Linux Directory Structure

Kevin B. O’BrienWashtenaw Linux Users Group

http://www.lugwash.org

General Notes● Linux derives from Unix and is designed

to be multi-user● This explains some otherwise puzzling

choices● This may have something to do with

Fedora proposing to change this Directory Structure

Drives 1● Linux derives from Unix● Different from Windows● Windows starts with drives, which are

explicit– C:– D:

Drives 2● Linux does not make drives explicit. ● You can have one physical drive, several

physical drives, partition the drives into multiple partitions, and normally you won’t be aware of it.

● In a file manager, you won’t see drives, just directories

● But if you look in fstab, you will see things like hda1, hda2, sda1, sda2. These are logical drives (either physical or from partitioning)

Root● In Windows, every drive has its own root

– C:\ is the root of the C drive● In Linux, there is only one root, no matter

how many drives you may have– In Linux, / is the root

● Root is ambiguous in one respect, since it can refer to the top of the file structure, and is also the name of the Administrator-type account in Linux.

Root 2● For our purposes, we will refer to root as

the top of the file structure in this presentation.

Comparison

/home/data

/var

•/boot•/bin•etc.

C:

D:

E:

F:

/

Disk Partitions

Linux Windows

/home

root

Watch out for...● Windows uses a backslash for

everything● Linux uses a forward slash for

everything● In Windows, the logical drive (e.g. C:) is

an important part of the directory structure

● In Linux, logical drives don't mean much. You can even mount a separate physical drive under a directory that is on another drive.

Watch out for 2

● In Windows, case does not matter. The C:\Windows directory is the same as the c:\windows directory.

● In Linux, everything is case sensitive. The convention in Linux is to use lower case for most things, but the important thing to remember is that /usr/bin is not the same thing as /USR/BIN.

Drives vs. directories

● In Windows, drives are directories● In Linux, you can have several drives all

under one overall directory, or nested one in another in the directory structure.

● In Linux, you tell the file system where you want a drive to appear by where it is mounted. See fstab for more on this.

Where did this come from?● This started with a Linux effort in August,

1993● In 1996 some BSD folks decided it should

be expanded to all Unix-like systems● This resulted in the Filesystem Hierarchy

Standard, which can be found at http://www.pathname.com/fhs/pub/fhs-2.3.html

Standard?● Of course, many distros vary from this to

some degree, some (Gobo) reject it entirely, and MacOS tends to substitute more “readable” names, like Library in place of lib, etc.

● Red Hat seems to be pretty faithful http://www.redhat.com/docs/manuals/linux/RHL-9-Manual/ref-guide/s1-filesystem-fhs.html

Standard? 2

● Distros can vary, but so can programs● When you install a program, it may not

follow the FHS in deciding where to place its files

● So this presentation should be considered a guide to how things more-or-less ought to work, but YMMV

/● This is the symbol for the root of the file

system in Linux● Every directory is “under” root,

ultimately● This is not the same as the user “root”,

which is the user with God-like powers over the system

● The user “root” does have a directory, called /root

/bin● Contains many of the commands used on

the command line● Examples include cat, chmod,dmesg, kill,

ls, mkdir more, ps, pwd, sed, su● the above commands, and many others,

must be in /bin to meet the standard.● Other commands can be optionally

included, such as tar. gzip, netstat, and ping

/bin 2● There are other commands that are not in

here● They are part of the shell (e.g. cd)● For most users, this will be bash, the

Bourne-Again Shell● You can tell by looking at the man pages● Commands in /bin will have their own man

page, commands in bash will have info in the bash man page

/boot

● Contains files needed for boot– kernel– Grub menu (good to know if you are dual-

booting)– Lilo boot sector backups

● Contains data that is used before the kernel starts executing user-mode programs

/dev● Kinda-sorta an equivalent to a mashup

of the Device Manager and C:\Windows\System in Windows

● Contains a file describing every device, and these files can send data to each device

● In Linux, everything is a file or a directory

● We mean it. We really mean it.

/dev 2

● hda1, hda2, etc. are partitions on the first physical IDE drive

● sda1, sda2, etc. are partitions on the first physical SATA drive

● /dev/cdrom is the optical drive● /dev/fd0 is the floppy drive, if you have

one● /dev/dsp is the speaker device

Fun with /dev

● Try this command out:cat /etc/lilo.conf > /dev/dsp

● The sound you will hear is the sound of your /etc/lilo.conf file

● If you don't have /etc/lilo.conf, substitute any other file you do have

● So, /dev/dsp is the speaker, and the speaker is the file /dev/dsp.

More /dev fun

● The most famous device in /dev is the ubiquitous /dev/null

● Anything sent to this device disappears into a black hole, and reappears in another galaxy, much to the puzzlement of the Lizard people of Zorg in the CmfR!&v Galaxy.

/etc● Perhaps the most important to understand● No binaries can be here, per the standard● This is just for configuration files● Examples include /etc/inittab, /etc/fstab,

/etc/passwd, /etc/hosts, /etc/x11,and /etc/opt

/etc 2

● These files are generally text files and can be edited using any text editor: emacs or vi on the command line, or whatever graphical equivalent (e.g. gedit, kate) your desktop offers

/etc/inittab

● Describes what takes place at bootup● Includes the runlevel of the system, and

which processes should be run at each runlevel

● Linux has seven runlevels, from 0-6

/etc/fstab

● Automatically mounts filesystems across multiple drives or partitions, or even from remote systems

● This is where you would place an entry if you added a hard drive

● This file tells the system what drive to access, and where to mount it in your system

/etc/hosts

● This is the famous hosts file, which matches up names with IP addresses

● This is like level 1 DNS. The system looks here first.

● This can be used to block sites by putting their URL in here and mapping it to 127.0.0.1. Great way to get rid of Double-Click.

/etc/passwd

● This is the password file, but it contains more: user name, user password, user ID, group ID, home directory, and shell. It can optionally contain the user's “real name”

● Each user is on its own line● Each user can select the shell they

want to use (most use bash these days)

/etc/opt/● This is a directory for the configuration files

for each system application you install. Each application gets its own subdirectory under /etc/opt/

● System applications are not just for one user, but for anyone logged in to the system

● This is not mandatory, so see the man pages or help for your application

/etc/x11● Configuration directory for x11, which is the

display system for graphical interfaces in Linux

● This can vary with different distros, so again you need to check

● /etc/x11/xorg.conf is the configuration file that lets you specify the resolutions your monitor and graphics card can display, for instance

/home

● This is where the home directories for all of the “ordinary” users are located.

● The exception is root, which has its own home directory, /root/

● Each user gets a directory with their user name: e.g. /home/baracko

● This can contain configuration files for applications that are user-specific

User-specific Configurations

● These files should be in the /home directory, and should begin with a “.” character (i.e. “dot files”)

● If more than one, the files should go into a directory with a name that begins with the “.” character (i.e. “dot directory), and then the files should not begin with a “.”

/home Partition?

● Your home directory is where you would place all of your documents, videos, MP3s, etc.

● It can get fairly large● It is also the stuff you want to back up,

and you don't want to lose● Putting it on its own partition, or even its

own physical drive, is not a bad idea

Reinstalling

● If you have a separate /home partition, you can reinstall (or do a clean upgrade) and still keep not only your data, but many of your file configurations

● Examples are Firefox bookmarks, Thunderbird or Evolution e-mail settings and mail files, etc.

/lib

● This is the location for shared library files that are used by system programs

● Shared library files are equivalent to Windows' “*.dll” files

● The files here are intended to be libraries for programs in /bin and /sbin, i.e. needed to boot the system and run the commands in the root file system

/lib 2

● Also in this directory are kernel modules● Other library locations for other

programs include /usr/lib and /usr/local/lib

● Generally these correspond to where the binary is installed, i.e. a binary in /usr/bin would put a library in /usr/lib, and a binary in /usr/local/bin would put a library in usr/local/lib

/media, /mnt● This is one where there is no general

agreement● Either directory can be a place to mount

removable media (e.g. CD, USB drive, Floppy disk)

● /mnt is the older way, and is still used for temporarily-mounted file systems

● At one time, these devices were often mounted in the root directory, but that is less common today

/media, /mnt 2● Either directory is allowed by the standard● Most current distro versions will mount

these devices automatically● When they are mounted, an entry will go in

the directory● For USB devices, you want to safely

unmount before removing. Current graphical file managers let you do so in the directory (right-click on the device).

/mnt vs. /etc/fstab● The distinction is temporary vs.

permanent● A file system that will be permanently

mounted should get an entry in fstab● A file system mounted temporarily will

get an entry in /mnt when you mount it● Again, everything in Linux is a file or a

directory. If you mount something on the command line, a file is created in /mnt.

/opt

● Intended as a place for “optional” software, i.e. add-on packages that are not part of the default installation

● Intended for use by sysadmins doing something locally, not for software developers creating packages

● Treat this as something deprecated, but possibly still there.

/proc

● Have we mentioned that everything in Linux is a file or a directory?

● Any time a process is created in Linux, a corresponding file goes in here

● Most users have no need to go here, but you're an adult, do what you want

● Gosh, what would happen if you deleted a file here?

/root

● Home directory for the root account● Normally, you don't want to be root, and

you don't want to go here● Give yourself a user account, and use

that for day-to-day stuff. It is much safer that way.

/sbin

● Place for System binaries● One of three such directories

– /sbin– /usr/sbin– /usr/local/sbin

● All three hold utilities used for system administration, and are intended for the root user

/sbin 2● /sbin should have those utilities

essential for booting, restoring, recovering, and/or repairing the system

● Programs used after /usr is known to be mounted (i.e. when there are no problems) go into /usr/sbin

● Locally installed sysadmin programs go in /usr/local/sbin

/tmp● Guess what this one is?● Yes, temporary files are placed here● Assume that anything in this directory

will be deleted whenever the system is booted

● If you want to have your own temporary directory and not lose files at reboot, create one in your home directory, i.e. /home/username/temp

/usr

● Lots of stuff in here● Back in the mists of prehistory, these

were the user directories, equivalent to what are now /home directories

● Now /usr is for shareable data● Not intended for software packages, in

general

/usr/bin

● Contains executable files for many Linux commands

● These are commands that are not part of the core Linux operating system

● They would go in /bin● Examples of commands in here: perl,

python

/usr/include

● General use include files, including header files, for C and C++ programming languages

/usr/lib

● Contains libraries for the C and C++ programming languages

● Object files, libraries, and internal files not intended to be executed directly by users or shell scripts

/usr/local

● For use by System Administrator when installing software locally

● Must not be over-written when system software is updated

● Generally has same subdirectories as /usr

/usr/sbin

● Non-essential standard system binaries, i.e. utilities

● Essential utilities go in /sbin

/usr/share● For read-only architecture independent

data files● Intended to be shared across platforms

(e.g. i386, Alpha, PPC)● Must not change● If the contents are variable, go to /var

e.g. a game file in /usr/share/games must be static. Game play logs go in /var/games.

/usr/share/man

● Primary location for man pages for the system

/usr/src

● Source code is placed here, for reference purposes only

● This includes the source code for the Linux kernel

/var

● This is for files that are expected to be updated and changed

● This includes:– mail directories– print spool– logs– web sites

/var 2

● Because these can be written to constantly, they can grow over time

● On a server, you may want to put /var on its own partition to limit the growth

● This can also prevent the /var directory from bringing down the server by using up all of the drive space.

/var/lock

● Contains lock files● These files prevent two users (or two

programs) from trying to access the same data at the same time

● You may need to delete a lock file from time to time

/var/log

● Contains the log files generated by programs

Fedora Proposal

● You may have noticed a lot of /bin directories in this strcuture

● So did Fedora developers Harald Hoyer and Kay Sievers

● Their proposal is called /usr merge

What is /usr merge?

● Move all executables into /usr/bin● Move all related libraries into either

/usr/lib or /usr/lib64, as needed

Problems with /usr merge

● LSB adheres to FHS● Would break most shell scripts that (for

example) start #!/bin/sh or #!/bin/bash● Would other distros go along? Doe

Fedora care if they do?

Some benefits to /usr merge● Matches what Solaris does already● Simplifies● Easier to run multiple instances of OS

on different machines in a network. This is particularly true with btrfs.