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<!DOCTYPE HTML PUBLIC "-//Norman Walsh//DTD DocBook HTML 1.0//EN">
<HTML
><HEAD
><TITLE
>Overview of the Directory Tree</TITLE
><META
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><DIV
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><H1
><A
NAME="DIR-TREE-OVERVIEW"
>Chapter 3. Overview of the Directory Tree</A
></H1
><DIV
CLASS="TOC"
><DL
><DT
><B
>Table of Contents</B
></DT
><DT
><A
HREF="c212.html#AEN218"
>Background</A
></DT
><DT
><A
HREF="x267.html"
>The root filesystem</A
></DT
><DT
><A
HREF="x338.html"
>The <TT
CLASS="FILENAME"
>/etc</TT
> directory</A
></DT
><DT
><A
HREF="x463.html"
>The <TT
CLASS="FILENAME"
>/dev</TT
> directory</A
></DT
><DT
><A
HREF="x472.html"
>The <TT
CLASS="FILENAME"
>/usr</TT
> filesystem</A
></DT
><DT
><A
HREF="x531.html"
>The <TT
CLASS="FILENAME"
>/var</TT
> filesystem</A
></DT
><DT
><A
HREF="x595.html"
>The <TT
CLASS="FILENAME"
>/proc</TT
> filesystem</A
></DT
></DL
></DIV
><BLOCKQUOTE
><P
>“ Two days later, there was Pooh, sitting
on his branch, dangling his legs, and there, beside him, were
four pots of honey...” (A.A.\ Milne) </P
></BLOCKQUOTE
><P
>This chapter describes the important parts of a standard
Linux directory tree, based on the FSSTND filesystem
standard. It outlines the normal way of breaking the directory
tree into separate filesystems with different purposes and gives
the motivation behind this particular split. Some alternative
ways of splitting are also described.</P
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="AEN218"
>Background</A
></H1
><P
>This chapter is loosely based on the <I
CLASS="CITETITLE"
>Linux
filesystem standard</I
>, FSSTND, version 1.2 (see
the bibliography), which attempts to set a standard for how
the directory tree in a Linux system is organized. Such a
standard has the advantage that it will be easier to write or
port software for Linux, and to administer Linux machines, since
everything will be in their usual places. There is no authority
behind the standard that forces anyone to comply with it, but it
has got the support of most, if not all, Linux distributions.
It is not a good idea to break with the FSSTND without very
compelling reasons. The FSSTND attempts to follow Unix tradition
and current trends, making Linux systems familiar to those with
experience with other Unix systems, and vice versa.</P
><P
>This chapter is not as detailed as the FSSTND. A system
administrator should also read the FSSTND for a complete
understanding.</P
><P
>This chapter does not explain all files in detail.
The intention is not to describe every file, but to give
an overview of the system from a filesystem point of view.
Further information on each file is available elsewhere in this
manual or the manual pages.</P
><P
>The full directory tree is intended to be breakable
into smaller parts, each on its own disk or partition,
to accomodate to disk size limits and to ease backup
and other system administration. The major parts are the
root, <TT
CLASS="FILENAME"
>/usr</TT
>, <TT
CLASS="FILENAME"
>/var</TT
>, and
<TT
CLASS="FILENAME"
>/home</TT
> filesystems (see
<A
HREF="c212.html#FSTREE"
>Figure 3-1</A
>). Each part has a different purpose.
The directory tree has been designed so that it works well in
a network of Linux machines which may share some parts of the
filesystems over a read-only device (e.g., a CD-ROM), or over
the network with NFS.</P
><DIV
CLASS="FIGURE"
><P
><B
><A
NAME="FSTREE"
>Figure 3-1. Parts of a Unix directory tree. Dashed lines indicate partition limits.</A
></B
></P
><P
><IMG
SRC="fstree.gif"></P
></DIV
><P
>The roles of the different parts of the directory tree are
described below.
<P
></P
><UL
><LI
><P
>The root filesystem is specific for
each machine (it is generally stored on a local disk,
although it could be a ramdisk or network drive as well)
and contains the files that are necessary for booting
the system up, and to bring it up to such a state that
the other filesystems may be mounted. The contents of
the root filesystem will therefore be sufficient for
the single user state. It will also contain tools for
fixing a broken system, and for recovering lost files
from backups.</P
></LI
><LI
><P
> The <TT
CLASS="FILENAME"
>/usr</TT
> filesystem
contains all commands, libraries, manual pages, and
other unchanging files needed during normal operation.
No files in <TT
CLASS="FILENAME"
>/usr</TT
> should be specific
for any given machine, nor should they be modified during
normal use. This allows the files to be shared over
the network, which can be cost-effective since it saves
disk space (there can easily be hundreds of megabytes in
<TT
CLASS="FILENAME"
>/usr</TT
>), and can make administration
easier (only the master <TT
CLASS="FILENAME"
>/usr</TT
> needs to
be changed when updating an application, not each machine
separately). Even if the filesystem is on a local disk,
it could be mounted read-only, to lessen the chance of
filesystem corruption during a crash.</P
></LI
><LI
><P
>The <TT
CLASS="FILENAME"
>/var</TT
>
filesystem contains files that change, such as spool
directories (for mail, news, printers, etc), log
files, formatted manual pages, and temporary files.
Traditionally everything in <TT
CLASS="FILENAME"
>/var</TT
>
has been somewhere below <TT
CLASS="FILENAME"
>/usr</TT
>, but
that made it impossible to mount <TT
CLASS="FILENAME"
>/usr</TT
>
read-only.</P
><P
></P
></LI
><LI
><P
> The <TT
CLASS="FILENAME"
>/home</TT
>
filesystem contains the users' home directories, i.e., all
the real data on the system. Separating home directories
to their own directory tree or filesystem makes backups
easier; the other parts often do not have to be backed
up, or at least not as often (they seldom change).
A big <TT
CLASS="FILENAME"
>/home</TT
> might have to be
broken on several filesystems, which requires adding an
extra naming level below <TT
CLASS="FILENAME"
>/home</TT
>,
e.g., <TT
CLASS="FILENAME"
>/home/students</TT
> and
<TT
CLASS="FILENAME"
>/home/staff</TT
>.</P
></LI
></UL
> </P
><P
>Although the different parts have been called filesystems
above, there is no requirement that they actually be on separate
filesystems. They could easily be kept in a single one if the
system is a small single-user system and the user wants to keep
things simple. The directory tree might also be divided into
filesystems differently, depending on how large the disks are, and
how space is allocated for various purposes. The important part,
though, is that all the standard <I
CLASS="EMPHASIS"
>names</I
>
work; even if, say, <TT
CLASS="FILENAME"
>/var</TT
> and
<TT
CLASS="FILENAME"
>/usr</TT
> are actually on the same
partition, the names <TT
CLASS="FILENAME"
>/usr/lib/libc.a</TT
>
and <TT
CLASS="FILENAME"
>/var/log/messages</TT
> must work, for
example by moving files below <TT
CLASS="FILENAME"
>/var</TT
>
into <TT
CLASS="FILENAME"
>/usr/var</TT
>, and
making <TT
CLASS="FILENAME"
>/var</TT
> a symlink to
<TT
CLASS="FILENAME"
>/usr/var</TT
>.</P
><P
>The Unix filesystem structure groups files according to purpose,
i.e., all commands are in one place, all data files in another,
documentation in a third, and so on. An alternative would be to
group files files according to the program they belong to, i.e.,
all Emacs files would be in one directory, all TeX in another,
and so on. The problem with the latter approach is that it
makes it difficult to share files (the program directory often
contains both static and shareable and changing and
non-shareable files), and sometimes to even find the files
(e.g., manual pages in a huge number of places, and making the
manual page programs find all of them is a maintenance
nightmare).</P
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