Linux Directory Structure

Linux Directory Structure

Kevin B. O’Brien
Washtenaw 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
Linux Windows

/home C:

Disk
/home/data Partitions D:
/
root
/var E:

•/boot
•/bin F:
•etc.

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:WindowsSystem 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.

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