for your company. There is a directory called /finance. Under that exist directories called sales, purchases, etc. Under the sales directory you have a subdirectory this_year representing sales for the current fiscal year, which data is stored on the hard drive.. Last year's data has been archived to a CD ROM. Thanks to the Linux virtual file system, you can create a directory under sales called last_year and mount the archive CD-ROM on this directory. Now all your sales data is available in one place! This is a very simplistic example, but it does illustrate one advantage of the virtual file system.