Linux Filesystem Demystified

(Click to enlarge)

from http://www.freeos.com/articles/3102/

There is only a single hierarchal directory structure. Everything starts from the root directory, represented by '/', and then expands into sub-directories. Where DOS/Windows had various partitions and then directories under those partitions, Linux places all the partitions
under the root directory by 'mounting' them under specific directories. Closest to root under Windows would be c:.

For best system performance, it is best to put /home, / (root) and file swap on it's own partition. Other users have gone as far as putting more than three partitions on their Linux install; Making a partition for /boot, /var and others.

Under Windows, the various partitions are detected at boot and assigned a drive letter. Under Linux, unless you mount a partition or a device, the system does not know of the existence of that partition or device. This might not seem to be the easiest way to provide access to your partitions or devices but it offers great flexibility.

This kind of layout, known as the unified filesystem, does offer several advantages over the approach that Windows uses. Let's take the example of the /usr directory. This directory off the root directory contains most of the system executables. With the Linux filesystem, you can choose to mount it off another partition or even off another machine over the network. The underlying system will not know the difference because /usr appears to be a local directory that is part of the local directory structure! How many times have you wished to move around executables and data under Windows, only to run into registry and system errors (Note: Linux doesn't have a registry like Windows)? Try moving c:windowssystem to another partition or drive.

Another thing to point out is the use of the frontslash '/' instead of the backslash '' as in DOS/Windows. So c:windowssystem would be /c/windows/system. Well, Linux is not going against convention here. Unix has been around a lot longer than Windows and was the standard a lot before Windows was. Rather, DOS took the different path, using '/' for command-line options and '' as the directory separator.

Linux also chooses to be case sensitive. What this means that the case, whether in capitals or not, of the characters becomes very important. So this is not the same as THIS or ThIs for that matter. This one feature probably causes the most problems for people just starting out on command line.

Below is a detailed summary of the Linux Filesystem Hierarchy:

/sbin - This directory contains all the binaries that are essential to the working of the system. These include system administration as well as maintenance and hardware configuration programs. Find lilo, fdisk, init, ifconfig etc here. These are the essential programs that are required by all the users. Another directory that contains system binaries is /usr/sbin. This directory contains other binaries of use to the system administrator. This is where you will find the network daemons for your system along with other binaries that only the system administrator has access to, but which are not required for system maintenance, repair etc.

/bin - In contrast to /sbin, the bin directory contains several useful commands that are used by both the system administrator as well as non-privileged users. This directory usually contains the shells like bash, csh etc. as well as much used commands like cp, mv, rm, cat, ls. There also is /usr/bin, which contains other user binaries. These binaries on the other hand are not essential for the user. The binaries in /bin however, a user cannot do without.

/boot - This directory contains the system.map file as well as the Linux kernel. Lilo or GRUB places the boot sector backups in this directory.

/dev - This is a very interesting directory that highlights one important characteristic of the Linux filesystem - everything is a file or a directory. Look through this directory and you should see hda1, hda2 etc, which represent the various partitions on the first master drive of the system. /dev/cdrom and /dev/fd0 represent your CDROM drive and your floppy drive. This may seem strange but it will make sense if you compare the characteristics of files to that of your hardware. Both can be read from and written to. Take /dev/dsp, for instance. This file represents your speaker device. So any data written to this file will be re-directed to your speaker. Try 'cat /etc/lilo.conf > /dev/dsp' and you should hear some sound on the speaker. That's the sound of your lilo.conf file! Similarly, sending data to and reading from /dev/tty. S0 (COM 1 ) will allow you to communicate with a device attached there - your modem.

/etc - This directory contains all the configuration files for your system. Your lilo.conf file lies in this directory as does hosts, resolv.conf and fstab. Under this directory will be X11 sub-directory which contains the configuration files for X. More importantly, the /etc/rc.d directory contains the system startup scripts. This is a good directory to backup often. It will definitely save you a lot of re-configuration later if you re-install or lose your current installation.

/home - Linux is a multi-user environment so each user is also assigned a specific directory which is accessible only to them and the system administrator. These are the user home directories, which can be found under /home/username. This directory also contains the user specific settings for programs like IRC, X, etc.

/lib - This contains all the shared libraries that are required by system programs. Windows equivalent to a shared library (lib) would be a DLL file.

/lost+found - Linux should always go through a proper shutdown. Sometimes your system might crash or a power failure might take the machine down. Either way, at the next boot, a lengthy filesystem check using fsck will be done. Fsck will go through the system and try to recover any corrupt files that it finds. The result of this recovery operation will be placed in this directory. The files recovered are not likely to be complete or make much sense but there always is a chance that something worthwhile is recovered.

/mnt - This is a generic mount point under which you mount your filesystems or devices. Mounting is the process by which you make a filesystem available to the system. After mounting your files will be accessible under the mount-point. This directory usually contains mount points or sub-directories where you mount your floppy, CD or USB Hard Drive. You can also create additional mount-points here if you want. There is no limitation to creating a mount-point anywhere on your system but convention says that you do not litter your file system with mount-points.

/media - This is also a generic mount for your removable media. Depending on the Linux distro, you may/may not see this. Usually, removable media, mounted partitions, DVD, etc mounted in /media will only show up in your menu such as 'Places'. Otherwise, if mounted in /mnt, it will more than likely show up on your Desktop as well too.

/opt - This directory contains all the software and add-on packages that are not part of the default installation. Generally you will find KDE and StarOffice here. Again, this directory is not used very often as it's mostly a standard in Unix installations.

/proc - This is a special directory on your system. The /proc File System is a pseudo file system residing in the virtual memory and maintains highly dynamic data on the state of your operating system. Most of the information in the /proc file system is updated to match the
current state of the operating system. The contents of the /proc file system can be read by anyone who has the requisite permissions

/root - This one is the home directory of the user root. This is not to be confused with the system root, which is directory at the highest level in the filesystem.

/tmp - This directory contains mostly files that are required temporarily. Many programs use this to create lock files and for temporary storage of data. On some systems, this directory is cleared out at boot or at shutdown.

/usr - This is one of the most important directories in the system as it contains all the user binaries. X and its supporting libraries can be found here. User programs like telnet, ftp etc are also placed here. /usr/doc contains useful system documentation. /usr/src/linux contains the
source code for the Linux kernel.

/var - This directory contains spooling data like mail and also the output from the printer daemon. The system logs are also kept here in /var/log/messages. You will also find the database for BIND in /var/named and for NIS in /var/yp.

Filesystem Commands to point out (with reference examples from the picture above):

1. When you are working within a shell terminal, you are always operating in a particular directory. To determine which directory you are in, use the pwd command:

student@linuxgym$ pwd
/home/student
student@linuxgym$

2. Your home directory is the directory you are in when you first open the terminal. To go to your home directory from anywhere, just type "cd":

student@linuxgym:$ pwd
/usr/local/bin
student@linuxgym:$ cd
student@linuxgym:$ pwd
/home/student
student@linuxgym:$

3. An absolute path name is one beginning with the "/" character, which signifies the root of the file system tree. Therefore, another way of going to your home directory is:

student@linuxgym:/etc$ cd /home/student
student@linuxgym:$ pwd
/home/student
student@linuxgym:$

4. A relative path is one which starts with the name of a directory connected to the current directory. For example, if you are in the /usr directory, then typing only "cd bin" (without preceding "bin" with "/") has the following effect:

student@linuxgym:$ pwd
/usr
student@linuxgym:$ cd bin
student@linuxgym:$ pwd
/usr/bin
student@linuxgym:$

and you go to /usr/bin rather than /usr/local/bin or /bin.

5. To go to the directory containing the current working directory (also called the parent directory) type:

student@linuxgym:$ pwd
/usr/bin
student@linuxgym:$ cd ..
student@linuxgym:$ pwd
/usr
student@linuxgym:$

6. The relative pathname of the current working directory is called "." (the full stop). Therefore typing:

student@linuxgym:$ pwd
/usr/bin
student@linuxgym:$ cd .
student@linuxgym:$ pwd
/usr/bin
student@linuxgym:$

does not change the current working directory.

You can also find a nice Index of Linux Commands and what they do -here. There is also a great directory guide for Unix/BSD/Linux commands for IT work or advanced users called Unix Toolbox.You can get the PDF version of it here.