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<appendix id="gfdl1.2">
<appendixinfo>
<title>GNU Free Documentation License</title>
<pubdate>Version 1.2, November 2002</pubdate>
<copyright><year>2000,2001,2002</year>
<holder>Free Software Foundation, Inc.</holder></copyright>
<legalnotice id="gfdl-legalnotice">
<para><address>Free Software Foundation, Inc.
<street>59 Temple Place, Suite 330</street>,
<city>Boston</city>,
<state>MA</state>
<postcode>02111-1307</postcode>
<country>USA</country>
</address></para>
<para>Everyone is permitted to copy and distribute verbatim
copies of this license document, but changing it is not
allowed.</para>
</legalnotice>
<releaseinfo>Version 1.2, November 2002</releaseinfo>
</appendixinfo>
<title>GNU Free Documentation License</title>
<section id="gfdl-0"><title>PREAMBLE</title>
<para>The purpose of this License is to make a manual, textbook, or
other functional and useful document "free" in the sense of freedom: to
assure everyone the effective freedom to copy and redistribute it, with
or without modifying it, either commercially or noncommercially.
Secondarily, this License preserves for the author and publisher a way
to get credit for their work, while not being considered responsible for
modifications made by others.</para>
<para>This License is a kind of "copyleft", which means that derivative
works of the document must themselves be free in the same sense. It
complements the GNU General Public License, which is a copyleft license
designed for free software.</para>
<para>We have designed this License in order to use it for manuals for
free software, because free software needs free documentation: a free
program should come with manuals providing the same freedoms that the
software does. But this License is not limited to software manuals; it
can be used for any textual work, regardless of subject matter or
whether it is published as a printed book. We recommend this License
principally for works whose purpose is instruction or reference.</para>
</section>
<section id="gfdl-1"><title>APPLICABILITY AND DEFINITIONS</title>
<para id="gfdl-doc">This License applies to any manual or other work, in
any medium, that contains a notice placed by the copyright holder saying
it can be distributed under the terms of this License. Such a notice
grants a world-wide, royalty-free license, unlimited in duration, to use
that work under the conditions stated herein. The "Document", below,
refers to any such manual or work. Any member of the public is a
licensee, and is addressed as "you". You accept the license if you
copy, modify or distribute the work in a way requiring permission under
copyright law.</para>
<para id="gfdl-mod-ver">A "Modified Version" of the Document means any
work containing the Document or a portion of it, either copied verbatim,
or with modifications and/or translated into another language.</para>
<para id="gfdl-secnd-sect">A "Secondary Section" is a named appendix or
a front-matter section of the Document that deals exclusively with the
relationship of the publishers or authors of the Document to the
Document's overall subject (or to related matters) and contains nothing
that could fall directly within that overall subject. (Thus, if the
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regarding them.</para>
<para id="gfdl-inv-sect">The "Invariant Sections" are certain Secondary
Sections whose titles are designated, as being those of Invariant
Sections, in the notice that says that the Document is released under
this License. If a section does not fit the above definition of
Secondary then it is not allowed to be designated as Invariant. The
Document may contain zero Invariant Sections. If the Document does not
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<para id="gfdl-cov-text">The "Cover Texts" are certain short passages of
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<para id="gfdl-transparent">A "Transparent" copy of the Document means a
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for input to text formatters. A copy made in an otherwise Transparent
file format whose markup, or absence of markup, has been arranged to
thwart or discourage subsequent modification by readers is not
Transparent. An image format is not Transparent if used for any
substantial amount of text. A copy that is not "Transparent" is called
"Opaque".</para>
<para>Examples of suitable formats for Transparent copies include plain
ASCII without markup, Texinfo input format, LaTeX input format, SGML or
XML using a publicly available DTD, and standard-conforming simple HTML,
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transparent image formats include PNG, XCF and JPG. Opaque formats
include proprietary formats that can be read and edited only by
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processing tools are not generally available, and the machine-generated
HTML, PostScript or PDF produced by some word processors for output
purposes only.</para>
<para id="gfdl-title-page">The "Title Page" means, for a printed book,
the title page itself, plus such following pages as are needed to hold,
legibly, the material this License requires to appear in the title page.
For works in formats which do not have any title page as such, "Title
Page" means the text near the most prominent appearance of the work's
title, preceding the beginning of the body of the text.</para>
<para id="gfdl-entitled">A section "Entitled XYZ" means a named subunit
of the Document whose title either is precisely XYZ or contains XYZ in
parentheses following text that translates XYZ in another language.
(Here XYZ stands for a specific section name mentioned below, such as
"Acknowledgements", "Dedications", "Endorsements", or "History".) To
"Preserve the Title" of such a section when you modify the Document
means that it remains a section "Entitled XYZ" according to this
definition.</para>
<para>The Document may include Warranty Disclaimers next to the notice
which states that this License applies to the Document. These Warranty
Disclaimers are considered to be included by reference in this License,
but only as regards disclaiming warranties: any other implication that
these Warranty Disclaimers may have is void and has no effect on the
meaning of this License.</para>
</section>
<section id="gfdl-2"><title>VERBATIM COPYING</title>
<para>You may copy and distribute the Document in any medium, either
commercially or noncommercially, provided that this License, the
copyright notices, and the license notice saying this License applies to
the Document are reproduced in all copies, and that you add no other
conditions whatsoever to those of this License. You may not use
technical measures to obstruct or control the reading or further copying
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compensation in exchange for copies. If you distribute a large enough
number of copies you must also follow the conditions in section 3.
</para>
<para>You may also lend copies, under the same conditions stated above,
and you may publicly display copies.</para>
</section>
<section id="gfdl-3"><title>COPYING IN QUANTITY</title>
<para>If you publish printed copies (or copies in media that commonly
have printed covers) of the Document, numbering more than 100, and the
Document's license notice requires Cover Texts, you must enclose the
copies in covers that carry, clearly and legibly, all these Cover Texts:
Front-Cover Texts on the front cover, and Back-Cover Texts on the back
cover. Both covers must also clearly and legibly identify you as the
publisher of these copies. The front cover must present the full title
with all words of the title equally prominent and visible. You may add
other material on the covers in addition. Copying with changes limited
to the covers, as long as they preserve the title of the Document and
satisfy these conditions, can be treated as verbatim copying in other
respects.</para>
<para>If the required texts for either cover are too voluminous to fit
legibly, you should put the first ones listed (as many as fit
reasonably) on the actual cover, and continue the rest onto adjacent
pages.</para>
<para>If you publish or distribute Opaque copies of the Document
numbering more than 100, you must either include a machine-readable
Transparent copy along with each Opaque copy, or state in or with each
Opaque copy a computer-network location from which the general
network-using public has access to download using public-standard
network protocols a complete Transparent copy of the Document, free of
added material. If you use the latter option, you must take reasonably
prudent steps, when you begin distribution of Opaque copies in quantity,
to ensure that this Transparent copy will remain thus accessible at the
stated location until at least one year after the last time you
distribute an Opaque copy (directly or through your agents or retailers)
of that edition to the public.</para>
<para>It is requested, but not required, that you contact the authors of
the Document well before redistributing any large number of copies, to
give them a chance to provide you with an updated version of the
Document.</para>
</section>
<section id="gfdl-4"><title>MODIFICATIONS</title>
<para>You may copy and distribute a Modified Version of the Document
under the conditions of sections 2 and 3 above, provided that you
release the Modified Version under precisely this License, with the
Modified Version filling the role of the Document, thus licensing
distribution and modification of the Modified Version to whoever
possesses a copy of it. In addition, you must do these things in the
Modified Version:</para>
<orderedlist id="gfdl-modif-cond" numeration="upperalpha">
<title>GNU FDL Modification Conditions</title>
<listitem><simpara>Use in the Title Page (and on the covers, if any) a
title distinct from that of the Document, and from those of previous
versions (which should, if there were any, be listed in the History
section of the Document). You may use the same title as a previous
version if the original publisher of that version gives permission.
</simpara></listitem>
<listitem><simpara>List on the Title Page, as authors, one or more
persons or entities responsible for authorship of the modifications in
the Modified Version, together with at least five of the principal
authors of the Document (all of its principal authors, if it has fewer
than five), unless they release you from this requirement.
</simpara></listitem>
<listitem><simpara>State on the Title page the name of the publisher of
the Modified Version, as the publisher.</simpara></listitem>
<listitem><simpara>Preserve all the copyright notices of the Document.
</simpara></listitem>
<listitem><simpara>Add an appropriate copyright notice for your
modifications adjacent to the other copyright notices.
</simpara></listitem>
<listitem><simpara>Include, immediately after the copyright notices, a
license notice giving the public permission to use the Modified
Version under the terms of this License, in the form shown in the
<link linkend="gfdl-addendum">Addendum</link> below.
</simpara></listitem>
<listitem><simpara>Preserve in that license notice the full lists of
Invariant Sections and required Cover Texts given in the Document's
license notice.</simpara></listitem>
<listitem><simpara>Include an unaltered copy of this License.
</simpara></listitem>
<listitem><simpara>Preserve the section Entitled "History", Preserve its
Title, and add to it an item stating at least the title, year, new
authors, and publisher of the Modified Version as given on the Title
Page. If there is no section Entitled "History" in the Document,
create one stating the title, year, authors, and publisher of the
Document as given on its Title Page, then add an item describing the
Modified Version as stated in the previous sentence.
</simpara></listitem>
<listitem><simpara>Preserve the network location, if any, given in the
Document for public access to a Transparent copy of the Document, and
likewise the network locations given in the Document for previous
versions it was based on. These may be placed in the "History"
section. You may omit a network location for a work that was
published at least four years before the Document itself, or if the
original publisher of the version it refers to gives permission.
</simpara></listitem>
<listitem><simpara>For any section Entitled "Acknowledgements" or
"Dedications", Preserve the Title of the section, and preserve in the
section all the substance and tone of each of the contributor
acknowledgements and/or dedications given therein.
</simpara></listitem>
<listitem><simpara>Preserve all the Invariant Sections of the Document,
unaltered in their text and in their titles. Section numbers or the
equivalent are not considered part of the section titles.
</simpara></listitem>
<listitem><simpara>Delete any section Entitled "Endorsements".
Such a section may not be included in the Modified Version.
</simpara></listitem>
<listitem><simpara>Do not retitle any existing section to be Entitled
"Endorsements" or to conflict in title with any Invariant Section.
</simpara></listitem>
<listitem><simpara>Preserve any Warranty Disclaimers.
</simpara></listitem>
</orderedlist>
<para>If the Modified Version includes new front-matter sections or
appendices that qualify as Secondary Sections and contain no material
copied from the Document, you may at your option designate some or all
of these sections as invariant. To do this, add their titles to the
list of Invariant Sections in the Modified Version's license notice.
These titles must be distinct from any other section titles.</para>
<para>You may add a section Entitled "Endorsements", provided it
contains nothing but endorsements of your Modified Version by various
parties--for example, statements of peer review or that the text has
been approved by an organization as the authoritative definition of a
standard.</para>
<para>You may add a passage of up to five words as a Front-Cover Text,
and a passage of up to 25 words as a Back-Cover Text, to the end of the
list of Cover Texts in the Modified Version. Only one passage of
Front-Cover Text and one of Back-Cover Text may be added by (or through
arrangements made by) any one entity. If the Document already includes
a cover text for the same cover, previously added by you or by
arrangement made by the same entity you are acting on behalf of, you may
not add another; but you may replace the old one, on explicit permission
from the previous publisher that added the old one.</para>
<para>The author(s) and publisher(s) of the Document do not by this
License give permission to use their names for publicity for or to
assert or imply endorsement of any Modified Version.</para>
</section>
<section id="gfdl-5"><title>COMBINING DOCUMENTS</title>
<para>You may combine the Document with other documents released under
this License, under the terms defined in <link linkend="gfdl-4">section
4</link> above for modified versions, provided that you include in the
combination all of the Invariant Sections of all of the original
documents, unmodified, and list them all as Invariant Sections of your
combined work in its license notice, and that you preserve all their
Warranty Disclaimers.</para>
<para>The combined work need only contain one copy of this License, and
multiple identical Invariant Sections may be replaced with a single
copy. If there are multiple Invariant Sections with the same name but
different contents, make the title of each such section unique by adding
at the end of it, in parentheses, the name of the original author or
publisher of that section if known, or else a unique number. Make the
same adjustment to the section titles in the list of Invariant Sections
in the license notice of the combined work.</para>
<para>In the combination, you must combine any sections Entitled
"History" in the various original documents, forming one section
Entitled "History"; likewise combine any sections Entitled
"Acknowledgements", and any sections Entitled "Dedications". You must
delete all sections Entitled "Endorsements".</para>
</section>
<section id="gfdl-6"><title>COLLECTIONS OF DOCUMENTS</title>
<para>You may make a collection consisting of the Document and other
documents released under this License, and replace the individual copies
of this License in the various documents with a single copy that is
included in the collection, provided that you follow the rules of this
License for verbatim copying of each of the documents in all other
respects.</para>
<para>You may extract a single document from such a collection, and
distribute it individually under this License, provided you insert a
copy of this License into the extracted document, and follow this
License in all other respects regarding verbatim copying of that
document.</para>
</section>
<section id="gfdl-7"><title>AGGREGATION WITH INDEPENDENT WORKS</title>
<para>A compilation of the Document or its derivatives with other
separate and independent documents or works, in or on a volume of a
storage or distribution medium, is called an "aggregate" if the
copyright resulting from the compilation is not used to limit the legal
rights of the compilation's users beyond what the individual works
permit. When the Document is included in an aggregate, this License does
not apply to the other works in the aggregate which are not themselves
derivative works of the Document.</para>
<para>If the Cover Text requirement of section 3 is applicable to these
copies of the Document, then if the Document is less than one half of
the entire aggregate, the Document's Cover Texts may be placed on covers
that bracket the Document within the aggregate, or the electronic
equivalent of covers if the Document is in electronic form. Otherwise
they must appear on printed covers that bracket the whole
aggregate.</para>
</section>
<section id="gfdl-8"><title>TRANSLATION</title>
<para>Translation is considered a kind of modification, so you may
distribute translations of the Document under the terms of section 4.
Replacing Invariant Sections with translations requires special
permission from their copyright holders, but you may include
translations of some or all Invariant Sections in addition to the
original versions of these Invariant Sections. You may include a
translation of this License, and all the license notices in the
Document, and any Warranty Disclaimers, provided that you also include
the original English version of this License and the original versions
of those notices and disclaimers. In case of a disagreement between the
translation and the original version of this License or a notice or
disclaimer, the original version will prevail.</para>
<para>If a section in the Document is Entitled "Acknowledgements",
"Dedications", or "History", the requirement (section 4) to Preserve its
Title (section 1) will typically require changing the actual
title.</para>
</section>
<section id="gfdl-9"><title>TERMINATION</title>
<para>You may not copy, modify, sublicense, or distribute the Document
except as expressly provided for under this License. Any other attempt
to copy, modify, sublicense or distribute the Document is void, and will
automatically terminate your rights under this License. However,
parties who have received copies, or rights, from you under this License
will not have their licenses terminated so long as such parties remain
in full compliance.</para>
</section>
<section id="gfdl-10"><title>FUTURE REVISIONS OF THIS LICENSE</title>
<para>The Free Software Foundation may publish new, revised versions of
the GNU Free Documentation License from time to time. Such new versions
will be similar in spirit to the present version, but may differ in
detail to address new problems or concerns. See
http://www.gnu.org/copyleft/.</para>
<para>Each version of the License is given a distinguishing version
number. If the Document specifies that a particular numbered version of
this License "or any later version" applies to it, you have the option
of following the terms and conditions either of that specified version
or of any later version that has been published (not as a draft) by the
Free Software Foundation. If the Document does not specify a version
number of this License, you may choose any version ever published (not
as a draft) by the Free Software Foundation.</para>
</section>
<section id="gfdl-addendum"><title>ADDENDUM: How to use this License for
your documents</title>
<para>To use this License in a document you have written, include a copy
of the License in the document and put the following copyright and
license notices just after the title page:</para>
<blockquote id="copyright-sample">
<title>Sample Invariant Sections list</title>
<para>Copyright (c) YEAR YOUR NAME.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.2
or any later version published by the Free Software Foundation;
with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
A copy of the license is included in the section entitled "GNU
Free Documentation License".
</para></blockquote>
<para>If you have Invariant Sections, Front-Cover Texts and Back-Cover
Texts, replace the "with...Texts." line with this:</para>
<blockquote id="inv-cover-sample">
<title>Sample Invariant Sections list</title>
<para>
with the Invariant Sections being LIST THEIR TITLES, with the
Front-Cover Texts being LIST, and with the Back-Cover Texts being LIST.
</para></blockquote>
<para>If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the
situation.</para>
<para>If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of free
software license, such as the GNU General Public License, to permit
their use in free software.</para>
</section>
</appendix>

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<preface id="preface">
<title>About This Book</title>
<blockquote><para><quote>Only two things are infinite, the universe
and human stupidity, and I'm not sure about the former.</quote>
Albert Einstein</para></blockquote>
<sect1 id="acknowledgements">
<title>Acknowledgments</title>
<sect2 id="acknowledgements-joanna"><title>Joanna's acknowledgments</title>
<para>Many people have helped me with this book, directly or
indirectly. I would like to especially thank Matt Welsh for
inspiration and LDP leadership, Andy Oram for getting me to work
again with much-valued feedback, Olaf Kirch for showing me that it
can be done, and Adam Richter at Yggdrasil and others for showing
me that other people can find it interesting as well.</para>
<para>Stephen Tweedie, H. Peter Anvin, Remy Card, Theodore
Ts'o, and Stephen Tweedie have let me borrow their work (and
thus make the book look thicker and much more impressive):
a comparison between the xia and ext2 filesystems, the device
list and a description of the ext2 filesystem. These aren't
part of the book any more. I am most grateful for this, and
very apologetic for the earlier versions that sometimes lacked
proper attribution.</para>
<para>In addition, I would like to thank Mark Komarinski for
sending his material in 1993 and the many system administration
columns in Linux Journal. They are quite informative and
inspirational.</para>
<para>Many useful comments have been sent by a large number
of people. My miniature black hole of an archive doesn't let
me find all their names, but some of them are, in alphabetical
order: Paul Caprioli, Ales Cepek, Marie-France Declerfayt,
Dave Dobson, Olaf Flebbe, Helmut Geyer, Larry Greenfield and
his father, Stephen Harris, Jyrki Havia, Jim Haynes, York Lam,
Timothy Andrew Lister, Jim Lynch, Michael J. Micek, Jacob Navia,
Dan Poirier, Daniel Quinlan, Jouni K Sepp&auml;nen, Philippe Steindl,
G.B. Stotte. My apologies to anyone I have forgotten.</para>
</sect2>
<sect2 id="acknowledgements-stephen"><title>Stephen's acknowledgments</title>
<para>I would like to thank Lars and Joanna for their hard
work on the guide.</para>
<para>In a guide like this one there are likely to be at least
some minor inaccuracies. And there are almost certainly going to
be sections that become out of date from time to time. If you
notice any of this then please let me know by sending me an email
to: <email>bagpuss@debian.org.NOSPAM</email>. I will take virtually
any form of input (diffs, just plain text, html, whatever), I am
in no way above allowing others to help me maintain such a large
text as this :) </para>
<para>Many thanks to Helen Topping Shaw for getting the red pen out
and making the text far better than it would otherwise have been.
Also thanks are due just for being wonderful.</para>
</sect2>
<sect2 id="acknowledgements-alex">
<title>Alex's Acknowledgments</title>
<para>I would like to thank Lars, Joanna, and Stephen for all the
great work that they have done on this document over the years. I
only hope that my contribution will be worthy of continuing the work
they started.</para>
<para>Like the previous maintainers, I openly welcome any comments,
suggestions, complains, corrections, or any other form of feedback
you may have. This document can only benefit from the suggestions
of those who use it.</para>
<para>There have been many people who have helped me on my journey
through the "Windows-Free" world, the person I feel I need to thank the
most is my first true UN*X mentor, Mike Velasco. Back in a time before
SCO became a "dirty word", Mike helped me on the path of tar's, cpio's,
and many, many man pages. Thanks Mike! You are the 'Sofa King'.<para>
</sect2>
</sect1>
<sect1 id="revision-hist">
<title>Revision History</title>
<para>
<revhistory>
<revision>
<revnumber>0.7</revnumber>
<date>2001-12-03</date>
<authorinitials>SS</authorinitials>
</revision>
<revision>
<revnumber>0.8</revnumber>
<date>2003-11-18</date>
<authorinitials>AW</authorinitials>
<revdescription><orderedlist spacing='compact'>
<listitem><simpara>Added a section on NTP
</simpara></listitem>
<listitem><simpara>Cleaned some SGML
</simpara></listitem>
<listitem><simpara>Added ext3 to the filesystem
section
</simpara></listitem>
</orderedlist></revdescription>
</revision>
<revision>
<revnumber>0.9</revnumber>
<date> </date>
<authorinitials>AW</authorinitials>
<revdescription><orderedlist spacing='compact'>
<listitem><simpara>Cleaned some SGML code, changed
doctype to lds.dsl, and added id tags
</simpara></listitem>
<listitem><simpara>Updated section on filesystem types,
and Filesystem comparison
</simpara></listitem>
<listitem><simpara>Updated partition type section
</simpara></listitem>
<listitem><simpara>Updated section on creating
partitions
</simpara></listitem>
<listitem><simpara>Wrote section on Logical Volume
Manager (LVM)
</simpara></listitem>
<listitem><simpara>Updated section on space allocation
</simpara></listitem>
<listitem><simpara>Added chapter on System Monitoring
</simpara></listitem>
<listitem><simpara>Added more command line utilities
</simpara></listitem>
<listitem><simpara>Verified Device list
</simpara></listitem>
<listitem><simpara>Modified email address for Authors
</simpara></listitem>
<listitem><simpara>Added references to more in-depth
documents where applicable
</simpara></listitem>
<listitem><simpara>Added notes on upcoming sections
</simpara></listitem>
<listitem><simpara>Indexed chapters 1 - 4, & part of 5
</simpara></listitem>
<listitem><simpara>Updated Misc Information throughout
the book
</simpara></listitem>
</orderedlist></revdescription>
</revision>
</revhistory>
</sect1>
<sect1 id="available-versions">
<title>Source and pre-formatted versions available</title>
<para>The source code and other machine readable formats
of this book can be found on the Internet via anonymous FTP at the
Linux Documentation Project home page <ulink
url="http://www.tldp.org/">http://www.tldp.org/</ulink>, or
at the home page of this book at
<ulink url="http://www.draxeman.com/sag.html">
http://www.draxeman/sag.html</ulink>. This book is available in at
least it's SGML source, as well as, HTML and PDF formats. Other
formats may be available.</para>
</sect1>
<sect1 id="typo-conventions">
<title>Typographical Conventions</title>
<para>Throughout this book, I have tried to use uniform
typographical conventions. Hopefully they aid readability. If
you can suggest any improvements please contact me.</para>
<para>Filenames are expressed as:
<filename>/usr/share/doc/foo</filename>.</para>
<para>Command names are expressed as: <command>fsck</command>
<para>Email addresses are expressed as:
<email>user@domain.com</email></para>
<para>URLs are expressed as: <ulink
url="http://www.tldp.org">http://www.tldp.org</ulink>
</para>
<para>I will add to this section as things come up whilst
editing. If you notice anything that should be added then
please let me know.</para>
</sect1>
</preface>

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<chapter id="intro">
<title>Introduction</title>
<blockquote><para><quote>In the beginning, the file was without
form, and void; and emptiness was upon the face of the bits.
And the Fingers of the Author moved upon the face of the
keyboard. And the Author said, Let there be words, and there
were words.</quote></para></blockquote>
<para>The Linux System Administrator's Guide,
describes the system administration aspects of using Linux.
It is intended for people who know next to nothing about system
administration (those saying ``what is it?''), but who have already
mastered at least the basics of normal usage. This manual
doesn't tell you how to install Linux; that is described in the
Installation and Getting Started document. See below for more
information about Linux manuals.</para>
<para>System administration covers all the things that you have to
do to keep a computer system in usable order. It includes
things like backing up files (and restoring them if necessary),
installing new programs, creating accounts for users (and deleting
them when no longer needed), making certain that the filesystem
is not corrupted, and so on. If a computer were, say, a house,
system administration would be called maintenance, and would
include cleaning, fixing broken windows, and other such things.
</para>
<para>The structure of this manual is such that many of the
chapters should be usable independently, so if you need information
about backups, for example, you can read just that chapter. However,
this manual is first and foremost a tutorial and can be read
sequentially or as a whole.</para>
<para>This manual is not intended to be used completely
independently. Plenty of the rest of the Linux documentation is also
important for system administrators. After all, a system
administrator is just a user with special privileges and duties.
Very useful resources are the manual pages, which should always be
consulted when you are not familiar with a command. If you do not
know which command you need, then the <command>apropos</command>
command can be used. Consult its manual page for more details.</para>
<para>While this manual is targeted at Linux, a general principle
has been that it should be useful with other UNIX based operating
systems as well. Unfortunately, since there is so much variance
between different versions of UNIX in general, and in system
administration in particular, there is little hope to cover
all variants. Even covering all possibilities for Linux is
difficult, due to the nature of its development.</para>
<para>There is no one official Linux distribution, so different
people have different setups and many people have a setup they
have built up themselves. This book is not targeted at any
one distribution. Distributions can and do vary considerably.
When possible, differences have been noted and alternatives
given. For a list of distributions <indexterm id="Linux-distro">
<primary>Linux</primary><secondary>Distributions</secondary></indexterm>
and some of their differences see
<ulink url="http://en.wikipedia.org/wiki/Comparison_of_Linux_distributions">
http://en.wikipedia.org/wiki/Comparison_of_Linux_distributions</ulink>.
</para>
<para>In trying to describe how things work, rather than just
listing ``five easy steps'' for each task, there is much information
here that is not necessary for everyone, but those parts are marked
as such and can be skipped if you use a preconfigured system.
Reading everything will, naturally, increase your understanding of
the system and should make using and administering it more
productive.</para>
<para>Understanding is the key to success with Linux. This book
could just provide recipes, but what would you do when confronted by
a problem this book had no recipe for? If the book can provide
understanding, then recipes are not required. The answers will be self
evident.</para>
<para>Like all other Linux related development, the work
to write this manual was done on a volunteer basis: I did it because
I thought it might be fun and because I felt it should be done.
However, like all volunteer work, there is a limit to how much time,
knowledge and experience people have. This means that the manual is
not necessarily as good as it would be if a wizard had been paid
handsomely to write it
and had spent millennia to perfect it. Be warned.</para>
<para>One particular point where corners have been cut is that
many things that are already well documented in other freely
available manuals are not always covered here. This applies
especially to program specific documentation, such as all the
details of using <command>mkfs</command>. Only the purpose of the
program and as much of its usage as is necessary for the purposes of
this manual is described. For further information, consult these
other manuals. Usually, all of the referred to documentation is
part of the full Linux
documentation set.</para>
<sect1 id="GNU-or-not">
<title>Linux or GNU/Linux, that is the question.</title>
<para>Many people feel that Linux should really be called GNU/Linux.
<indexterm id="ch01-GNU-or-not"><primary>Linux</primary><secondary>GNU
</secondary></indexterm>
This is because Linux is only the kernel, not the applications that run
on it. Most of the basic command line utilities were written by the
Free Software Foundation while developing their GNU operating system.
Among those utilities are some of the most basic commands like cp, mv
lsof, and dd.</para>
<para>In a nutshell, what happened was, the FSF <indexterm id="fsf">
<primary>Free Software Foundation</primary></indexterm>started developing GNU
by writing things like compliers, C libraries, and basic command line
utilities before the kernel. Linus Torvalds, started Linux by writing
the Linux kernel first and using applications written for GNU.</para>
<para>I do not feel that this is the proper forum to debate what name
people should use when referring to Linux. I mention it here, because
I feel it is important to understand the relationship between GNU and
Linux, and to also explain why some Linux is sometimes referred to as
GNU/Linux. The document will be simply referring to it as Linux.
</para>
<para>GNU's side of the issue is discussed on their website:</para>
<para>The relationship -
<ulink url="http://www.gnu.org/gnu/linux-and-gnu.html">
http://www.gnu.org/gnu/linux-and-gnu.html</ulink></para>
<para>Why Linux should be GNU/Linux - <ulink
url="http://www.gnu.org/gnu/why-gnu-linux.html">
http://www.gnu.org/gnu/why-gnu-linux.html</ulink></para>
<para>GNU/Linux FAQ's - <ulink
url="http://www.gnu.org/gnu/gnu-linux-faq.html">
http://www.gnu.org/gnu/gnu-linux-faq.html</ulink></para>
<para>Here are some Alternate views:</para>
<para><ulink url="http://librenix.com/?inode=2312">
http://librenix.com/?inode=2312</ulink></para>
<para><ulink url="http://www.topology.org/linux/lingl.html">
http://www.topology.org/linux/lingl.html</ulink></para>
<para><ulink url="http://atulchitnis.net/writings/gnulinux.php">
http://atulchitnis.net/writings/gnulinux.php</ulink></para>
</sect1>
<sect1>
<title>Trademarks</title>
<para>Microsoft, Windows, Windows NT, Windows 2000, and Windows XP
are trademarks and/or registered trademarks of Microsoft Corporation.
</para>
<para>Red Hat is a trademark of Red Hat, Inc., in the United States
and other countries.</para>
<para>SuSE is a trademark of Novell.</para>
<para>Linux is a registered trademark of Linus Torvalds.</para>
<para>UNIX is a registered trademark in the United States and other
countries, licensed exclusively through X/Open Company Ltd.</para>
<para>GNU is a registered trademark of the Free Software Foundation.
</para>
<para>Other product names mentioned herein may be trademarks and/or
registered trademarks of their respective companies. </para>
</chapter>

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<chapter id="overview">
<title>Overview of a Linux System</title>
<blockquote><para><quote>God saw everything that he
had made, and saw that it was very good. </quote> -- Bible
King James Version. Genesis 1:31</para></blockquote>
<para>This chapter gives an overview of a Linux system. First,
the major services provided by the operating system are described.
Then, the programs that implement these services are described
with a considerable lack of detail. The purpose of this chapter
is to give an understanding of the system as a whole, so that
each part is described in detail elsewhere.</para>
<sect1 id="various-parts">
<title>Various parts of an operating system</title>
<para>UNIX and 'UNIX-like' operating systems (such as Linux) consist
of a <glossterm>kernel</glossterm> and some
<glossterm>system programs</glossterm>. There are also some
<glossterm>application programs</glossterm> for doing work.
The kernel <indexterm id="ch02-kern-over"><primary>kernel</primary>
<secondary>overview</secondary></indexterm>is the heart of the operating
system. In fact, it is often mistakenly considered to be the
operating system itself, but it is not. An operating system provides
provides many more services than a plain kernel.</para>
<para>It keeps track of files on the disk, starts programs and runs
them concurrently, assigns memory and other resources to various
processes, receives packets from and sends packets to the network,
and so on. The kernel does very little by itself, but it provides
tools with which all services can be built. It also prevents anyone
from accessing the hardware directly, forcing everyone to use the
tools it provides.
This way the kernel provides some protection for users from each
other. The tools provided by the kernel are used via
<glossterm>system calls<glossterm>. See manual page section 2 for more
information on these. </para>
<para>The system programs use the tools provided by the kernel to
implement the various services required from an operating system.
System programs, and all other programs, run `on top of the
kernel', in what is called the <glossterm>user mode</glossterm>.
The difference between system and application programs is
one of intent: applications are intended for getting useful
things done (or for playing, if it happens to be a game),
whereas system programs are needed to get the system working.
A word processor is an application; <command>mount</command>
is a system program. The difference is often somewhat blurry,
however, and is important only to compulsive categorizers.</para>
<para>An operating system can also contain compilers and their
corresponding libraries (GCC and the C library in particular under
Linux), although not all programming languages need be part of
the operating system. Documentation, and sometimes even games,
can also be part of it. Traditionally, the operating system has
been defined by the contents of the installation tape or disks;
with Linux it is not as clear since it is spread all over the
FTP sites of the world.</para>
</sect1>
<sect1 id="kernel-parts">
<title>Important parts of the kernel</title>
<para>The Linux kernel <indexterm id="ch02-kern-over2"><primary>kernel
</primary><secondary>overview</secondary></indexterm> consists of several important
parts: process
management, memory management, hardware device drivers, filesystem
drivers, network management, and various other bits and pieces.
<xref linkend="kerneloverview">
shows some of them.</para>
<figure id="kerneloverview" float="1">
<title>Some of the more important parts of the Linux kernel</title>
<graphic fileref="overview-kernel.png">
</figure>
<para>Probably the most important parts of the kernel (nothing else
works without them) are memory management and
process management. Memory management <indexterm id="kern-mem">
<primary>kernel</primary><secondary>memory management</secondary>
</indexterm> takes care of assigning
memory areas and swap space areas to processes, parts of the
kernel, and for the buffer cache. Process management
<indexterm id="kern-proc-mgt"><primary>kernel</primary><secondary>
process management</secondary></indexterm> creates
processes, and implements multitasking by switching the
active process on the processor.</para>
<para>At the lowest level, the kernel contains a hardware device
driver <indexterm id="hardware-driver"><primary>kernel</primary>
<secondary>driver</secondary></indexterm>for each kind of hardware
it supports. Since the world is
full of different kinds of hardware, the number of hardware device
drivers is large. There are often many otherwise similar pieces
of hardware that differ in how they are controlled by software.
The similarities make it possible to have general classes of
drivers that support similar operations; each member of the class
has the same interface to the rest of the kernel but differs in
what it needs to do to implement them. For example, all disk
drivers look alike to the rest of the kernel, i.e., they all
have operations like `initialize the drive', `read sector N',
and `write sector N'.</para>
<para>Some software services provided by the kernel itself have
similar properties, and can therefore be abstracted into classes.
For example, the various network protocols have been abstracted
into one programming interface, the BSD socket library. Another
example is the <glossterm>virtual filesystem</glossterm>
<indexterm id="VFS"><primary>kernel</primary><secondary>virtual
filesystem (VFS)</secondary></indexterm> (VFS)
layer that abstracts the filesystem operations away from their
implementation. Each filesystem type provides an implementation
of each filesystem operation. When some entity tries to use
a filesystem, the request goes via the VFS, which routes the
request to the proper filesystem driver.</para>
<para>A more in-depth discussion of kernel internals can be found
at <ulink url="http://www.tldp.org/LDP/lki/index.html">
http://www.tldp.org/LDP/lki/index.html</ulink>. This document was
written for the 2.4 kernel. When I find one for the 2.6 kernel, I
will list it here.</para>
</sect1>
<sect1 id="major-services">
<title>Major services in a UNIX system</title>
<para>This section describes some of the more important UNIX
services, but without much detail. They are described more
thoroughly in later chapters.</para>
<sect2 id="init">
<title><command>init</command></title>
<para>The single most important service in a UNIX system is
provided by <command>init</command><indexterm id="ch02-init">
<primary>init</primary></indexterm> <command>init</command>
is started as the first process of every UNIX system, as the last
thing the kernel does when it boots. When <command>init</command>
starts, it continues the boot process by doing various startup
chores (checking and mounting filesystems, starting daemons,
etc).</para>
<para>The exact list of things that <command>init</command>
does depends on which flavor it is; there are several to choose
from. <command>init</command> usually provides the concept of
<glossterm>single user mode</glossterm><indexterm id="ch02-single-user">
<primary>runlevels</primary><secondary>1 - single user
</secondary>, in which no one can
log in and root uses a shell at the console; the usual mode is
called <glossterm>multiuser mode</glossterm><indexterm id="multi-user">
<primary>runlevels</primary><secondary>3 - multi-user</secondary>
</indexterm>. Some flavors
generalize this as <glossterm>run levels</glossterm>; single
and multiuser modes are considered to be two run levels, and
there can be additional ones as well, for example, to run X on
the console.</para>
<para>Linux allows for up to 10
<glossterm>runlevels</glossterm><indexterm id="rlevels">
<primary>runlevels</primary></indexterm>, 0-9, but usually only some of
these are defined by default. Runlevel 0 <indexterm id="rlevel0">
<primary>runlevels</primary><secondary>0 - shutdown</secondary>
</indexterm>is defined as ``system
halt''. Runlevel 1 <indexterm id="rlevel1">
<primary>runlevels</primary><secondary>1 - single-user</secondary>
</indexterm>is defined as ``single user mode''. Runlevel 3
<indexterm id="rlevel3"><primary>runlevels</primary>
<secondary>3 - multi-user</secondary></indexterm> is defined as
"multi user" because it is the runlevel that the system boot into
under normal day to day conditions. Runlevel 5 <indexterm id="rlevel5">
<primary>runlevels</primary><secondary>5 - multi-user with GUI
</secondary> is typically the same as 3 except that a GUI
<indexterm id="ch02-xwindows"><primary>GUI</primary></indexterm>
gets started also.
Runlevel 6 <indexterm id="rlevel6"><primary>runlevels</primary>
<secondary>6 - reboot</secondary></indexterm>is defined as ``system
reboot''. Other runlevels are
dependent on how your particular distribution has defined them,
and they vary significantly between distributions. Looking at
the contents of <filename>/etc/inittab</filename>
<indexterm id="ch02-inittab"><primary>inittab</primary></indexterm>
usually will <indexterm id="ch02-rlevel-inittab"><primary>runlevels
</primary><secondary>inittab</secondary></indexterm>
give some hint what the predefined runlevels are and what they
have been defined as.</para>
<para>In normal operation, <command>init</command>
<indexterm id="ch02-init2"><primary>commands</primary>
<secondary>init</secondary></indexterm> makes
sure <command>getty</command><indexterm id="ch02-getty">
<primary>commands</primary><secondary>getty</secondary></indexterm>
is working (to allow users to log in)
and to adopt orphan processes (processes whose parent has died; in
UNIX <emphasis>all</emphasis> processes <emphasis>must</emphasis>
be in a single tree, so orphans must be adopted).</para>
<para>When the system is shut down, it is <command>init</command>
that is in charge of killing all other processes, unmounting all
filesystems and stopping the processor, along with anything else
it has been configured to do.</para>
</sect2>
<sect2 id="terminal-logins">
<title>Logins from terminals</title>
<para>Logins from terminals (via serial lines) and the console
(when not running X) are provided by the <command>getty</command>
<indexterm id="ch02-getty2"><primary>getty</primary></indexterm>
program. <command>init</command><indexterm id="ch02-init3">
<primary>init</primary></indexterm> starts a separate instance of
<command>getty</command> for each terminal upon which logins are to
be allowed. <command>getty</command> reads the username and runs
the <command>login</command><indexterm id="ch02-login">
<primary>login</primary></indexterm>program, which reads the password.
If the username and password are correct, <command>login</command> runs
the shell. When the shell terminates, i.e., the user logs out, or
when <command>login</command> terminated because the username and
password didn't match, <command>init</command> notices this and
starts a new instance of <command>getty</command>. The kernel has no
notion of logins, this is all handled by the
<glossterm>system programs</glossterm>.</para>
</sect2>
<sect2 id="syslog">
<title>Syslog</title>
<para>The kernel and many <glossterm>system programs</glossterm>
produce error, warning, and other messages. It is often important
that these messages can be viewed later, even much later, so they
should be written to a file. The program doing this is
<command>syslog</command> <indexterm id="ch02-syslog"><primary>syslog
</primary></indexterm>. It can be configured to sort the
messages to different files according to writer or degree of
importance. For example, kernel messages are often directed to a
separate file from the others, since kernel messages are often more
important and need to be read
regularly to spot problems.</para>
<para><xref linkend="system-logs"> will provide more on
this.</para>
</sect2>
<sect2 id="cron">
<title>Periodic command execution: <command>cron</command> and
<command>at</command></title>
<para>Both users and system administrators often need
to run commands periodically. For example, the system administrator
might want to run a command to clean the directories with temporary
files (<filename>/tmp</filename> and <filename>/var/tmp</filename>)
from old files, to keep the disks from filling up, since not all
programs clean up after
themselves correctly.</para>
<para>The <command>cron</command> <indexterm id="ch02-cron">
<primary>cron</primary></indexterm> service is set up to do this.
Each user can have a <filename>crontab</filename>
<indexterm id="ch02-crontab"><primary>cron</primary><secondary>crontab
</secondary></indexterm> file, where she
lists the commands she wishes to execute and the times they should
be executed. The <command>cron</command> daemon takes care of
starting the commands when specified.</para>
<para>The <command>at</command> <indexterm id="ch02-at"><primary>at
</primary></indexterm> service is similar to
<command>cron</command>, but it is once only: the command is
executed at the given time, but it is not repeated.</para>
<para>We will go more into this later. See the manual pages
cron(1), crontab(1), crontab(5), at(1) and atd(8) for more in
depth information.</para>
<para><xref linkend="task-automation"> will cover this.</para>
</sect2>
<sect2 id="gui">
<title>Graphical user interface</title>
<para><indexterm id="ch02-gui2"><primary>GUI</primary></indexterm>
UNIX and Linux don't incorporate the user interface
into the kernel; instead, they let it be implemented by user level
programs. This applies for both text mode and graphical
environments.</para>
<para>This arrangement makes the system more flexible, but has
the disadvantage that it is simple to implement a different user
interface for each program, making the system harder to
learn.</para>
<para>The graphical environment primarily used with Linux
is called the X Window System <indexterm id="ch02-xwin"><primary>GUI
</primary><secondary>X Windows</secondary></indexterm> (X for short).
X also does not implement a user interface; it only implements a
window system, i.e., tools with which a graphical user interface can
be implemented. Some popular window managers are: fvwm <indexterm id="fvwm">
<primary>GUI</primary><secondary>fvwm</secondary></indexterm>, icewm
<indexterm id="icewm"><primary>GUI</primary><secondary>icewm</secondary>
</indexterm>, blackbox <indexterm id="blackbox"><primary>GUI</primary>
<secondary>blackbox</secondary></indexterm>, and windowmaker
<indexterm id="windowmaker"><primary>GUI</primary><secondary>windowmaker
</secondary></indexterm>. There are also two popular desktop managers,
KDE <indexterm id="ch02-kde"><primary>KDE</primary></indexterm> and Gnome.
<indexterm id="ch02-gnome"><primary>GNOME</primary></indexterm></para>
</sect2>
<sect2 id="networking">
<title>Networking</title>
<para>Networking <indexterm id="ch02-net"><primary>networking</primary>
</indexterm>is the act of connecting two or more computers
so that they can communicate with each other. The actual methods
of connecting and communicating are slightly complicated, but
the end result is very useful.</para>
<para>UNIX operating systems have many networking features.
Most basic services (filesystems, printing, backups, etc) can
be done over the network. This can make system administration
easier, since it allows centralized administration, while
still reaping in the benefits of microcomputing and distributed
computing, such as lower costs and better fault tolerance.</para>
<para>However, this book merely glances at networking; see the
<citetitle>Linux Network Administrators' Guide</citetitle>
<ulink url="http://www.tldp.org/LDP/nag2/index.html">
http://www.tldp.org/LDP/nag2/index.html</ulink>
<indexterm id="ch02-nag"><primary>networking</primary><secondary>Network
Admin Guide (NAG)</secondary></indexterm> for
more information, including a basic description of how networks
operate.</para>
</sect2>
<sect2 id="network-logins">
<title>Network logins</title>
<para>Network logins<indexterm id="logging-in">
<primary>logging in</primary></indexterm> work a little differently
than normal logins. For each person logging in via the network
there is a separate virtual network connection,
and there can be any number of these depending on the available
bandwidth. It is therefore not possible to run a separate
<command>getty</command><indexterm id="ch02-getty3"><primary>getty
</primary></indexterm> for each possible virtual connection.
There are also several different ways to log in via a network,
<command>telnet</command><indexterm id="ch02-telnet"><primary>telnet
</primary></indexterm> and <command>ssh</command>
<indexterm id="ch02-ssh"><primary>ssh</primary></indexterm> being
the major ones in TCP/IP networks.</para>
<para>These days many Linux system administrators consider
<command>telnet</command> and <command>rlogin</command> to be
insecure and prefer <command>ssh</command>, the ``secure shell'',
which encrypts traffic going over the network, thereby making it far
less likely that the malicious can ``sniff'' your connection and gain
sensitive data like usernames and passwords. It is highly recommended
you use <command>ssh</command> rather than <command>telnet</command>
or <command>rlogin</command>.</para>
<para>Network logins<indexterm id="logging-in2">
<primary>Logging in</primary></indexterm> have, instead of a herd
of <command>getty</command>s<indexterm id="ch02-getty4">
<primary>getty</primary></indexterm>, a single daemon per way
of logging in
(<command>telnet</command><indexterm id="ch02-telnet2">
<primary>telnet</primary></indexterm> and <command>ssh</command>
<indexterm id="ch02-ssh2"><primary>ssh</primary></indexterm> have
separate daemons) that listens for all incoming login attempts.
When it notices one, it starts a new instance of itself to
handle that single attempt; the original instance continues to
listen for other attempts. The new instance works similarly
to <command>getty</command>.</para>
</sect2>
<sect2 id="nfs">
<title>Network file systems</title>
<para>One of the more useful things that can be done with
networking services is sharing files via a <glossterm>network
file system</glossterm>. Depending on your network this could
be done over the Network File System (NFS)<indexterm id="ch02-nfs">
<primary>Network File System (NFS)</primary></indexterm>, or over
the Common Internet File System (CIFS)<indexterm id="ch02-cifs">
<primary>Common Internet File System (CIFS)</primary></indexterm>.
NFS is typically a 'UNIX' based service. In Linux, NFS is supported
by the kernel<indexterm id="ch02-kern-nfs"><primary>kernel</primary>
<secondary>NFS</secondary></indexterm>. CIFS however is not. In Linux,
CIFS is supported by Samba<indexterm id="ch02-samba"><primary>Samba
</primary></indexterm> <ulink url="http://www.samba.org">
http://www.samba.org</ulink>.
<para>With a network file system any file operations done by
a program on one machine are sent over the network to another
computer. This fools the program to think that all the files
on the other computer are actually on the computer the program
is running on. This makes information sharing extremely simple,
since it requires no modifications to programs.</para>
<para>This will be covered in more detail in
<xref linkend="net-attached">.</para>
</sect2>
<sect2 id="mail">
<title>Mail</title>
<para>Electronic mail<indexterm id="ch02-email">
<primary>email</primary></indexterm> is the most popularly used method for
communicating via computer. An electronic letter is stored in a
file using a special format, and special mail programs are used
to send and read the letters.</para>
<para>Each user has an <glossterm>incoming mailbox</glossterm>
(a file in the special format), where all new mail is stored.
When someone sends mail, the mail program locates the receiver's
mailbox and appends the letter to the mailbox file. If the
receiver's mailbox is in another machine, the letter is sent to
the other machine, which delivers it to the mailbox as it best
sees fit.</para>
<para>The mail system consists of many programs. The
delivery of mail to local or remote mailboxes is done by one
program (the <glossterm>mail transfer agent</glossterm> (MTA)
<indexterm id="ch02-email2"><primary>mail transfer agent (MTA)
</primary></indexterm>, e.g., <command>sendmail</command>
<indexterm id="ch02-email3"><primary>mail transfer agent (MTA)
</primary><secondary>sendmail</secondary></indexterm>
or <command>postfix</command><indexterm id="ch02-email4">
<primary>mail transfer agent (MTA)</primary>
<secondary>postfix</secondary></indexterm>
), while the programs users use are many and varied
(<glossterm>mail user agent</glossterm> (MUA)
<indexterm id="ch02-email5"><primary>mail user agent</primary>
</indexterm>, e.g., <command>pine</command>
<indexterm id="ch02-email6"><primary>mail user agent</primary>
<secondary>pine</secondary></indexterm>, or
<command>evolution</command>
<indexterm id="ch02-email7"><primary>mail user agent</primary>
<secondary>evolution</secondary></indexterm>.
The mailboxes are usually stored
in <filename>/var/spool/mail</filename> until the user's MUA
retrieves them.</para>
<para>For more information on setting up and running mail services
you can read the Mail Administrator HOWTO at
<ulink url="http://www.tldp.org/HOWTO/Mail-Administrator-HOWTO.html">
http://www.tldp.org/HOWTO/Mail-Administrator-HOWTO.html</ulink>, or
visit the sendmail or postfix's website.
<ulink url="http://www.sendmail.org/">http://www.sendmail.org/</ulink>,
or <ulink url="http://www.postfix.org/">http://www.postfix.org/
</ulink>.</para>
</sect2>
<sect2 id="printing">
<title>Printing</title>
<para>Only one person can use a printer<indexterm id="ch02-print">
<primary>printing</primary></indexterm> at one time, but it is
uneconomical not to share printers between users. The printer is
therefore managed by software that implements a <glossterm>print
queue</glossterm><indexterm id="ch02-print2"><primary>printing
</primary><secondary>queue</secondary></indexterm>: all print
jobs are put into a queue and
whenever the printer is done with one job, the next one is sent
to it automatically. This relieves the users from organizing
the print queue and fighting over control of the printer.
Instead, they form a new queue <emphasis>at</emphasis>
the printer, waiting for their printouts, since no one ever seems to
be able to get the queue software to know exactly when anyone's printout
is really finished. This is a great boost to intra-office social
relations.</para>
<para>The print queue software also <glossterm>spools</glossterm>
<indexterm id="ch02-print3"><primary>printing</primary>
<secondary>spools</secondary></indexterm> the printouts on
disk, i.e., the text is kept in a file while
the job is in the queue. This allows an application program
to spit out the print jobs quickly to the print queue software;
the application does not have to wait until the job is actually
printed to continue. This is really convenient, since it
allows one to print out one version, and not have to wait for
it to be printed before one can make a completely revised new
version.</para>
<para>You can refer to the Printing-HOWTO located at
<ulink url="http://www.tldp.org/HOWTO/Printing-HOWTO/index.html">
http://www.tldp.org/HOWTO/Printing-HOWTO/index.html</ulink>
for more help in setting up printers.</para>
</sect2>
<sect2 id="fs-layout">
<title>The filesystem layout</title>
<para>The filesystem<indexterm id="ch02-filesystem">
<primary>filesystem</primary></indexterm> is divided into many parts;
usually along the lines of a root filesystem with
<filename>/bin</filename>
<indexterm id="ch02-filesystem2"><primary>filesystem</primary>
<secondary>/bin</secondary></indexterm>,
<filename>/lib</filename>
<indexterm id="ch02-filesystem3"><primary>filesystem</primary>
<secondary>/lib</secondary></indexterm>,
<filename>/etc</filename>
<indexterm id="ch02-filesystem4"><primary>filesystem</primary>
<secondary>/etc</secondary></indexterm>,
<filename>/dev</filename>
<indexterm id="ch02-filesystem5"><primary>filesystem</primary>
<secondary>/dev</secondary></indexterm>, and a few others;
a <filename>/usr</filename>
<indexterm id="ch02-filesystem6"><primary>filesystem</primary>
<secondary>/usr</secondary></indexterm> filesystem with
programs and unchanging data;
<filename>/var</filename>
<indexterm id="ch02-filesystem7"><primary>filesystem</primary>
<secondary>/var</secondary></indexterm> filesystem with changing
data (such as log files); and a
<filename>/home</filename>
<indexterm id="ch02-filesystem8"><primary>filesystem</primary>
<secondary>/home</secondary></indexterm> for everyone's personal
files. Depending on the hardware configuration and the decisions
of the system administrator, the division can be different;
it can even be all in one filesystem.</para>
<para><xref linkend="dir-tree-overview"> describes the filesystem
layout in some little detail; the Filesystem Hierarchy Standard
<indexterm id="ch02-fhs"><primary>Filesystem Hierarchy Standard (FHS)
</primary></indexterm>. covers
it in somewhat more detail. This can be found on the web at:
<ulink url="http://www.pathname.com/fhs/">
http://www.pathname.com/fhs/</ulink></para>
</sect2>
</sect1>
</chapter>

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<chapter id="device-list">
<title>Hardware, Devices, and Tools</title>
<blockquote><para><quote>Knowledge speaks, but wisdom listens.</quote>
Jimi Hendrix</para></blockquote>
<para>This chapter gives an overview of what a device file is, and how to
create one. The canonical list of device files is
<filename>/usr/src/linux/Documentation/devices.txt</filename>
<indexterm id="ch04-kern-dev"><primary>kernel</primary><secondary>devices</secondary>
</indexterm> if you have
the Linux kernel source code installed on your system. The devices listed
here are correct as of kernel version 2.6.8.</para>
<sect1 id="hwutils">
<title>Hardware Utilities </title>
<sect2 id="makedev">
<title>The <command>MAKEDEV</command><indexterm id="ch04-makedev">
<primary>commands</primary><secondary>MAKEDEV</secondary>
</indexterm> Script</title>
<para>Most device files will already be created and will be there
ready to use after you install your Linux system. If by some chance
you need to create one which is not provided then you should first
try to use the <command>MAKEDEV</command> script. This script is
usually located in <filename>/dev/MAKEDEV</filename> but might also
have a copy (or a symbolic link) in
<filename>/sbin/MAKEDEV</filename>. If it turns out not to be in
your path then you will need to specify the path to it
explicitly.</para>
<para>In general the command is used as:
<screen>
<prompt>#</prompt> <userinput>/dev/MAKEDEV -v ttyS0</userinput>
<computeroutput>create ttyS0 c 4 64 root:dialout 0660</computeroutput>
</screen>
This will create the device file <filename>/dev/ttyS0</filename>
<indexterm id="ch04-ttys0"><primary>filesystem</primary>
<secondary>/dev</secondary><tertiary>/dev/ttyS0</tertiary></indexterm>
with major node 4 and minor node 64 as a character device with
access permissions 0660 with owner root and group dialout.</para>
<para><filename>ttyS0</filename> is a serial port. The major and
minor node numbers are numbers understood by the kernel. The kernel
refers to hardware devices as numbers, this would be very difficult
for us to remember, so we use filenames. Access permissions of 0660
means read and write permission for the owner (root in this case)
and read and write permission for members of the group (dialout in
this case) with no access for anyone else.</para>
</sect2>
<sect2 id="mknod">
<title>The <command>mknod</command><indexterm id="ch04-mknod">
<primary>commands</primary><secondary>mknod</secondary>
</indexterm> command</title>
<para><command>MAKEDEV</command><indexterm id="ch04-makedev2">
<primary>commands</primary><secondary>MAKEDEV</secondary>
</indexterm> is the preferred way of creating
device files which are not present. However sometimes the
<command>MAKEDEV</command> script will not know about the device
file you wish to create. This is where the <command>mknod</command>
command comes in. In order to use <command>mknod</command> you need
to know the major and minor node numbers for the device you wish to
create. The <filename>devices.txt</filename><indexterm id="kern-doc">
<primary>kernel</primary><secondary>documentation</secondary>
<tertiary>devices.txt</tertiary></indexterm> file in the kernel
source documentation is the canonical source of this
information.</para>
<para>To take an example, let us suppose that our version of the
<command>MAKEDEV</command><indexterm id="ch04-makedev3">
<primary>commands</primary><secondary>MAKEDEV</secondary>
</indexterm> script does not know how to create the
<filename>/dev/ttyS0</filename><indexterm id="ch04-ttys02">
<primary>filesystem</primary><secondary>/dev</secondary>
<tertiary>/dev/ttyS0</tertiary></indexterm> device file. We need
to use <command>mknod</command><indexterm id="ch04-mknod2">
<primary>commands</primary><secondary>mknod</secondary>
</indexterm> to create it. We know from looking at the
<filename>devices.txt</filename><indexterm id="kern-doc2">
<primary>kernel</primary><secondary>documentation</secondary>
<tertiary>devices.txt</tertiary></indexterm> that it should be a character
device with major number 4 and minor number 64. So we now know all
we need to create the file.
<screen>
<prompt>#</prompt> <userinput>mknod /dev/ttyS0 c 4 64</userinput>
<prompt>#</prompt> <userinput>chown root.dialout /dev/ttyS0</userinput>
<prompt>#</prompt> <userinput>chmod 0644 /dev/ttyS0</userinput>
<prompt>#</prompt> <userinput>ls -l /dev/ttyS0</userinput>
<computeroutput>
crw-rw---- 1 root dialout 4, 64 Oct 23 18:23 /dev/ttyS0
</computeroutput>
</screen>
As you can see, many more steps are required to create the file. In
this example you can see the process required however. It is
unlikely in the extreme that the ttyS0 file would not be provided by
the <command>MAKEDEV</command><indexterm id="ch04-makedev4">
<primary>commands</primary><secondary>MAKEDEV</secondary>
</indexterm> script, but it suffices to illustrate
the point.</para>
</sect2>
<sect2 id="lspci">
<title>The <command>lspci</command><indexterm id="ch04-lspci">
<primary>commands</primary><secondary>lspci</secondary>
</indexterm> command</title>
<para>lspci</para>
<para>TO BE ADDED</para>
</sect2>
<sect2 id="lsdev">
<title>The <command>lsdev</command><indexterm id="ch04-lsdev">
<primary>commands</primary><secondary>lsdev</secondary>
</indexterm> command</title>
<para>lsdev</para>
<para>TO BE ADDED</para>
</sect2>
<sect2 id="lsusb">
<title>The <command>lsusb</command><indexterm id="ch04-lsusb">
<primary>commands</primary><secondary>lsusb</secondary>
</indexterm> command</title>
<para>lsusb</para>
<para>TO BE ADDED</para>
</sect2>
<sect2 id="lsraid">
<title>The <command>lsraid</command><indexterm id="ch04-lsraid">
<primary>commands</primary><secondary>lsraid</secondary>
</indexterm> command</title>
<para>lsraid</para>
<para>TO BE ADDED</para>
</sect2>
<sect2 id="hdparm">
<title>The <command>hdparm</command><indexterm id="ch04-hdparm">
<primary>commands</primary><secondary>hdparm</secondary>
</indexterm> command</title>
<para>hdparm</para>
<para>TO BE ADDED</para>
</sect2>
<sect2 id="more-hwresources">
<title>More Hardware Resources</title>
<para>More information on what hardware resources the kernel is using
can be found in the <filename>/proc</filename> directory. Refer to
<xref linkend="proc-fs"> in chapter 3.
</para>
</sect2>
</sect1>
<sect1>
<title>Kernel Modules<indexterm id="ch04-modules"><primary>kernel</primary>
<secondary>modules</secondary></indexterm></title>
<para>This section will discuss kernel modules.</para>
<para>TO BE ADDED</para>
<sect2>
<title>lsmod<indexterm id="ch04-lsmod1"><primary>commands</primary>
<secondary>lsmod</secondary></indexterm><indexterm id="ch04-lsmod2">
<primary>kernel</primary><secondary>modules</secondary>
<tertiary>lsmod</tertiary></indexterm></title>
<para>lsmod</para>
<para>TO BE ADDED</para>
</sect2>
<sect2>
<title>insmod<indexterm id="ch04-insmod1"><primary>commands</primary>
<secondary>insmod</secondary></indexterm><indexterm id="ch04-insmod2">
<primary>kernel</primary><secondary>modules</secondary>
<tertiary>insmod</tertiary></indexterm></title>
<para>insmod</para>
<para>TO BE ADDED</para>
</sect2>
<sect2>
<title>depmod<indexterm id="ch04-depmod1"><primary>commands</primary>
<secondary>depmod</secondary></indexterm><indexterm id="ch04-depmod2">
<primary>kernel</primary><secondary>modules</secondary>
<tertiary>depmod</tertiary></indexterm></title>
<para>depmod</para>
<para>TO BE ADDED</para>
</sect2>
<sect2>
<title>rmmod<indexterm id="ch04-rmmod1"><primary>commands</primary>
<secondary>rmmod</secondary></indexterm><indexterm id="ch04-rmmod2">
<primary>kernel</primary><secondary>modules</secondary>
<tertiary>rmmod</tertiary></indexterm></title>
<para>rmmod</para>
<para>TO BE ADDED</para>
</sect2>
<sect2>
<title>modprobe<indexterm id="ch04-modprobe1"><primary>commands</primary>
<secondary>modprobe</secondary></indexterm><indexterm id="ch04-modprobe2">
<primary>kernel</primary><secondary>modules</secondary>
<tertiary>modprobe</tertiary></indexterm></title>
<para>modprobe</para>
<para>TO BE ADDED</para>
</sect2>
</sect1>
</chapter>

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<chapter id="memory-management">
<title>Memory Management</title>
<blockquote><para><quote>Minnet, jag har tappat mitt minne,
&auml;r jag svensk eller finne, kommer inte ih&aring;g...</quote>
(Bosse &Ouml;sterberg)
</para>
<para>A Swedish drinking song, (rough) translation: ``Memory, I
have lost my memory. Am I Swedish or Finnish? I can't
remember''</para>
</blockquote>
<para> This section describes the Linux memory management
features, i.e., virtual memory and the disk buffer cache.
The purpose and workings and the things the system administrator
needs to take into consideration are described.</para>
<sect1 id="vm-intro">
<title>What is virtual memory?</title>
<para>Linux supports <glossterm>virtual memory</glossterm>, that
is, using a disk as an extension of RAM so that the effective
size of usable memory grows correspondingly. The kernel will
write the contents of a currently unused block of memory to the
hard disk so that the memory can be used for another purpose.
When the original contents are needed again, they are read back
into memory. This is all made completely transparent to the
user; programs running under Linux only see the larger amount of
memory available and don't notice that parts of them reside on
the disk from time to time. Of course, reading and writing the
hard disk is slower (on the order of a thousand times slower)
than using real memory, so the programs don't run as fast.
The part of the hard disk that is used as virtual memory is
called the <glossterm>swap space</glossterm>.</para>
<para>Linux can use either a normal file in the filesystem or a
separate partition for swap space. A swap partition is
faster, but it is easier to change the size of a swap file
(there's no need to repartition the whole hard disk, and
possibly install everything from scratch). When you know how
much swap space you need, you should go for a swap partition,
but if you are uncertain, you can use a swap file first, use
the system for a while so that you can get a feel for how much
swap you need, and then make a swap partition when you're
confident about its size.</para>
<para>You should also know that Linux allows one to use several swap
partitions and/or swap files at the same time. This means
that if you only occasionally need an unusual amount of swap space,
you can set up an extra swap file at such times, instead of
keeping the whole amount allocated all the time.</para>
<para>A note on operating system terminology: computer science
usually distinguishes between swapping (writing the whole process
out to swap space) and paging (writing only fixed size parts,
usually a few kilobytes, at a time). Paging is usually more
efficient, and that's what Linux does, but traditional Linux
terminology talks about swapping anyway.
</para>
</sect1>
<sect1 id="swap-space">
<title>Creating a swap space</title>
<para>A swap file is an ordinary file; it is in no way special
to the kernel. The only thing that matters to the kernel is that it
has no holes, and that it is prepared for use with
<command>mkswap</command>. It must reside on a local disk, however;
it can't reside in a filesystem that has been mounted
over NFS due to implementation reasons.</para>
<para>The bit about holes is important. The swap file reserves
the disk space so that the kernel can quickly swap out a page
without having to go through all the things that are necessary
when allocating a disk sector to a file. The kernel merely
uses any sectors that have already been allocated to the file.
Because a hole in a file means that there are no disk sectors
allocated (for that place in the file), it is not good for the
kernel to try to use them.</para>
<para>One good way to create the swap file without holes is through
the following command:
<screen>
<prompt>$</prompt> <userinput>dd if=/dev/zero of=/extra-swap bs=1024
count=1024</userinput>
<computeroutput>1024+0 records in
1024+0 records out</computeroutput>
<prompt>$</prompt>
</screen>
where <filename>/extra-swap</filename> is the name of the swap
file and the size of is given after the <literal>count=</literal>.
It is best for the size to be a multiple of 4, because the
kernel writes out <glossterm>memory pages</glossterm>, which
are 4 kilobytes in size. If the size is not a multiple of 4,
the last couple of kilobytes may be unused.</para>
<para>A swap partition is also not special in any way. You create
it just like any other partition; the only difference is that
it is used as a raw partition, that is, it will not contain any
filesystem at all. It is a good idea to mark swap partitions
as type 82 (Linux swap); this will the make partition listings
clearer, even though it is not strictly necessary to the
kernel.</para>
<para>After you have created a swap file or a swap partition, you
need to write a signature to its beginning; this contains some
administrative information and is used by the kernel. The
command to do this is <command>mkswap</command>, used like this:
<screen>
<prompt>$</prompt> <userinput>mkswap /extra-swap 1024</userinput>
<computeroutput>Setting up swapspace, size = 1044480
bytes</computeroutput>
<prompt>$</prompt>
</screen>
Note that the swap space is still not in use yet: it exists,
but the kernel does not use it to provide virtual memory.</para>
<para>You should be very careful when using
<command>mkswap</command>, since it does not check that the
file or partition isn't used for anything else. <emphasis>You
can easily overwrite important files and partitions with
<command>mkswap</command>!</emphasis> Fortunately, you should
only need to use <command>mkswap</command> when you install
your system.</para>
<para>The Linux memory manager limits the size of each swap space to
2 GB. You can, however, use up to
8 swap spaces simultaneously, for a total of 16GB.
</para>
</sect1>
<sect1 id="using-swap">
<title>Using a swap space</title>
<para>An initialized swap space is taken into use with
<command>swapon</command>. This command tells the kernel that
the swap space can be used. The path to the swap space is given
as the argument, so to start swapping on a temporary swap file
one might use the following command.
<screen>
<prompt>$</prompt> <userinput>swapon /extra-swap</userinput>
<prompt>$</prompt>
</screen>
Swap spaces can be used automatically by listing them in
the <filename>/etc/fstab</filename> file.
<screen>
/dev/hda8 none swap sw 0 0
/swapfile none swap sw 0 0
</screen>
The startup scripts will run the command <command>swapon
-a</command>, which will start swapping on all the swap
spaces listed in <command>/etc/fstab</command>. Therefore,
the <command>swapon</command> command is usually used only when
extra swap is needed.</para>
<para>You can monitor the use of swap spaces with
<command>free</command>. It will tell the total amount of swap
space used.
<screen>
<prompt>$</prompt> <userinput>free</userinput>
<computeroutput> total used free shared
buffers
Mem: 15152 14896 256 12404 2528
-/+ buffers: 12368 2784
Swap: 32452 6684 25768</computeroutput>
<prompt>$</prompt>
</screen>
The first line of output (<literal>Mem:</literal>) shows the
physical memory. The total column does not show the physical
memory used by the kernel, which is usually about a megabyte.
The used column shows the amount of memory used (the second
line does not count buffers). The free column shows completely
unused memory. The shared column shows the amount of memory
shared by several processes; the more, the merrier. The buffers
column shows the current size of the disk buffer cache.</para>
<para>That last line (<literal>Swap:</literal>) shows similar
information for the swap spaces. If this line is all zeroes,
your swap space is not activated.</para>
<para>The same information is available via
<command>top</command>, or using the proc filesystem in file
<filename>/proc/meminfo</filename>. It is currently difficult
to get information on the use of a specific swap space.</para>
<para>A swap space can be removed from use with
<command>swapoff</command>. It is usually not necessary to do it,
except for temporary swap spaces. Any pages in use in the swap
space are swapped in first; if there is not sufficient physical
memory to hold them, they will then be swapped out (to some other
swap space). If there is not enough virtual memory to hold all
of the pages Linux will start to thrash; after a long while it
should recover, but meanwhile the system is unusable. You should
check (e.g., with <command>free</command>) that there is enough
free memory before removing a swap space from use.</para>
<para>All the swap spaces that are used automatically
with <command>swapon -a</command> can be removed from use
with <command>swapoff -a</command>; it looks at the file
<filename>/etc/fstab</filename> to find what to remove.
Any manually used swap spaces will remain in use.</para>
<para>Sometimes a lot of swap space can be in use even though
there is a lot of free physical memory. This can happen for
instance if at one point there is need to swap, but later a big
process that occupied much of the physical memory terminates
and frees the memory. The swapped-out data is not automatically
swapped in until it is needed, so the physical memory may remain
free for a long time. There is no need to worry about this,
but it can be comforting to know what is happening. </para>
</sect1>
<sect1 id="sharing-swap">
<title>Sharing swap spaces with other operating systems</title>
<para>Virtual memory is built into many operating systems.
Since they each need it only when they are running, i.e., never at
the same time, the swap spaces of all but the currently running
one are being wasted. It would be more efficient for them to
share a single swap space. This is possible, but can require a
bit of hacking. The Tips-HOWTO at
<ulink url="http://www.tldp.org/HOWTO/Tips-HOWTO.html">
http://www.tldp.org/HOWTO/Tips-HOWTO.html</ulink>, which contains
some advice on how to
implement this. </para>
</sect1>
<sect1 id="swap-allocation">
<title>Allocating swap space</title>
<para>Some people will tell you that you should allocate twice as
much swap space as you have physical memory, but this is a bogus
rule. Here's how to do it properly:
<itemizedlist>
<listitem>
<para> Estimate your total memory needs. This is the largest
amount of memory you'll probably need at a time, that is the
sum of the memory requirements of all the programs you want to
run at the same time. This can be done by running at the same
time all the programs you are likely to ever be running at the
same time. </para>
<para>For instance, if you want to run X, you should allocate
about 8 MB for it, gcc wants several megabytes (some
files need an unusually large amount, up to tens of
megabytes, but usually about four should do), and so on.
The kernel will use about a megabyte by itself, and the
usual shells and other small utilities perhaps a few
hundred kilobytes (say a megabyte together). There is
no need to try to be exact, rough estimates are fine,
but you might want to be on the pessimistic side.</para>
<para>Remember that if there are going to be several people
using the system at the same time, they are all going
to consume memory. However, if two people run the same
program at the same time, the total memory consumption
is usually not double, since code pages and shared
libraries exist only once.</para>
<para>The <command>free</command> and <command>ps</command>
commands are useful for estimating the memory needs.
</listitem>
<listitem>
<para>Add some security to the estimate in step 1. This is because
estimates of program sizes will probably be wrong, because
you'll probably forget some programs you want to run, and to
make certain that you have some extra space just in case. A
couple of megabytes should be fine. (It is better to allocate
too much than too little swap space, but there's no need to
over-do it and allocate the whole disk, since unused swap space
is wasted space; see later about adding more swap.) Also,
since it is nicer to deal with even numbers, you can round the
value up to the next full megabyte.</para>
</listitem>
<listitem>
<para>Based on the computations above, you know how much memory
you'll be needing in total. So, in order to allocate swap
space, you just need to subtract the size of your physical
memory from the total memory needed, and you know how much
swap space you need. (On some versions of UNIX, you need to
allocate space for an image of the physical memory as well, so
the amount computed in step 2 is what you need and you shouldn't
do the subtraction.)</para>
</listitem>
<listitem>
<para>If your calculated swap space is very much larger than your
physical memory (more than a couple times larger), you should
probably invest in more physical memory, otherwise performance
will be too low.</para>
</itemizedlist>
<para>It's a good idea to have at least some swap space, even if
your calculations indicate that you need none. Linux uses
swap space somewhat aggressively, so that as much physical
memory as possible can be kept free. Linux will swap out
memory pages that have not been used, even if the memory
is not yet needed for anything. This avoids waiting for
swapping when it is needed: the swapping can be done
earlier, when the disk is otherwise idle.</para>
<para>Swap space can be divided among several disks. This
can sometimes improve performance, depending on the
relative speeds of the disks and the access patterns
of the disks. You might want to experiment with a few
schemes, but be aware that doing the experiments
properly is quite difficult. You should not believe
claims that any one scheme is superior to any other,
since it won't always be true.
</para>
</sect1>
<sect1 id="buffer-cache">
<title>The buffer cache</title>
<para>Reading from a disk
is very slow compared to accessing (real) memory. In addition,
it is common to read the same part of a disk several times
during relatively short periods of time. For example, one
might first read an e-mail message, then read the letter into
an editor when replying to it, then make the mail program read
it again when copying it to a folder. Or, consider how often
the command <command>ls</command> might be run on a system with
many users. By reading the information from disk only once
and then keeping it in memory until no longer needed, one can
speed up all but the first read. This is called <glossterm>disk
buffering</glossterm>, and the memory used for the purpose is
called the <glossterm>buffer cache</glossterm>.</para>
<para>Since memory is, unfortunately, a finite, nay, scarce
resource, the buffer cache usually cannot be big enough (it
can't hold all the data one ever wants to use). When the cache
fills up, the data that has been unused for the longest time
is discarded and the memory thus freed is used for the new
data.</para>
<para>Disk buffering works for writes as well. On the one hand,
data that is written is often soon read again (e.g., a source
code file is saved to a file, then read by the compiler),
so putting data that is written in the cache is a good idea.
On the other hand, by only putting the data into the cache, not
writing it to disk at once, the program that writes runs quicker.
The writes can then be done in the background, without slowing
down the other programs.</para>
<para>Most operating systems have buffer caches (although
they might be called something else), but not all of
them work according to the above principles. Some are
<glossterm>write-through</glossterm>: the data is written to disk
at once (it is kept in the cache as well, of course). The cache
is called <glossterm>write-back</glossterm> if the writes are done
at a later time. Write-back is more efficient than write-through,
but also a bit more prone to errors: if the machine crashes,
or the power is cut at a bad moment, or the floppy is removed
from the disk drive before the data in the cache waiting to be
written gets written, the changes in the cache are usually lost.
This might even mean that the filesystem (if there is one) is
not in full working order, perhaps because the unwritten data
held important changes to the bookkeeping information.</para>
<para>Because of this, you should never turn off the
power without using a proper shutdown procedure
or remove a floppy from the
disk drive until it has been unmounted (if it was mounted)
or after whatever program is using it has signaled that it
is finished and the floppy drive light doesn't shine anymore.
The <command>sync</command> command <glossterm>flushes</glossterm>
the buffer, i.e., forces all unwritten data to be written to disk,
and can be used when one wants to be sure that everything is
safely written. In traditional UNIX systems, there is a program
called <command>update</command> running in the background
which does a <command>sync</command> every 30 seconds, so
it is usually not necessary to use <command>sync</command>.
Linux has an additional daemon, <command>bdflush</command>,
which does a more imperfect sync more frequently to avoid the
sudden freeze due to heavy disk I/O that <command>sync</command>
sometimes causes.</para>
<para>Under Linux, <command>bdflush</command> is started by
<command>update</command>. There is usually no reason to worry
about it, but if <command>bdflush</command> happens to die for
some reason, the kernel will warn about this, and you should
start it by hand (<command>/sbin/update</command>).</para>
<para>The cache does not actually buffer files, but blocks, which
are the smallest units of disk I/O (under Linux, they are usually
1 KB). This way, also directories, super blocks, other filesystem
bookkeeping data, and non-filesystem disks are cached.</para>
<para>The effectiveness of a cache is primarily decided by its
size. A small cache is next to useless: it will hold so little
data that all cached data is flushed from the cache before it
is reused. The critical size depends on how much data is read
and written, and how often the same data is accessed. The only
way to know is to experiment.</para>
<para>If the cache is of a fixed size, it is not very good to have
it too big, either, because that might make the free memory too
small and cause swapping (which is also slow). To make the most
efficient use of real memory, Linux automatically uses all free
RAM for buffer cache, but also automatically makes the cache
smaller when programs need more memory.</para>
<para>Under Linux, you do not need to do anything to make use
of the cache, it happens completely automatically. Except for
following the proper procedures for shutdown and removing
floppies, you do not need to worry about it. </para>
</sect1>
</chapter>

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<chapter id="system-monitoring">
<title>System Monitoring</title>
<blockquote><para><quote>That's Hall Monitor to you!</quote>Spongebob
Squarepants</para></blockquote>
<para>One of the most important responsibilities a system administrator
has, is monitoring their systems. As a system administrator you'll need
the ability to find out what is happening on your system at any given
time. Whether it's the percentage of system's resources currently used,
what commands are being run, or who is logged on. This chapter will cover
how to monitor your system, and in some cases, how to resolve problems
that may arise.</para>
<para>When a performance issue arises, there are 4 main areas to consider:
CPU, Memory, Disk I/O, and Network. The ability to determine where the
bottleneck is can save you a lot of time.</para>
<sect1 id="system-resources">
<title>System Resources</title>
<para>Being able to monitor the performance of your system
is essential. If system resources become to low it can cause a lot of
problems. System resources can be taken up by individual users, or by
services your system may host such as email or web pages. The ability to
know what is happening can help determine whether system upgrades are needed,
or if some services need to be moved to another machine.</para>
<sect2 id="top">
<title>The <command>top</command> command.</title>
<para>The most common of these commands is <command>top</command>.
The <command>top</command> will display a continually updating report
of system resource usage.
<screen>
<prompt>#</prompt> <userinput>top</userinput>
<computeroutput> 12:10:49 up 1 day, 3:47, 7 users, load average: 0.23, 0.19, 0.10
125 processes: 105 sleeping, 2 running, 18 zombie, 0 stopped
CPU states: 5.1% user 1.1% system 0.0% nice 0.0% iowait 93.6% idle
Mem: 512716k av, 506176k used, 6540k free, 0k shrd, 21888k buff
Swap: 1044216k av, 161672k used, 882544k free 199388k cached
PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME CPU COMMAND
2330 root 15 0 161M 70M 2132 S 4.9 14.0 1000m 0 X
2605 weeksa 15 0 8240 6340 3804 S 0.3 1.2 1:12 0 kdeinit
3413 weeksa 15 0 6668 5324 3216 R 0.3 1.0 0:20 0 kdeinit
18734 root 15 0 1192 1192 868 R 0.3 0.2 0:00 0 top
1619 root 15 0 776 608 504 S 0.1 0.1 0:53 0 dhclient
1 root 15 0 480 448 424 S 0.0 0.0 0:03 0 init
2 root 15 0 0 0 0 SW 0.0 0.0 0:00 0 keventd
3 root 15 0 0 0 0 SW 0.0 0.0 0:00 0 kapmd
4 root 35 19 0 0 0 SWN 0.0 0.0 0:00 0 ksoftirqd_CPU0
9 root 25 0 0 0 0 SW 0.0 0.0 0:00 0 bdflush
5 root 15 0 0 0 0 SW 0.0 0.0 0:00 0 kswapd
10 root 15 0 0 0 0 SW 0.0 0.0 0:00 0 kupdated
11 root 25 0 0 0 0 SW 0.0 0.0 0:00 0 mdrecoveryd
15 root 15 0 0 0 0 SW 0.0 0.0 0:01 0 kjournald
81 root 25 0 0 0 0 SW 0.0 0.0 0:00 0 khubd
1188 root 15 0 0 0 0 SW 0.0 0.0 0:00 0 kjournald
1675 root 15 0 604 572 520 S 0.0 0.1 0:00 0 syslogd
1679 root 15 0 428 376 372 S 0.0 0.0 0:00 0 klogd
1707 rpc 15 0 516 440 436 S 0.0 0.0 0:00 0 portmap
1776 root 25 0 476 428 424 S 0.0 0.0 0:00 0 apmd
1813 root 25 0 752 528 524 S 0.0 0.1 0:00 0 sshd
1828 root 25 0 704 548 544 S 0.0 0.1 0:00 0 xinetd
1847 ntp 15 0 2396 2396 2160 S 0.0 0.4 0:00 0 ntpd
1930 root 24 0 76 4 0 S 0.0 0.0 0:00 0 rpc.rquotad
</computeroutput>
</screen></para>
<para>The top portion of the report lists information such as
the system time, uptime, CPU usage, physical ans swap memory usage,
and number of processes. Below that is a list of the processes sorted
by CPU utilization.</para>
<para>You can modify the output of <command>top</command> while
is is running. If you hit an <option>i</option>, top will no longer
display idle processes. Hit <option>i</option> again to see them
again. Hitting <option>M</option> will sort by memory usage,
<option>S</option> will sort by how long they processes have been
running, and <option>P</option> will sort by CPU usage again.</para>
<para>In addition to viewing options, you can also modify processes
from within the <command>top</command> command. You can use
<option>u</option> to view processes owned by a specific user,
<option>k</option> to kill processes, and <option>r</option> to
renice them.</para>
<para>For more in-depth information about processes you can look in
the <filename>/proc</filename> filesystem. In the <filename>/proc</filename>
filesystem you will find a series of sub-directories with numeric names.
These directories are associated with the processes ids of currently
running processes. In each directory you will find a series of files
containing information about the process.</para>
<para>YOU MUST TAKE EXTREME CAUTION TO NOT MODIFY THESE FILES, DOING
SO MAY CAUSE SYSTEM PROBLEMS!</para>
</sect2>
<sect2 id="iostat">
<title>The <command>iostat</command> command.</title>
<para>The <command>iostat</command> will display the current CPU load
average and disk I/O information. This is a great command to monitor
your disk I/O usage.
<screen>
<prompt>#</prompt> <userinput>iostat</userinput>
<computeroutput>Linux 2.4.20-24.9 (myhost) 12/23/2003
avg-cpu: %user %nice %sys %idle
62.09 0.32 2.97 34.62
Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
dev3-0 2.22 15.20 47.16 1546846 4799520
</computeroutput>
</screen>
For 2.4 kernels the devices is names using the device's major
and minor number. In this case the device listed is <filename>
/dev/hda</filename>. To have <command>iostat</command> print this
out for you, use the <option>-x</option>.
<screen>
<prompt>#</prompt> <userinput>iostat -x</userinput>
<computeroutput>Linux 2.4.20-24.9 (myhost) 12/23/2003
avg-cpu: %user %nice %sys %idle
62.01 0.32 2.97 34.71
Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s rkB/s wkB/s avgrq-sz avgqu-sz await svctm %util
/dev/hdc 0.00 0.00 .00 0.00 0.00 0.00 0.00 0.00 0.00 2.35 0.00 0.00 14.71
/dev/hda 1.13 4.50 .81 1.39 15.18 47.14 7.59 23.57 28.24 1.99 63.76 70.48 15.56
/dev/hda1 1.08 3.98 .73 1.27 14.49 42.05 7.25 21.02 28.22 0.44 21.82 4.97 1.00
/dev/hda2 0.00 0.51 .07 0.12 0.55 5.07 0.27 2.54 30.35 0.97 52.67 61.73 2.99
/dev/hda3 0.05 0.01 .02 0.00 0.14 0.02 0.07 0.01 8.51 0.00 12.55 2.95 0.01
</computeroutput>
</screen>
<para>The <command>iostat</command> man page contains a detailed
explanation of what each of these columns mean.</para>
</sect2>
<sect2 id="ps">
<title>The <command>ps</command> command</title>
<para>The <command>ps</command> will provide you a list of
processes currently running. There is a wide variety of options
that this command gives you.</para>
<para>A common use would be to list all processes currently running.
To do this you would use the <command>ps -ef</command> command.
(Screen output from this command is too large to include, the following
is only a partial output.)
<screen>
UID PID PPID C STIME TTY TIME CMD
root 1 0 0 Dec22 ? 00:00:03 init
root 2 1 0 Dec22 ? 00:00:00 [keventd]
root 3 1 0 Dec22 ? 00:00:00 [kapmd]
root 4 1 0 Dec22 ? 00:00:00 [ksoftirqd_CPU0]
root 9 1 0 Dec22 ? 00:00:00 [bdflush]
root 5 1 0 Dec22 ? 00:00:00 [kswapd]
root 6 1 0 Dec22 ? 00:00:00 [kscand/DMA]
root 7 1 0 Dec22 ? 00:01:28 [kscand/Normal]
root 8 1 0 Dec22 ? 00:00:00 [kscand/HighMem]
root 10 1 0 Dec22 ? 00:00:00 [kupdated]
root 11 1 0 Dec22 ? 00:00:00 [mdrecoveryd]
root 15 1 0 Dec22 ? 00:00:01 [kjournald]
root 81 1 0 Dec22 ? 00:00:00 [khubd]
root 1188 1 0 Dec22 ? 00:00:00 [kjournald]
root 1675 1 0 Dec22 ? 00:00:00 syslogd -m 0
root 1679 1 0 Dec22 ? 00:00:00 klogd -x
rpc 1707 1 0 Dec22 ? 00:00:00 portmap
root 1813 1 0 Dec22 ? 00:00:00 /usr/sbin/sshd
ntp 1847 1 0 Dec22 ? 00:00:00 ntpd -U ntp
root 1930 1 0 Dec22 ? 00:00:00 rpc.rquotad
root 1934 1 0 Dec22 ? 00:00:00 [nfsd]
root 1942 1 0 Dec22 ? 00:00:00 [lockd]
root 1943 1 0 Dec22 ? 00:00:00 [rpciod]
root 1949 1 0 Dec22 ? 00:00:00 rpc.mountd
root 1961 1 0 Dec22 ? 00:00:00 /usr/sbin/vsftpd /etc/vsftpd/vsftpd.conf
root 2057 1 0 Dec22 ? 00:00:00 /usr/bin/spamd -d -c -a
root 2066 1 0 Dec22 ? 00:00:00 gpm -t ps/2 -m /dev/psaux
bin 2076 1 0 Dec22 ? 00:00:00 /usr/sbin/cannaserver -syslog -u bin
root 2087 1 0 Dec22 ? 00:00:00 crond
daemon 2195 1 0 Dec22 ? 00:00:00 /usr/sbin/atd
root 2215 1 0 Dec22 ? 00:00:11 /usr/sbin/rcd
weeksa 3414 3413 0 Dec22 pts/1 00:00:00 /bin/bash
weeksa 4342 3413 0 Dec22 pts/2 00:00:00 /bin/bash
weeksa 19121 18668 0 12:58 pts/2 00:00:00 ps -ef
</screen>
</para>
<para>The first column shows who owns the process. The second
column is the process ID. The Third column is the parent process
ID. This is the process that generated, or started, the process.
The forth column is the CPU usage (in
percent). The fifth column is the start time, of date if the process
has been running long enough. The sixth column is the tty associated
with the process, if applicable. The seventh column is the cumulitive
CPU usage (total amount of CPU time is has used while running). The
eighth column is the command itself.</para>
<para>With this information you can see exacly what is running on
your system and kill run-away processes, or those that are causing
problems.</para>
</sect2>
<sect2 id="vmstat">
<title>The <command>vmstat</command> command</title>
<para>The <command>vmstat</command> command will provide a report
showing statistics for system processes, memory, swap,
I/O, and the CPU. These statistics are generated using data from the
last time the command was run to the present. In the case of the
command never being run, the data will be from the last reboot until
the present.<para>
<screen>
<prompt>#</prompt> <userinput>vmstat</userinput>
<computeroutput>
procs memory swap io system cpu
r b w swpd free buff cache si so bi bo in cs us sy id
0 0 0 181604 17000 26296 201120 0 2 8 24 149 9 61 3 36
</computeroutput>
</screen></para>
<para>The following was taken from the
<command>vmstat</command> man page.</para>
<blockquote><para><literallayout>
FIELD DESCRIPTIONS
Procs
r: The number of processes waiting for run time.
b: The number of processes in uninterruptable sleep.
w: The number of processes swapped out but otherwise runnable. This
field is calculated, but Linux never desperation swaps.
Memory
swpd: the amount of virtual memory used (kB).
free: the amount of idle memory (kB).
buff: the amount of memory used as buffers (kB).
Swap
si: Amount of memory swapped in from disk (kB/s).
so: Amount of memory swapped to disk (kB/s).
IO
bi: Blocks sent to a block device (blocks/s).
bo: Blocks received from a block device (blocks/s).
System
in: The number of interrupts per second, including the clock.
cs: The number of context switches per second.
CPU
These are percentages of total CPU time.
us: user time
sy: system time
id: idle time</literallayout>
</para></blockquote>
</sect2>
<sect2 id="lsof">
<title>The <command>lsof</command> command</title>
<para>The <command>lsof</command> command will print out a list of
every file that is in use. Since Linux considers everythihng a file,
this list can be very long. However, this command
can be useful in diagnosing problems. An example of this is if you wish
to unmount a filesystem, but you are being told that it is in use. You
could use this command and <command>grep</command> for the name of the
filesystem to see who is using it.</para>
<para>Or suppose you want to see all files in use by a particular process.
To do this you would use <command>lsof -p -processid-</command>.</para>
</sect2>
<sect2 id="more-utils">
<title>Finding More Utilities</title>
<para>To learn more about what command line tools are available, Chris
Karakas has wrote a reference guide titled <ulink
url="http://www.karakas-online.de/gnu-linux-tools-summary/"> GNU/Linux
Command-Line Tools Summary</ulink>. It's a good resource for learning
what tools are out there and how to do a number of tasks.</para>
</sect2>
<sect1 id="fs-usage">
<title>Filesystem Usage</title>
<para>Many reports are currently talking about how cheap storage has
gotten, but if you're like most of us it isn't cheap enough. Most of
us have a limited amount of space, and need to be able to monitor it
and control how it's used.</para>
<sect2 id="df">
<title>The df command</title>
<para>The <command>df</command> is the simplest tool available to
view disk usage. Simply type in <command>df</command> and you'll
be shown disk usage for all your mounted filesystems in 1K blocks
<screen>
<prompt>user@server:~> <prompt><userinput>df</userinput>
<computeroutput>
Filesystem 1K-blocks Used Available Use% Mounted on
/dev/hda3 5242904 759692 4483212 15% /
tmpfs 127876 8 127868 1% /dev/shm
/dev/hda1 127351 33047 87729 28% /boot
/dev/hda9 10485816 33508 10452308 1% /home
/dev/hda8 5242904 932468 4310436 18% /srv
/dev/hda7 3145816 32964 3112852 2% /tmp
/dev/hda5 5160416 474336 4423928 10% /usr
/dev/hda6 3145816 412132 2733684 14% /var
</computeroutput>
</screen></para>
<para>You can also use the <command>-h</command> to see the output in
"human-readable" format. This will be in K, Megs, or Gigs depending
on the size of the filesystem. Alternately, you can also use the
<command>-B</command> to specify block size.</para>
<para>In addition to space usage, you could use the
<command>-i</command> option to view the number of used and available
inodes.
<screen>
<prompt>user@server:~> </prompt><userinput>df -i</userinput>
<computeroutput>
Filesystem Inodes IUsed IFree IUse% Mounted on
/dev/hda3 0 0 0 - /
tmpfs 31969 5 31964 1% /dev/shm
/dev/hda1 32912 47 32865 1% /boot
/dev/hda9 0 0 0 - /home
/dev/hda8 0 0 0 - /srv
/dev/hda7 0 0 0 - /tmp
/dev/hda5 656640 26651 629989 5% /usr
/dev/hda6 0 0 0 - /var
</computeroutput>
</screen></para>
</sect2>
<sect2 id="du">
<title>The du command</title>
<para>Now that you know how much space has been used on a filesystem
how can you find out where that data is? To view usage by a directory
or file you can use <command>du</command>. Unless you specify a
filename <command>du</command> will act recursively. For example:
<screen>
<prompt>user@server:~> </prompt><userinput>du file.txt</userinput>
<computeroutput>
1300 file.txt
</computeroutput>
</screen>
Or like the <command>df</command> I can use the <command>-h</command>
and get the same output in "human-readable" form.
<screen>
<prompt>user@server:~> </prompt><userinput>du -h file.txt</userinput>
<computeroutput>
1.3M file.txt
</computeroutput>
</screen>
</para>
<para>Unless you specify a filename <command>du</command> will act
recursively.
<screen>
<prompt>user@server:~> </prompt><userinput>du -h /usr/local</userinput>
<computeroutput>
4.0K /usr/local/games
16K /usr/local/include/nessus/net
180K /usr/local/include/nessus
208K /usr/local/include
62M /usr/local/lib/nessus/plugins/.desc
97M /usr/local/lib/nessus/plugins
164K /usr/local/lib/nessus/plugins_factory
97M /usr/local/lib/nessus
12K /usr/local/lib/pkgconfig
2.7M /usr/local/lib/ladspa
104M /usr/local/lib
112K /usr/local/man/man1
4.0K /usr/local/man/man2
4.0K /usr/local/man/man3
4.0K /usr/local/man/man4
16K /usr/local/man/man5
4.0K /usr/local/man/man
</computeroutput>
</screen></para>
<para>If you just want a summary of that directory you can use the
<command>-s</command> option.
<screen>
<prompt>user@server:~> </prompt><userinput>du -hs /usr/local</userinput>
<computeroutput>
210M /usr/local
</computeroutput>
</screen></para>
</sect2>
<sect2 id="quotas">
<title>Quotas</title>
<para>For more information about quotas you can read
<ulink url="http://www.tldp.org/HOWTO/Quota.html">The Quota HOWTO
</ulink>.
</sect2>
</sect1>
<sect1 id="monitoring-users">
<title>Monitoring Users</title>
<blockquote><para>Just because you're paranoid doesn't mean they
AREN'T out to get you... Source Unknown</para></blockquote>
<para>From time to time there are going to be occasions where you will
want to know exactly what people are doing on your system. Maybe you
notice that a lot of RAM is being used, or a lot of CPU activity.
You are going to want to see who is on the system, what they are
running, and what kind of resources they are using.</para>
<sect2 id="who">
<title>The who command</title>
<para>The easiest way to see who is on the system is to do a
<command>who</command> or <command>w</command>. The -->
<command>who</command> is a simple tool that lists out who is logged -->
on the system and what port or terminal they are logged on at.
<screen>
<prompt>user@server:~></prompt> <userinput> who</userinput>
<computeroutput>
bjones pts/0 May 23 09:33
wally pts/3 May 20 11:35
aweeks pts/1 May 22 11:03
aweeks pts/2 May 23 15:04
</computeroutput>
</screen></para>
</sect2>
<sect2 id="ps-u">
<title>The ps command -again!</title>
<para>In the previous section we can see that user aweeks is logged
onto both <filename>pts/1</filename> and <filename>pts/2</filename>,
but what if we want to see what they are doing? We could to a
<command>ps -u aweeks</command> and get the following output
<screen>
<prompt>user@server:~> </prompt><userinput>ps -u aweeks</userinput>
<computeroutput>
20876 pts/1 00:00:00 bash
20904 pts/2 00:00:00 bash
20951 pts/2 00:00:00 ssh
21012 pts/1 00:00:00 ps
</computeroutput>
</screen>
From this we can see that the user is doing a <command>ps</command>
<command>ssh</command>.</para>
<para>This is a much more consolidated use of the
<command>ps</command> than discussed previously.<para>
</sect2>
<sect2 id="w">
<title>The w command</title>
<para>Even easier than using the <command>who</command> and
<command>ps -u</command> commands is to use the <command>w</command>.
<command>w</command> will print out not only who is on the system,
but also the commands they are running.
<screen>
<prompt>user@server:~> </prompt><userinput>w</userinput>
<computeroutput>
aweeks :0 09:32 ?xdm? 30:09 0.02s -:0
aweeks pts/0 09:33 5:49m 0.00s 0.82s kdeinit: kded
aweeks pts/2 09:35 8.00s 0.55s 0.36s vi sag-0.9.sgml
aweeks pts/1 15:03 59.00s 0.03s 0.03s /bin/bash
</computeroutput>
</screen></para>
<para>From this we can see that I have a <command>kde</command> session
running, I'm working in this document :-), and have another terminal
open sitting idle at a bash prompt.</para>
</sect2>
<sect2 id="skill">
<title>The skill command</title>
<para> To Be Added</para>
</sect2>
<sect2 id="nice">
<title>nice and renice</title>
<para>To Be Added</para>
</sect2>
</sect1>
</chapter>

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<chapter id="boots-and-shutdowns">
<title>Boots And Shutdowns</title>
<blockquote><para><literallayout>
Start me up
Ah... you've got to... you've got to
Never, never never stop
Start it up
Ah... start it up, never, never, never
You make a grown man cry,
you make a grown man cry
(Rolling Stones)
</literallayout></para></blockquote>
<para> This section explains what goes on when a Linux system is
brought up and taken down, and how it should be done properly.
If proper procedures are not followed, files might be corrupted
or lost.</para>
<sect1 id="boot-overview">
<title>An overview of boots and shutdowns</title>
<para>The act of turning on a computer system and causing its
operating system to be loaded
is called <glossterm>booting</glossterm>. The name comes from
an image of the computer pulling itself up from its bootstraps,
but the act itself slightly more realistic.</para>
<para>During bootstrapping, the computer first loads a small piece
of code called the <glossterm>bootstrap loader</glossterm>, which
in turn loads and starts the operating system. The bootstrap
loader is usually stored in a fixed location on a hard disk
or a floppy. The reason for this two step process is that
the operating system is big and complicated, but the first
piece of code that the computer loads must be very small (a
few hundred bytes), to avoid making the firmware unnecessarily
complicated.</para>
<para>Different computers do the bootstrapping differently.
For PCs, the computer (its BIOS) reads in the first sector
(called the <glossterm>boot sector</glossterm>) of a floppy or
hard disk. The bootstrap loader is contained within this sector.
It loads the operating system from elsewhere on the disk (or
from some other place).</para>
<para>After Linux has been loaded, it initializes the hardware and
device drivers, and then runs <command>init</command>.
<command>init</command>
starts other processes to allow users to log in, and do things.
The details of this part will be discussed below.</para>
<para>In order to shut down a Linux system, first all processes
are told to terminate (this makes them close any files and
do other necessary things to keep things tidy), then filesystems
and swap areas are unmounted, and finally a message is printed
to the console that the power can be turned off. If the proper
procedure is not followed, terrible things can and will happen;
most importantly, the filesystem buffer cache might not be flushed,
which means that all data in it is lost and the filesystem on
disk is inconsistent, and therefore possibly unusable.
</para>
</sect1>
<sect1 id="boot-process">
<title>The boot process in closer look</title>
<para>When a PC is booted, the BIOS will do various tests to
check that everything looks all right, and will then start the actual
booting. This process is called the <glossterm>power on self test
</glossterm>, or POST for short. It will choose a disk
drive (typically the first floppy drive, if there is a floppy
inserted, otherwise the first hard disk, if one is installed
in the computer; the order might be configurable, however)
and will then read its very first sector. This is called the
<glossterm>boot sector</glossterm>; for a hard disk, it is also
called the <glossterm>master boot record</glossterm>, since a
hard disk can contain several partitions, each with their own
boot sectors.</para>
<para>The boot sector contains a small program (small enough to
fit into one sector) whose responsibility is to read the actual
operating system from the disk and start it. When booting Linux
from a floppy disk, the boot sector contains code that just reads
the first few hundred blocks (depending on the actual kernel
size, of course) to a predetermined place in memory. On a Linux
boot floppy, there is no filesystem, the kernel is just stored
in consecutive sectors, since this simplifies the boot process.
It is possible, however, to boot from a floppy with a filesystem,
by using LILO, the LInux LOader, or GRUB, the GRand Unifying
Bootloader.</para>
<para>When booting from the hard disk, the code in the master
boot record will examine the partition table (also in the master
boot record), identify the active partition (the partition that is
marked to be bootable), read the boot sector from that partition,
and then start the code in that boot sector. The code in the
partition's boot sector does what a floppy disk's boot sector
does: it will read in the kernel from the partition and start it.
The details vary, however, since it is generally not useful to
have a separate partition for just the kernel image, so the
code in the partition's boot sector can't just read the disk
in sequential order, it has to find the sectors wherever the
filesystem has put them. There are several ways around this
problem, but the most common way is to use a boot loader like
LILO or GRUB. (The details
about how to do this are irrelevant for this discussion, however;
see the LILO or GRUB documentation for more information; it is most
thorough.)</para>
<para>When booting, the bootloader will normally go right ahead
and read in and boot the default kernel. It is also possible
to configure the boot loader to be able to boot one of several kernels,
or even other operating systems than Linux, and it is possible
for the user to choose which kernel or operating system is to
be booted at boot time. LILO, for example, can be configured so that if one
holds down the <keycap>alt</keycap>, <keycap>shift</keycap>, or
<keycap>ctrl</keycap> key at boot time (when LILO is loaded),
LILO will ask what is to be booted and not boot the default
right away. Alternatively, the bootloader can be configured so that it
will always ask, with an optional timeout that will cause the
default kernel to be booted.</para>
<para>It is also possible to give a <glossterm>kernel
command line argument</glossterm>, after the name of the kernel
or operating system. For a list of possible options you can read
<ulink url="http://www.tldp.org/HOWTO/BootPrompt-HOWTO.html">
http://www.tldp.org/HOWTO/BootPrompt-HOWTO.html</ulink>.</para>
<para>Booting from floppy and from hard disk have both their
advantages, but generally booting from the hard disk is nicer,
since it avoids the hassle of playing around with floppies.
It is also faster. Most Linux distributions will setup the bootloader
for you during the install process.</para>
<para>After the Linux kernel has been read into the memory, by
whatever means, and is started for real, roughly the following
things happen:
<itemizedlist>
<listitem><para>
The Linux kernel is installed compressed, so it will first
uncompress itself. The beginning of the kernel image
contains a small program that does this.
</para></listitem>
<listitem><para>
If you have a super-VGA card that Linux
recognizes and that has some special text modes (such as 100
columns by 40 rows), Linux asks you which mode
you want to use. During the kernel compilation, it is
possible to preset a video mode, so that this is never asked.
This can also be done with LILO, GRUB or <command>rdev</command>.
</para></listitem>
<listitem><para>
After this, the kernel checks what other hardware there is
(hard disks, floppies, network adapters, etc), and configures
some of its device drivers appropriately; while it does this,
it outputs messages about its findings. For example, when I
boot, I it looks like this:
<screen>
<computeroutput>
LILO boot:
Loading linux.
Console: colour EGA+ 80x25, 8 virtual consoles
Serial driver version 3.94 with no serial options enabled
tty00 at 0x03f8 (irq = 4) is a 16450
tty01 at 0x02f8 (irq = 3) is a 16450
lp_init: lp1 exists (0), using polling driver
Memory: 7332k/8192k available (300k kernel code, 384k reserved, 176k
data)
Floppy drive(s): fd0 is 1.44M, fd1 is 1.2M
Loopback device init
Warning WD8013 board not found at i/o = 280.
Math coprocessor using irq13 error reporting.
Partition check:
hda: hda1 hda2 hda3
VFS: Mounted root (ext filesystem).
Linux version 0.99.pl9-1 (root@haven) 05/01/93 14:12:20
</computeroutput>
</screen>
The exact texts are different on different systems, depending
on the hardware, the version of Linux being used, and how
it has been configured.
</para></listitem>
<listitem><para> Then the kernel will try to mount the root
filesystem. The place is configurable at compilation time, or
any time with <command>rdev</command> or the bootloader. The filesystem
type is detected automatically. If the mounting of the root
filesystem fails, for example because you didn't remember to
include the corresponding filesystem driver in the kernel, the
kernel panics and halts the system (there isn't much it can do,
anyway). </para>
<para>The root filesystem is usually mounted read-only (this can
be set in the same way as the place). This makes it possible
to check the filesystem while it is mounted; it is not a good
idea to check a filesystem that is mounted read-write.
</para></listitem>
<listitem><para> After this, the kernel starts
the program <command>init</command> (located in
<filename>/sbin/init</filename>) in the background (this will
always become process number 1). <command>init</command> does
various startup chores. The exact things it does depends on how
it is configured; see <xref linkend="init"> for more information
(not yet written). It will at least start some essential
background daemons. </para></listitem>
<listitem><para> <command>init</command> then switches to
multi-user mode, and starts a <command>getty</command> for virtual
consoles and serial lines. <command>getty</command> is the
program which lets people log in via virtual consoles and serial
terminals. <command>init</command> may also start some other
programs, depending on how it is configured. </para></listitem>
<listitem><para> After this, the boot is complete, and the system
is up and running normally. </para></listitem>
</itemizedlist>
</para>
<sect2 id="bootloaders">
<title>A Word About Bootloaders</title>
<para>TO BE ADDED</para>
<para>This section will give an overview of the difference between
GRUB and LILO.</para>
<para>For more information on LILO, you can read
<ulink url="http://www.tldp.org/HOWTO/LILO.html">
http://www.tldp.org/HOWTO/LILO.html</ulink></para>
<para>For more information on GRUB, you can visit
<ulink url="http://www.gnu.org/software/grub/grub.html">
http://www.gnu.org/software/grub/grub.html</ulink></para>
</sect2>
</sect1>
<sect1 id="shutdown">
<title>More about shutdowns</title>
<para>It is important to follow the correct procedures when you shut
down a Linux system. If you fail do so, your filesystems probably
will become trashed and the files probably will become scrambled.
This is because Linux has a disk cache that won't write things
to disk at once, but only at intervals. This greatly improves
performance but also means that if you just turn off the power
at a whim the cache may hold a lot of data and that what is on
the disk may not be a fully working filesystem (because only
some things have been written to the disk).</para>
<para>Another reason against just flipping the power switch is that
in a multi-tasking system there can be lots of things going on
in the background, and shutting the power can be quite
disastrous. By using the proper shutdown sequence, you ensure
that all background processes can save their data.</para>
<para>The command for properly shutting down a Linux system
is <command>shutdown</command>. It is usually used in one of
two ways.</para>
<para>If you are running a system where you are the only user,
the usual way of using <command>shutdown</command> is to quit
all running programs, log out on all virtual consoles, log
in as root on one of them (or stay logged in as root if you
already are, but you should change to root's home directory or
the root directory, to avoid problems with unmounting), then
give the command <command>shutdown -h now</command> (substitute
<literal>now</literal> with a plus sign and a number in minutes
if you want a delay, though you usually don't on a single user
system).</para>
<para>Alternatively, if your system has many users, use the command
<command>shutdown -h +time message</command>, where
<literal>time</literal>
is the
time in minutes until the system is halted, and
<literal>message</literal>
is a short explanation of why the system is shutting down.
<screen>
<prompt>#</prompt> <userinput>shutdown -h +10 'We will install a new
disk. System should
> be back on-line in three hours.'</userinput>
<prompt>#</prompt>
</screen>
This will warn everybody that the system will shut down in
ten minutes, and that they'd better get lost or lose data.
The warning is printed to every terminal on which someone is
logged in, including all <command>xterm</command>s:
<screen>
<computeroutput>
Broadcast message from root (ttyp0) Wed Aug 2 01:03:25 1995...
We will install a new disk. System should
be back on-line in three hours.
The system is going DOWN for system halt in 10 minutes !!
</computeroutput>
</screen>
The warning is automatically repeated a few times before the boot,
with shorter and shorter intervals as the time runs out.</para>
<para>When the real shutting down starts after any delays, all
filesystems (except the root one) are unmounted, user processes
(if anybody is still logged in) are killed, daemons are shut down,
all filesystem are unmounted, and generally everything settles
down. When that is done, <command>init</command> prints out a
message that you can power down the machine. Then, and only then,
should you move your fingers towards the power switch.</para>
<para>Sometimes, although rarely on any good system, it is
impossible to shut down properly. For instance, if the kernel
panics and crashes and burns and generally misbehaves, it might
be completely impossible to give any new commands, hence shutting
down properly is somewhat difficult, and just about everything
you can do is hope that nothing has been too severely damaged
and turn off the power. If the troubles are a bit less severe
(say, somebody hit your keyboard with an axe), and the kernel
and the <command>update</command> program still run normally,
it is probably a good idea to wait a couple of minutes to give
<command>update</command> a chance to flush the buffer cache,
and only cut the power after that.</para>
<para>In the old days, some people like to shut down using the command
<command>sync</command> three times, waiting for the disk I/O to stop,
then turn off the power. If there are no running programs, this is
equivalent to using <command>shutdown</command>. However, it
does not unmount any filesystems and this can lead to problems
with the ext2fs ``clean filesystem'' flag. The triple-sync
method is <emphasis>not recommended</emphasis>.</para>
<para>(In case you're wondering: the reason for three syncs is
that in the early days of UNIX, when the commands were
typed separately, that usually gave sufficient time for most
disk I/O to be finished.)
</para>
</sect1>
<sect1 id="rebooting">
<title>Rebooting</title>
<para>Rebooting means booting the system again. This can be
accomplished by first shutting it down completely, turning
power off, and then turning it back on. A simpler way is to
ask <command>shutdown</command> to reboot the system, instead
of merely halting it. This is accomplished by using the
<option>-r</option> option to <command>shutdown</command>,
for example, by giving the command <command>shutdown -r
now</command>.</para>
<para>Most Linux systems run <command>shutdown -r now</command>
when ctrl-alt-del is pressed on the keyboard. This reboots the
system. The action on ctrl-alt-del is configurable, however, and
it might be better to allow for some delay before the reboot on
a multiuser machine. Systems that are physically accessible to
anyone might even be configured to do nothing when ctrl-alt-del
is pressed. </para>
</sect1>
<sect1 id="single-user">
<title>Single user mode</title>
<para>The <command>shutdown</command> command can also be used
to bring the system down to single user mode, in which no one
can log in, but root can use the console. This is useful for
system administration tasks that can't be done while the system is
running normally.</para>
</sect1>
<sect1 id="emerg-boot-floppy">
<title>Emergency boot floppies</title>
<para>It is not always possible to boot a computer from the hard
disk.
For example, if you make a mistake in configuring LILO, you might
make your system unbootable. For these situations, you need an
alternative way of booting that will always work (as long as the
hardware works). For typical PCs, this means booting from the
floppy drive.</para>
<para>Most Linux distributions allow one to create an
<glossterm>emergency boot floppy</glossterm> during installation.
It is a good idea to do this. However, some such boot disks
contain only the kernel, and assume you will be using the programs
on the distribution's installation disks to fix whatever problem
you have. Sometimes those programs aren't enough; for example,
you might have to restore some files from backups made with
software not on the installation disks.</para>
<para>Thus, it might be necessary to create a custom root floppy
as well. The Bootdisk HOWTO by Graham Chapman contains instructions
for doing this. You can find this HOWTO at
<ulink url="http://www.tldp.org/HOWTO/Bootdisk-HOWTO/index.html">
http://www.tldp.org/HOWTO/Bootdisk-HOWTO/index.html</ulink>.
You must, of course, remember to keep your emergency boot and
root floppies up to date.</para>
<para>You can't use the floppy drive you use to mount the root
floppy for anything else. This can be inconvenient if you only
have one floppy drive. However, if you have enough memory, you
can configure your boot floppy to load the root disk to a ramdisk
(the boot floppy's kernel needs to be specially configured for
this). Once the root floppy has been loaded into the ramdisk,
the floppy drive is free to mount other disks. </para>
</chapter>

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<chapter id="init-intro">
<title><command>init</command></title>
<blockquote><para><quote>Uuno on numero yksi</quote>
(Slogan for a series of Finnish movies.)</para></blockquote>
<para>This chapter describes the <command>init</command> process,
which is the first user level process started by the kernel.
<command>init</command> has many important duties, such as
starting <command>getty</command> (so that users can log in),
implementing run levels, and taking care of orphaned processes.
This chapter explains how <command>init</command> is configured
and how you can make use of the different run levels.</para>
<sect1 id="init-process">
<title><command>init</command> comes first</title>
<para><command>init</command> is one of those programs that
are absolutely essential to the operation of a Linux system,
but that you still can mostly ignore. A good Linux distribution
will come with a configuration for <command>init</command>
that will work for most systems, and on these systems there is
nothing you need to do about <command>init</command>. Usually,
you only need to worry about <command>init</command> if you hook
up serial terminals, dial-in (not dial-out) modems, or if you
want to change the default run level.</para>
<para>When the kernel has started itself (has been loaded
into memory, has started running, and has initialized all
device drivers and data structures and such), it finishes its
own part of the boot process by starting a user level program,
<command>init</command>. Thus, <command>init</command> is always
the first process (its process number is always 1).</para>
<para>The kernel looks for <command>init</command>
in a few locations that have been historically used
for it, but the proper location for it (on a Linux
system) is <filename>/sbin/init</filename>. If the
kernel can't find <command>init</command>, it tries to run
<filename>/bin/sh</filename>, and if that also fails, the startup
of the system fails.</para>
<para>When <command>init</command> starts, it finishes the
boot process by doing a number of administrative tasks, such
as checking filesystems, cleaning up <filename>/tmp</filename>,
starting various services, and starting a <command>getty</command>
for each terminal and virtual console where users should be able
to log in (see <xref linkend="log-in-and-out">).</para>
<para>After the system is properly up, <command>init</command>
restarts <command>getty</command> for each terminal
after a user has logged out (so that the next user can log
in). <command>init</command> also adopts orphan processes: when
a process starts a child process and dies before its child, the
child immediately becomes a child of <command>init</command>.
This is important for various technical reasons, but it is good
to know it, since it makes it easier to understand process lists
and process tree graphs.
There are a few variants of <command>init</command>
available. Most Linux distributions
use <command>sysvinit</command> (written by Miquel
van Smoorenburg), which is based on the System V
<command>init</command> design. The BSD versions of Unix have
a different <command>init</command>. The primary difference
is run levels: System V has them, BSD does not (at least
traditionally). This difference is not essential. We'll look
at <command>sysvinit</command> only. </para>
</sect1>
<sect1 id="config-init">
<title>Configuring <command>init</command> to start
<command>getty</command>: the
<filename>/etc/inittab</filename> file</title>
<para>When it starts up, <command>init</command> reads the
<filename>/etc/inittab</filename>
configuration file. While the system is running, it will
re-read it, if sent the HUP signal (<command>kill -HUP 1</command>);
this feature makes it unnecessary to boot the system to make
changes to the <command>init</command> configuration take
effect.</para>
<para>The <filename>/etc/inittab</filename> file is
a bit complicated. We'll start with the simple case
of configuring <command>getty</command> lines. Lines in
<filename>/etc/inittab</filename> consist of four colon-delimited
fields:
<screen>
id:runlevels:action:process
</screen>
The fields are described below. In addition,
<filename>/etc/inittab</filename> can contain empty lines, and
lines that begin with a number sign (`<literal>#</literal>');
these are both ignored.
<glosslist>
<glossentry><glossterm>id</glossterm>
<glossdef><para>
This identifies the line in the file. For
<command>getty</command> lines, it specifies the terminal
it runs on (the characters after <filename>/dev/tty</filename>
in the device file name). For other lines,
it doesn't matter (except for length restrictions),
but it should be unique.
</para></glossdef></glossentry>
<glossentry><glossterm>runlevels</glossterm>
<glossdef><para>
The run levels the line should be considered
for. The run levels are given as single digits,
without delimiters. (Run levels are described
in the next section.)
</para></glossdef></glossentry>
<glossentry><glossterm>action</glossterm>
<glossdef><para>
What action should be taken by the line, e.g.,
<literal>respawn</literal> to run the command in the
next field again, when it exits, or <literal>once</literal>
to run it just once.
</para></glossdef></glossentry>
<glossentry><glossterm>process</glossterm>
<glossdef><para>
The command to run.
</para></glossdef></glossentry>
</glosslist>
To start a <command>getty</command> on the first virtual terminal
(<filename>/dev/tty1</filename>), in all the normal multi-user
run levels (2-5), one would write the following line:
<screen>
1:2345:respawn:/sbin/getty 9600 tty1
</screen>
The first field says that this is the line for
<filename>/dev/tty1</filename>.
The second field says that it applies to run levels 2, 3, 4,
and 5. The third field means that the command should be run
again, after it exits (so that one can log in, log out, and
then log in again). The last field is the command that runs
<command>getty</command> on the first virtual terminal.</para>
<para>Different versions of <command>getty</command> are run
differently. Consult your manual page, and make sure it is
the correct manual page.</para>
<para>If you wanted to add terminals or dial-in modem lines to a
system, you'd add more lines to <filename>/etc/inittab</filename>,
one for each terminal or dial-in line. For more details, see the
manual pages <command>init</command>, <filename>inittab</filename>,
and <command>getty</command>.</para>
<para>If a command fails when it starts,
and <command>init</command> is configured to
<literal>restart</literal> it, it will use a lot of
system resources: <command>init</command> starts it,
it fails, <command>init</command> starts it, it fails,
<command>init</command> starts it, it fails, and so on, ad
infinitum. To prevent this, <command>init</command> will keep
track of how often it restarts a command, and if the frequency
grows to high, it will delay for five minutes before restarting
again. </para>
</sect1>
<sect1 id="run-levels-intro">
<title>Run levels</title>
<para>A <glossterm>run level</glossterm> is a state of
<command>init</command> and the whole system that defines what
system services are operating. Run levels are identified by
numbers. Some system administrators
use run levels to define which subsystems are working, e.g.,
whether X is running, whether the network is operational, and
so on. Others have all subsystems always running or start and
stop them individually, without changing run levels, since run
levels are too coarse for controlling their systems. You need
to decide for yourself, but it might be easiest to follow the
way your Linux distribution does things.</para>
<para>The following table defines how most Linux Distributions
define the different run levels. However, run-levels 2 through 5
can be modified to suit your own tastes.</para>
<table id="run-levels-table">
<title>Run level numbers</title>
<tgroup cols=2>
<tbody>
<row> <entry>0</entry> <entry>Halt the system.</entry> </row>
<row> <entry>1</entry> <entry>Single-user mode (for special
administration).</entry> </row>
<row> <entry>2</entry> <entry>Local Multiuser with Networking
but without network service (like NFS)</entry> </row>
<row> <entry>3</entry> <entry>Full Multiuser with Networking
</entry> </row>
<row> <entry>4</entry> <entry>Not Used
</entry> </row>
<row> <entry>5</entry> <entry>Full Multiuser with Networking
and X Windows(GUI)</entry> </row>
<row> <entry>6</entry> <entry>Reboot.</entry> </row>
</tbody>
</tgroup>
</table>
<para>Services that get started at a certain runtime are determined
by the contents of the various <filename>rcN.d</filename> directories.
Most distributions locate these directories either at
<filename>/etc/init.d/rcN.d</filename> or
<filename>/etc/rcN.d</filename>. (Replace the N with the run-level
number.)<para>
<para>In each run-level you will find a series of if links pointing
to start-up scripts located in <filename>/etc/init.d</filename>.
The names of these links all start as either K or S, followed by a
number. If the name of the link starts with an S, then that indicates
the service will be started when you go into that run level. If the
name of the link starts with a K, the service will be killed (if
running).</para>
<para>The number following the K or S indicates the order the scripts
will be run. Here is a sample of what an
<filename>/etc/init.d/rc3.d</filename> may look like.
<screen>
<prompt>#</prompt> <userinput>ls -l /etc/init.d/rc3.d</userinput>
<computeroutput>
lrwxrwxrwx 1 root root 10 2004-11-29 22:09 K12nfsboot -> ../nfsboot
lrwxrwxrwx 1 root root 6 2005-03-29 13:42 K15xdm -> ../xdm
lrwxrwxrwx 1 root root 9 2004-11-29 22:08 S01pcmcia -> ../pcmcia
lrwxrwxrwx 1 root root 9 2004-11-29 22:06 S01random -> ../random
lrwxrwxrwx 1 root root 11 2005-03-01 11:56 S02firewall -> ../firewall
lrwxrwxrwx 1 root root 10 2004-11-29 22:34 S05network -> ../network
lrwxrwxrwx 1 root root 9 2004-11-29 22:07 S06syslog -> ../syslog
lrwxrwxrwx 1 root root 10 2004-11-29 22:09 S08portmap -> ../portmap
lrwxrwxrwx 1 root root 9 2004-11-29 22:07 S08resmgr -> ../resmgr
lrwxrwxrwx 1 root root 6 2004-11-29 22:09 S10nfs -> ../nfs
lrwxrwxrwx 1 root root 12 2004-11-29 22:40 S12alsasound -> ../alsasound
lrwxrwxrwx 1 root root 8 2004-11-29 22:09 S12fbset -> ../fbset
lrwxrwxrwx 1 root root 7 2004-11-29 22:10 S12sshd -> ../sshd
lrwxrwxrwx 1 root root 8 2005-02-01 09:24 S12xntpd -> ../xntpd
lrwxrwxrwx 1 root root 7 2004-12-02 20:34 S13cups -> ../cups
lrwxrwxrwx 1 root root 6 2004-11-29 22:09 S13kbd -> ../kbd
lrwxrwxrwx 1 root root 13 2004-11-29 22:10 S13powersaved -> ../powersaved
lrwxrwxrwx 1 root root 9 2004-11-29 22:09 S14hwscan -> ../hwscan
lrwxrwxrwx 1 root root 7 2004-11-29 22:10 S14nscd -> ../nscd
lrwxrwxrwx 1 root root 10 2004-11-29 22:10 S14postfix -> ../postfix
lrwxrwxrwx 1 root root 6 2005-02-04 13:27 S14smb -> ../smb
lrwxrwxrwx 1 root root 7 2004-11-29 22:10 S15cron -> ../cron
lrwxrwxrwx 1 root root 8 2004-12-22 20:35 S15smbfs -> ../smbfs
</computeroutput>
<prompt>
</screen>
<para>How run levels start are configured in
<filename>/etc/inittab</filename> by lines like the following:
<screen>
l2:2:wait:/etc/init.d/rc 2
</screen>
The first field is an arbitrary label, the second one means
that this applies for run level 2. The third field means
that <command>init</command> should run the command in the
fourth field once, when the run level is entered, and that
<command>init</command> should wait for it to complete. The
<filename>/etc/init.d/rc</filename> command runs whatever
commands are necessary to start and stop services to enter run
level 2.</para>
<para>The command in the fourth field does all the hard work of
setting up a run level. It starts services that aren't already
running, and stops services that shouldn't be running in the
new run level any more. Exactly what the command is, and how run
levels are configured, depends on the Linux distribution.</para>
<para>When <command>init</command> starts, it looks for a line
in <filename>/etc/inittab</filename> that specifies the default
run level:
<screen>
id:2:initdefault:
</screen>
You can ask <command>init</command> to go to a non-default run
level at startup by giving the kernel a command line argument
of <literal>single</literal> or <literal>emergency</literal>.
Kernel command line arguments can be given via LILO, for example.
This allows you to choose the single user mode (run level 1).</para>
<para>While the system is running, the <command>telinit</command>
command can change the run level. When the run level is
changed, <command>init</command> runs the relevant command from
<filename>/etc/inittab</filename>. </para>
</sect1>
<sect1 id="inittab">
<title>Special configuration in
<filename>/etc/inittab</filename></title>
<para>The <filename>/etc/inittab</filename> has some special
features that allow <command>init</command> to react to special
circumstances. These special features are marked by special
keywords in the third field. Some examples:
<glosslist>
<glossentry><glossterm><literal>powerwait</literal></glossterm>
<glossdef><para>
Allows <command>init</command> to shut the system
down, when the power fails. This assumes the use of
a UPS, and software that watches the UPS and informs
<command>init</command> that the power is off.
</para></glossdef></glossentry>
<glossentry><glossterm><literal>ctrlaltdel</literal></glossterm>
<glossdef><para>
Allows <command>init</command> to reboot the system, when
the user presses ctrl-alt-del on the console keyboard.
Note that the system administrator can configure the
reaction to ctrl-alt-del to be something else instead,
e.g., to be ignored, if the system is in a public
location. (Or to start <command>nethack</command>.)
</para></glossdef></glossentry>
<glossentry><glossterm><literal>sysinit</literal></glossterm>
<glossdef><para>
Command to be run when the system is booted. This command
usually cleans up <filename>/tmp</filename>, for example.
</para></glossdef></glossentry>
</glosslist>
The list above is not exhaustive. See your
<filename>inittab</filename> manual page for all possibilities,
and for details on how to use the above ones. </para>
</sect1>
<sect1 id="boot-single-user">
<title>Booting in single user mode</title>
<para>An important run level is <glossterm>single user
mode</glossterm> (run level 1),
in which only the system administrator is using the machine
and as few system services, including logins, as possible are
running. Single user mode is necessary for a few administrative
tasks, such as running <command>fsck</command> on a
<filename>/usr</filename> partition, since this requires that
the partition be unmounted, and that can't happen, unless just
about all system services are killed.</para>
<para>A running system can be taken to single user mode by using
<command>telinit</command> to request run level 1. At bootup,
it can be entered by giving the word <literal>single</literal>
or <literal>emergency</literal> on the kernel command line: the
kernel gives the command line to <command>init</command> as well,
and <command>init</command> understands from that word that it
shouldn't use the default run level. (The kernel command line is
entered in a way that depends on how you boot the system.)</para>
<para>Booting into single user mode is sometimes necessary so
that one can run <command>fsck</command> by hand, before anything
mounts or otherwise touches a broken <filename>/usr</filename>
partition (any activity on a broken filesystem is likely to
break it more, so <command>fsck</command> should be run as soon
as possible).</para>
<para>The bootup scripts <command>init</command> runs
will automatically enter single user mode, if the automatic
<command>fsck</command> at bootup fails. This is an attempt to
prevent the system from using a filesystem that is so broken that
<command>fsck</command> can't fix it automatically. Such breakage
is relatively rare, and usually involves a broken hard disk or an
experimental kernel release, but it's good to be prepared.</para>
<para>As a security measure, a properly configured system
will ask for the root password before starting the shell in
single user mode. Otherwise, it would be simple to just enter
a suitable line to LILO to get in as root. (This will break if
<filename>/etc/passwd</filename> has been broken by filesystem
problems, of course, and in that case you'd better have a boot
floppy handy.)</para>
</chapter>

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<chapter id="log-in-and-out">
<title>Logging In And Out</title>
<blockquote><para><quote>I don't care to belong to a club
that accepts people like me as a member.</quote>
(Groucho Marx)</para></blockquote>
<para>
This section describes what happens when a user logs
in or out. The various interactions of background processes,
log files, configuration files, and so on are described in
some detail.
</para>
<sect1 id="login-via-terminal">
<title>Logins via terminals</title>
<para><xref linkend="terminal-logins"> shows how logins happen via
terminals. First, <command>init</command> makes sure there is
a <command>getty</command> program for the terminal connection
(or console). <command>getty</command> listens at the terminal
and waits for the user to notify that he is ready to login in
(this usually means that the user must type something). When it
notices a user, <command>getty</command> outputs a welcome message
(stored in <filename>/etc/issue</filename>), and prompts for
the username, and finally runs the <command>login</command>
program. <command>login</command> gets the username as a
parameter, and prompts the user for the password. If these
match, <command>login</command> starts the shell configured
for the user; else it just exits and terminates the process
(perhaps after giving the user another chance at entering the
username and password). <command>init</command> notices that
the process terminated, and starts a new <command>getty</command>
for the terminal.
</para>
<figure id="terminal-logins-table" float="1">
<title>Logins via terminals: the interaction of
<command>init</command>,
<command>getty</command>, <command>login</command>, and the
shell.</title>
<graphic fileref="logins-via-terminals.png">
</figure>
<para> Note that the only new process is the
one created by <command>init</command> (using the
<function>fork</function> system call); <command>getty</command>
and <command>login</command> only replace the program running in
the process (using the <function>exec</function> system call).
</para>
<para> A separate program, for noticing the user, is needed
for serial lines, since it can be (and traditionally was)
complicated to notice when a terminal becomes active.
<command>getty</command> also adapts to the speed and other
settings of the connection, which is important especially for
dial-in connections, where these parameters may change from call
to call. </para>
<para> There are several versions of <command>getty</command>
and <command>init</command> in use, all with their good and
bad points. It is a good idea to learn about the versions on
your system, and also about the other versions (you could use the
Linux Software Map to search them). If you don't have dial-ins,
you probably don't have to worry about <command>getty</command>,
but <command>init</command> is still important. </para>
</sect1>
<sect1 id="login-via-network">
<title>Logins via the network</title>
<para>Two computers in the same network are usually linked via a
single physical cable. When they communicate over the network,
the programs in each computer that take part in the communication
are linked via a <glossterm>virtual connection</glossterm>, a sort
of imaginary cable. As far as the programs at either end of the
virtual connection are concerned, they have a monopoly on their
own cable. However, since the cable is not real, only imaginary,
the operating systems of both computers can have several virtual
connections share the same physical cable. This way, using just
a single cable, several programs can communicate without having
to know of or care about the other communications. It is even
possible to have several computers use the same cable; the virtual
connections exist between two computers, and the other computers
ignore those connections that they don't take part in. </para>
<para> That's a complicated and over-abstracted description of
the reality. It might, however, be good enough to understand
the important reason why network logins are somewhat different
from normal logins. The virtual connections are established
when there are two programs on different computers that wish
to communicate. Since it is in principle possible to login
from any computer in a network to any other computer, there is
a huge number of potential virtual communications. Because of
this, it is not practical to start a <command>getty</command>
for each potential login. </para>
<para> There is a single process inetd (corresponding to
<command>getty</command>) that handles all network logins.
When it notices an incoming network login (i.e., it notices
that it gets a new virtual connection to some other computer),
it starts a new process to handle that single login. The original
process remains and continues to listen for new logins. </para>
<para> To make things a bit more complicated, there is
more than one communication protocol for network logins.
The two most important ones are <command>telnet</command> and
<command>rlogin</command>. In addition to logins, there are many
other virtual connections that may be made (for FTP, Gopher, HTTP,
and other network services). It would be ineffective to have a
separate process listening for a particular type of connection,
so instead there is only one listener that can recognize the type
of the connection and can start the correct type of program to
provide the service. This single listener is called
<command>inetd</command>;
see the <citetitle>Linux Network Administrators' Guide</citetitle>
for more information. </para>
</sect1>
<sect1 id="what-login-does">
<title>What <command>login</command> does</title>
<para>The <command>login</command> program takes care of
authenticating the user (making sure that the username and
password match), and of setting up an initial environment for
the user by setting permissions for the serial line and starting
the shell. </para>
<para> Part of the initial setup is outputting the contents of
the file <filename>/etc/motd</filename> (short for message of the
day) and checking for electronic mail. These can be disabled
by creating a file called <filename>.hushlogin</filename> in
the user's home directory. </para>
<para> If the file <filename>/etc/nologin</filename>
exists, logins are disabled. That file is typically
created by <command>shutdown</command> and relatives.
<command>login</command> checks for this file, and will
refuse to accept a login if it exists. If it does exist,
<command>login</command> outputs its contents to the terminal
before it quits. </para>
<para> <command>login</command> logs all failed login attempts in
a system log file (via <command>syslog</command>). It also logs
all logins by root. Both of these can be useful when tracking
down intruders. </para>
<para> Currently logged in people are listed in
<filename>/var/run/utmp</filename>. This file is valid only
until the system is next rebooted or shut down; it is cleared
when the system is booted. It lists each user and the terminal
(or network connection) he is using, along with some other useful
information. The <command>who</command>, <command>w</command>,
and other similar commands look in <filename>utmp</filename>
to see who are logged in. </para>
<para> All successful logins are recorded into
<filename>/var/log/wtmp</filename>. This file will grow without
limit, so it must be cleaned regularly, for example by having
a weekly <command>cron</command> job to clear it.
The <command>last</command> command browses
<filename>wtmp</filename>. </para>
<para> Both <filename>utmp</filename> and
<filename>wtmp</filename> are in a binary format (see the
<filename>utmp</filename> manual page); it is unfortunately not
convenient to examine them without special programs. </para>
</sect1>
<sect1 id="X-xdm">
<title>X and xdm</title>
<para> XXX X implements logins via xdm; also: xterm -ls </para>
<para>TO BE ADDED</para>
</sect1>
<sect1 id="access-control">
<title>Access control</title>
<para> The user database is traditionally contained in the
<filename>/etc/passwd</filename> file. Some systems use
<glossterm>shadow passwords</glossterm>, and have moved the
passwords to <command>/etc/shadow</command>. Sites with many
computers that share the accounts use NIS or some other method
to store the user database; they might also automatically copy
the database from one central location to all other computers.
</para>
<para> The user database contains not only the passwords, but
also some additional information about the users, such as their
real names, home directories, and login shells. This other
information needs to be public, so that anyone can read it.
Therefore the password is stored encrypted. This does have
the drawback that anyone with access to the encrypted password
can use various cryptographic methods to guess it, without
trying to actually log into the computer. Shadow passwords try
to avoid this by moving the password into another file, which
only root can read (the password is still stored encrypted).
However, installing shadow passwords later onto a system that
did not support them can be difficult. </para>
<para> With or without passwords, it is important to make
sure that all passwords in a system are good, i.e., not easily
guessed. The <command>crack</command> program can be used
to crack passwords; any password it can find is by definition
not a good one. While <command>crack</command> can be run
by intruders, it can also be run by the system administrator
to avoid bad passwords. Good passwords can also be enforced
by the <command>passwd</command> program; this is in fact more
effective in CPU cycles, since cracking passwords requires quite
a lot of computation. </para>
<para> The user group database is kept in
<filename>/etc/group</filename>; for systems with shadow
passwords, there can be a <filename>/etc/shadow.group</filename>.
</para>
<para> root usually can't login via most terminals
or the network, only via terminals listed in the
<filename>/etc/securetty</filename> file. This makes it necessary
to get physical access to one of these terminals. It is, however,
possible to log in via any terminal as any other user, and use
the <command>su</command> command to become root. </para>
</sect1>
<sect1 id="shell-startup">
<title>Shell startup</title>
<para> When an interactive login shell starts, it automatically
executes one or more pre-defined files. Different shells execute
different files; see the documentation of each shell for further
information. </para>
<para> Most shells first run some global file, for example, the
Bourne shell (<command>/bin/sh</command>) and its derivatives
execute <filename>/etc/profile</filename>; in addition,
they execute <filename>.profile</filename> in the user's
home directory. <filename>/etc/profile</filename> allows the
system administrator to have set up a common user environment,
especially by setting the <envar>PATH</envar> to include local
command directories in addition to the normal ones. On the other
hand, <filename>.profile</filename> allows the user to customize
the environment to his own tastes by overriding, if necessary,
the default environment. </para>
</chapter>

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<chapter id="managing-users">
<title>Managing user accounts</title>
<blockquote><para><quote>The similarities of sysadmins and drug
dealers: both measure stuff in Ks, and both have users.</quote>
(Old, tired computer joke.)</para></blockquote>
<para> This chapter explains how to create new user accounts,
how to modify the properties of those accounts, and how to remove
the accounts. Different Linux systems have different tools for
doing this.</para>
<sect1 id="account">
<title>What's an account?</title>
<para> When a computer is used by many people it is usually
necessary to differentiate between the users, for example, so that
their private files can be kept private. This is important even
if the computer can only be used by a single person at a time,
as with most microcomputers. Thus, each user is given a unique
username, and that name is used to log in.</para>
<para> There's more to a user than just a name, however. An
<glossterm>account</glossterm> is all the files, resources,
and information belonging to one user. The term hints at banks,
and in a commercial system each account usually has some money
attached to it, and that money vanishes at different speeds
depending on how much the user stresses the system. For example,
disk space might have a price per megabyte and day, and processing
time might have a price per second. </para>
</sect1>
<sect1 id="adduser">
<title>Creating a user</title>
<para> The Linux kernel itself treats users are mere numbers.
Each user is identified by a unique integer, the <glossterm>user
id</glossterm> or <glossterm>uid</glossterm>, because numbers are
faster and easier for a computer to process than textual names.
A separate database outside the kernel assigns a textual name,
the <glossterm>username</glossterm>, to each user id. The database
contains additional information as well. </para>
<para> To create a user, you need to add information about
the user to the user database, and create a home directory for
him. It may also be necessary to educate the user, and set up
a suitable initial environment for him. </para>
<para> Most Linux distributions come with a program for
creating accounts. There are several such programs available.
Two command line alternatives are <command>adduser</command>
and <command>useradd</command>; there may be a GUI tool as well.
Whatever the program, the result is that there is little if
any manual work to be done. Even if the details are many and
intricate, these programs make everything seem trivial. However,
<xref linkend="manual-adduser"> describes how to do it by hand.
</para>
<sect2 id="etc-passwd">
<title><filename>/etc/passwd</filename> and other informative
files</title>
<para> The basic user database in a Unix system is the text file,
<filename>/etc/passwd</filename> (called the <glossterm>password
file</glossterm>), which lists all valid usernames and their
associated information. The file has one line per username,
and is divided into seven colon-delimited fields:
<itemizedlist>
<listitem><para>Username.</para></listitem>
<listitem><para>Previously this was where the user's password was stored.
</para></listitem>
<listitem><para>Numeric user id.</para></listitem>
<listitem><para>Numeric group id.</para></listitem>
<listitem><para>Full name or other description of
account.</para></listitem>
<listitem><para>Home directory.</para></listitem>
<listitem><para>Login shell (program to run at
login).</para></listitem>
</itemizedlist>
The format is explained in more detail on the
<filename>passwd</filename> manual page. </para>
<para>
Most Linux systems use <glossterm>shadow passwords</glossterm>.
As mentioned, previously passwords were stored in the
<filename>/etc/passwd</filename> file. This newer method
of storing the password: the encrypted
password is stored in a separate file,
<filename>/etc/shadow</filename>,
which only root can read. The <filename>/etc/passwd</filename>
file only contains a special marker in the second field.
Any program that needs to verify a user is setuid, and
can therefore access the shadow password file. Normal
programs, which only use the other fields in the password
file, can't get at the password.
</para>
</sect2>
<sect2 id="uid-gid">
<title>Picking numeric user and group ids</title>
<para> On most systems it doesn't matter what the numeric user
and group ids are, but if you use the Network filesystem (NFS),
you need to have the same uid and gid on all systems. This
is because NFS also identifies users with the numeric uids.
If you aren't using NFS, you can let your account creation tool
pick them automatically. </para>
<para> If you are using NFS, you'll have to be invent a mechanism
for synchronizing account information. One alternative is to
the NIS system (see XXX network-admin-guide). </para>
<para> However, you should try to avoid re-using numeric uids
(and textual usernames), because the new owner of the uid (or
username) may get access to the old owner's files (or mail,
or whatever). </para>
</sect2>
<!--
%\subsection{Managing groups}
%
% \meta Debian creates a new group for each user; give reason for
this;
% give reasons against.
-->
<sect2 id="etc-skel">
<title>Initial environment: <filename>/etc/skel</filename></title>
<para> When the home directory for a new user is created, it is
initialized with files from the <filename>/etc/skel</filename>
directory. The system administrator can create files in
<filename>/etc/skel</filename> that will provide a nice
default environment for users. For example, he might create a
<filename>/etc/skel/.profile</filename> that sets the EDITOR
environment variable to some editor that is friendly towards
new users. </para>
<para> However, it is usually best to try to keep
<filename>/etc/skel</filename> as small as possible, since it
will be next to impossible to update existing users' files. For
example, if the name of the friendly editor changes, all existing
users would have to edit their <filename>.profile</filename>. The
system administrator could try to do it automatically, with a
script, but that is almost certain going to break someone's file.
</para>
<para> Whenever possible, it is better to put global configuration
into global files, such as <filename>/etc/profile</filename>. This
way it is possible to update it without breaking users'
own setups. </para>
</sect2>
<sect2 id="manual-adduser">
<title>Creating a user by hand</title>
<para> To create a new account manually, follow these steps:
<itemizedlist>
<listitem><para> Edit <filename>/etc/passwd</filename> with
<command>vipw</command> and add a new line for the new account. Be
careful with the syntax. <emphasis>Do not edit directly with an
editor!</emphasis> <command>vipw</command> locks the file, so
that other commands won't try to update it at the same time. You
should make the password field be `<literal>*</literal>', so
that it is impossible to log in. </para></listitem>
<listitem><para> Similarly, edit <filename>/etc/group</filename>
with <command>vigr</command>, if you need to create a new group
as well. </para></listitem>
<listitem><para> Create the home directory of the user with
<command>mkdir</command>. </para></listitem>
<listitem><para> Copy the files from
<filename>/etc/skel</filename> to the new home directory.
</para></listitem>
<listitem><para> Fix ownerships and permissions with
<command>chown</command> and <command>chmod</command>. The
<option>-R</option> option is most useful. The correct
permissions vary a little from one site to another, but usually
the following commands do the right thing:
<screen>
<userinput>cd /home/newusername
chown -R username.group .
chmod -R go=u,go-w .
chmod go= .</userinput>
</screen>
</para></listitem>
<listitem><para> Set the password with <command>passwd</command>.
</para></listitem>
</itemizedlist>
</para>
<para> After you set the password in the last step, the account
will work. You shouldn't set it until everything else has been
done, otherwise the user may inadvertently log in while you're
still copying the files. </para>
<para>
It is sometimes necessary to create dummy
accounts
that are not used by people. For example, to set up an anonymous
FTP server (so that anyone can download files from it, without
having to get an account first), you need to create an account
called ftp. In such cases, it is usually not necessary to set
the password (last step above). Indeed, it is better not to, so
that no-one can use the account, unless they first become root,
since root can become any user. </para>
</sect2>
</sect1>
<!--
%\section{Educating a new user}
%
% \meta
% make sure they know how to get help
% large sites might want to write a small booklet (or even just
% a couple of pages) with important stuff: how to log in
% and out, how to change password, which systems there are,
% how to use mail, list of people that answer questions
-->
<sect1 id="user-properties">
<title>Changing user properties</title>
<para>
There are a few commands for changing various
properties of an account (i.e., the relevant field
in <filename>/etc/passwd</filename>):
<glosslist>
<glossentry><glossterm><command>chfn</command></glossterm>
<glossdef><para> Change the full name field.
</para></glossdef></glossentry>
<glossentry><glossterm><command>chsh</command></glossterm>
<glossdef><para> Change the login shell.
</para></glossdef></glossentry>
<glossentry><glossterm><command>passwd</command></glossterm>
<glossdef><para>Change the password.
</para></glossdef></glossentry>
</glosslist>
The super-user may use these commands to change the properties
of any account. Normal users can only change the properties
of their own account. It may sometimes be necessary to disable
these commands (with <command>chmod</command>) for normal users,
for example in an environment with many novice users. </para>
<para>
Other tasks need to be done by hand. For example, to
change the username, you need to edit
<filename>/etc/passwd</filename>
directly (with <command>vipw</command>, remember). Likewise, to add
or remove the user to more groups, you need to edit
<filename>/etc/group</filename> (with <command>vigr</command>). Such
tasks tend to
be rare, however, and should be done with caution: for
example, if
you change the username, e-mail will no longer reach the
user, unless you also create a mail alias.
</para>
</sect1>
<sect1 id="deluser">
<title>Removing a user</title>
<para> To remove a user, you first remove all
his files, mailboxes, mail aliases, print jobs,
<command>cron</command> and <command>at</command> jobs,
and all other references to the user. Then you remove the
relevant lines from <filename>/etc/passwd</filename> and
<filename>/etc/group</filename> (remember to remove the username
from all groups it's been added to). It may be a good idea to
first disable the account (see below), before you start removing
stuff, to prevent the user from using the account while it is
being removed. </para>
<para>
Remember that users may have files outside their home
directory. The <command>find</command> command can find them:
<screen>
find / -user username
</screen>
However, note that the above command will take a
<emphasis>long</emphasis> time, if you have large disks. If you
mount network disks, you need to be careful so that you won't
trash the network or the server. </para>
<para> Some Linux distributions come with special
commands to do this; look for <command>deluser</command> or
<command>userdel</command>. However, it is easy to do it by
hand as well, and the commands might not do everything. </para>
</sect1>
<sect1 id="disable-user">
<title>Disabling a user temporarily</title>
<para> It is sometimes necessary to temporarily disable an
account, without removing it. For example, the user might not
have paid his fees, or the system administrator may suspect that
a cracker has got the password of that account. </para>
<para> The best way to disable an account is to change its shell
into a special program that just prints a message. This way,
whoever tries to log into the account, will fail, and will
know why. The message can tell the user to contact the system
administrator so that any problems may be dealt with. </para>
<para>
It would also be possible to change the username
or password to something else, but then the user
won't know what is going on. Confused users mean more
work.
</para>
<para> A simple way to create the special programs is to write
`tail scripts':
<screen>
#!/usr/bin/tail +2
This account has been closed due to a security breach.
Please call 555-1234 and wait for the men in black to arrive.
</screen>
The first two characters (`<literal>#!</literal>') tell the
kernel that the rest of the line is a command that needs to be
run to interpret this file. The <command>tail</command> command
in this case outputs everything except the first line to the
standard output. </para>
<para>
If user billg is suspected of a security breach,
the system administrator would do something like this:
<screen>
<prompt>#</prompt> <userinput>chsh -s
/usr/local/lib/no-login/security billg</userinput>
<prompt>#</prompt> <userinput>su - tester</userinput>
This account has been closed due to a security breach.
Please call 555-1234 and wait for the men in black to arrive.
<prompt>#</prompt>
</screen>
The purpose of the <command>su</command> is to test that the
change worked, of course. </para>
<para> Tail scripts should be kept in a separate directory,
so that their names don't interfere with normal user commands.
</para>
</sect1>
<!--
%\section{Accounting}
%
% \meta
% sac et al
-->
</chapter>

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<chapter id="backups-intro">
<title>Backups</title>
<blockquote><para><literallayout>
Hardware is indeterministically reliable.
Software is deterministically unreliable.
People are indeterministically unreliable.
Nature is deterministically reliable.
</literallayout></para></blockquote>
<para> This chapter explains about why, how, and when to make
backups, and how to restore things from backups.</para>
<sect1 id="backups">
<title>On the importance of being backed up</title>
<para> Your data is valuable. It will cost you time and effort
re-create it, and that costs money or at least personal grief
and tears; sometimes it can't even be re-created, e.g., if it
is the results of some experiments. Since it is an investment,
you should protect it and take steps to avoid losing it. </para>
<para> There are basically four reasons why you might lose data:
hardware failures, software bugs, human action, or natural
disasters. Although modern hardware tends to be quite reliable, it
can still break seemingly spontaneously. The most critical piece
of hardware for storing data is the hard disk, which relies on
tiny magnetic fields remaining intact in a world filled with
electromagnetic noise. Modern software doesn't even tend to
be reliable; a rock solid program is an exception, not a rule.
Humans are quite unreliable, they will either make a mistake, or
they will be malicious and destroy data on purpose. Nature might
not be evil, but it can wreak havoc even when being good. All in
all, it is a small miracle that anything works at all. </para>
<para> Backups are a way to protect the investment in data.
By having several copies of the data, it does not matter as much
if one is destroyed (the cost is only that of the restoration
of the lost data from the backup). </para>
<para> It is important to do backups properly. Like everything
else that is related to the physical world, backups will fail
sooner or later. Part of doing backups well is to make sure
they work; you don't want to notice that your backups didn't work.
Adding insult to injury, you might have a bad crash just as
you're making the backup; if you have only one backup medium,
it might destroyed as well, leaving you with the smoking ashes
of hard work.
Or you might notice, when trying to restore, that you forgot to
back up something important, like the user database on a 15000
user site. Best of all, all your backups might be working
perfectly, but the last known tape drive reading the kind of
tapes you used was the one that now has a bucketful of water
in it. </para>
<para> When it comes to backups, paranoia is in the job
description. </para>
</sect1>
<sect1 id="backup-media">
<title>Selecting the backup medium</title>
<para> The most important decision regarding backups is the choice
of backup medium. You need to consider cost, reliability, speed,
availability, and usability. </para>
<para> Cost is important, since you should preferably have
several times more backup storage than what you need for the data.
A cheap medium is usually a must. </para>
<para> Reliability is extremely important, since a broken
backup can make a grown man cry. A backup medium must be able
to hold data without corruption for years. The way you use the
medium affects it reliability as a backup medium. A hard disk
is typically very reliable, but as a backup medium it is not
very reliable, if it is in the same computer as the disk you
are backing up. </para>
<para> Speed is usually not very important, if backups can be done
without interaction. It doesn't matter if a backup takes two
hours, as long as it needs no supervision. On the other hand,
if the backup can't be done when the computer would otherwise
be idle, then speed is an issue. </para>
<para> Availability is obviously necessary, since you can't
use a backup medium if it doesn't exist. Less obvious is the
need for the medium to be available even in the future, and on
computers other than your own. Otherwise you may not be able
to restore your backups after a disaster. </para>
<para> Usability is a large factor in how often backups are made.
The easier it is to make backups, the better. A backup medium
mustn't be hard or boring to use. </para>
<para> The typical alternatives are floppies and tapes.
Floppies are very cheap, fairly reliable, not very fast,
very available, but not very usable for large amounts of data.
Tapes are cheap to somewhat expensive, fairly reliable, fairly
fast, quite available, and, depending on the size of the tape,
quite comfortable. </para>
<para> There are other alternatives. They are usually not very
good on availability, but if that is not a problem, they can
be better in other ways. For example, magneto-optical disks
can have good sides of both floppies (they're random access,
making restoration of a single file quick) and tapes (contain
a lot of data). </para>
</sect1>
<sect1 id="backup-tools">
<title>Selecting the backup tool</title>
<para> There are many tools that can be used to make
backups. The traditional UNIX tools used for backups
are <command>tar</command>, <command>cpio</command>, and
<command>dump</command>. In addition, there are large number
of third party packages (both freeware and commercial) that
can be used. The choice of backup medium can affect the choice
of tool. </para>
<para> <command>tar</command> and <command>cpio</command> are
similar, and mostly equivalent from a backup point of view.
Both are capable of storing files on tapes, and retrieving
files from them. Both are capable of using almost any media,
since the kernel device drivers take care of the low level
device handling and the devices all tend to look alike to user
level programs. Some UNIX versions of <command>tar</command>
and <command>cpio</command> may have problems with unusual files
(symbolic links, device files, files with very long pathnames, and
so on), but the Linux versions should handle all files correctly.
</para>
<para> <command>dump</command> is different in that it reads
the filesystem directly and not via the filesystem. It is
also written specifically for backups; <command>tar</command>
and <command>cpio</command> are really for archiving files,
although they work for backups as well. </para>
<para> Reading the filesystem directly has some advantages.
It makes it possible to back files up without affecting their time
stamps; for <command>tar</command> and <command>cpio</command>,
you would have to mount the filesystem read-only first.
Directly reading the filesystem is also more effective, if
everything needs to be backed up, since it can be done with
much less disk head movement. The major disadvantage is that
it makes the backup program specific to one filesystem type;
the Linux <command>dump</command> program understands the ext2
filesystem only. </para>
<para> <command>dump</command> also directly supports
backup levels (which we'll be discussing below); with
<command>tar</command> and <command>cpio</command> this has to
be implemented with other tools. </para>
<para> A comparison of the third party backup tools is beyond
the scope of this book. The Linux Software Map lists many of
the freeware ones. </para>
</sect1>
<sect1 id="simple-backups">
<title>Simple backups</title>
<para> A simple backup scheme is to back up everything once,
then back up everything that has been modified since the
previous backup. The first backup is called a <glossterm>full
backup</glossterm>, the subsequent ones are <glossterm>incremental
backups</glossterm>. A full backup is often more laborious
than incremental ones, since there is more data to write to the
tape and a full backup might not fit onto one tape (or floppy).
Restoring from incremental backups can be many times more work
than from a full one. Restoration can be optimized so that
you always back up everything since the previous full backup;
this way, backups are a bit more work, but there should never
be a need to restore more than a full backup and an incremental
backup. </para>
<para> If you want to make backups every day and have six
tapes, you could use tape 1 for the first full backup (say, on
a Friday), and tapes 2 to 5 for the incremental backups (Monday
through Thursday). Then you make a new full backup on tape 6
(second Friday), and start doing incremental ones with tapes 2
to 5 again. You don't want to overwrite tape 1 until you've got
a new full backup, lest something happens while you're making
the full backup. After you've made a full backup to tape 6,
you want to keep tape 1 somewhere else, so that when your other
backup tapes are destroyed in the fire, you still have at least
something left. When you need to make the next full backup,
you fetch tape 1 and leave tape 6 in its place. </para>
<para> If you have more than six tapes, you can use the extra
ones for full backups. Each time you make a full backup, you
use the oldest tape. This way you can have full backups from
several previous weeks, which is good if you want to find an old,
now deleted file, or an old version of a file. </para>
<sect2 id="tar-backups">
<title>Making backups with <command>tar</command></title>
<para>
A full backup can easily be made with <command>tar</command>:
<screen>
<prompt>#</prompt> <userinput>tar --create --file /dev/ftape
/usr/src</userinput>
<computeroutput>tar: Removing leading / from absolute path names in
the archive</computeroutput>
<prompt>#</prompt>
</screen>
The example above uses the GNU version of <command>tar</command>
and its long option names. The traditional version of
<command>tar</command> only understands single character
options. The GNU version can also handle backups that don't
fit on one tape or floppy, and also very long paths; not all
traditional versions can do these things. (Linux only uses
GNU <command>tar</command>.) </para>
<para> If your backup doesn't fit on one tape, you need to use
the <option>--multi-volume</option> (<option>-M</option>) option:
<screen>
<prompt>#</prompt> <userinput>tar -cMf /dev/fd0H1440
/usr/src</userinput>
<computeroutput>tar: Removing leading / from absolute path names in
the archive
Prepare volume #2 for /dev/fd0H1440 and hit return:</computeroutput>
<prompt>#</prompt>
</screen>
Note that you should format the floppies before you begin the
backup, or else use another window or virtual terminal and do
it when <command>tar</command> asks for a new floppy. </para>
<para> After you've made a backup, you should check that it is OK,
using the <option>--compare</option> (<option>-d</option>) option:
<screen>
<prompt>#</prompt> <userinput>tar --compare --verbose -f
/dev/ftape</userinput>
<computeroutput>usr/src/
usr/src/linux
usr/src/linux-1.2.10-includes/
....</computeroutput>
<prompt>#</prompt>
</screen>
Failing to check a backup means that you will not notice that your
backups aren't working until after you've lost the original data.
</para>
<para> An incremental backup can be done with
<command>tar</command> using the <option>--newer</option>
(<option>-N</option>) option:
<screen>
<prompt>#</prompt> <userinput>tar --create --newer '8 Sep 1995'
--file /dev/ftape /usr/src
--verbose</userinput>
<computeroutput>tar: Removing leading / from absolute path names in
the archive
usr/src/
usr/src/linux-1.2.10-includes/
usr/src/linux-1.2.10-includes/include/
usr/src/linux-1.2.10-includes/include/linux/
usr/src/linux-1.2.10-includes/include/linux/modules/
usr/src/linux-1.2.10-includes/include/asm-generic/
usr/src/linux-1.2.10-includes/include/asm-i386/
usr/src/linux-1.2.10-includes/include/asm-mips/
usr/src/linux-1.2.10-includes/include/asm-alpha/
usr/src/linux-1.2.10-includes/include/asm-m68k/
usr/src/linux-1.2.10-includes/include/asm-sparc/
usr/src/patch-1.2.11.gz</computeroutput>
<prompt>#</prompt>
</screen>
Unfortunately, <command>tar</command> can't notice when a file's
inode information has changed, for example, that its permission
bits have been changed, or when its name has been changed.
This can be worked around using <command>find</command> and
comparing current filesystem state with lists of files that have
been previously backed up. Scripts and programs for doing this
can be found on Linux ftp sites. </para>
</sect2>
<sect2 id="tar-restore">
<title>Restoring files with <command>tar</command></title>
<para> The <option>--extract</option> (<option>-x</option>)
option for <command>tar</command> extracts files:
<screen>
<prompt>#</prompt> <userinput>tar --extract --same-permissions
--verbose --file
/dev/fd0H1440</userinput>
<computeroutput>usr/src/
usr/src/linux
usr/src/linux-1.2.10-includes/
usr/src/linux-1.2.10-includes/include/
usr/src/linux-1.2.10-includes/include/linux/
usr/src/linux-1.2.10-includes/include/linux/hdreg.h
usr/src/linux-1.2.10-includes/include/linux/kernel.h
...</computeroutput>
<prompt>#</prompt>
</screen>
You also extract only specific files or directories (which
includes all their files and subdirectories) by naming on the
command line:
<screen>
<prompt>#</prompt> <userinput>tar xpvf /dev/fd0H1440
usr/src/linux-1.2.10-includes/include/linux/hdreg.h</userinput>
<computeroutput>usr/src/linux-1.2.10-includes/include/linux/hdreg.h</computeroutput>
<prompt>#</prompt>
</screen>
Use the <option>--list</option> (<option>-t</option>) option,
if you just want to see what files are on a backup volume:
<screen>
<prompt>#</prompt> <userinput>tar --list --file
/dev/fd0H1440</userinput>
<computeroutput>usr/src/
usr/src/linux
usr/src/linux-1.2.10-includes/
usr/src/linux-1.2.10-includes/include/
usr/src/linux-1.2.10-includes/include/linux/
usr/src/linux-1.2.10-includes/include/linux/hdreg.h
usr/src/linux-1.2.10-includes/include/linux/kernel.h
...</computeroutput>
<prompt>#</prompt>
</screen>
Note that <command>tar</command> always reads the backup volume
sequentially, so for large volumes it is rather slow. It is not
possible, however, to use random access database techniques when
using a tape drive or some other sequential medium. </para>
<para> <command>tar</command> doesn't handle deleted files
properly. If you need to restore a filesystem from a full and
an incremental backup, and you have deleted a file between
the two backups, it will exist again after you have done the
restore. This can be a big problem, if the file has sensitive
data that should no longer be available. </para>
</sect2>
</sect1>
<sect1 id="multi-level-backups">
<title>Multilevel backups</title>
<para> The simple backup method outlined in the previous section
is often quite adequate for personal use or small sites. For more
heavy duty use, multilevel backups are more appropriate. </para>
<para> The simple method has two backup levels: full and
incremental backups. This can be generalized to any number of
levels. A full backup would be level 0, and the different levels
of incremental backups levels 1, 2, 3, etc. At each incremental
backup level you back up everything that has changed since the
previous backup at the same or a previous level. </para>
<para> The purpose for doing this is that it allows a longer
<glossterm>backup history</glossterm> cheaply. In the example in
the previous section, the backup history went back to the previous
full backup. This could be extended by having more tapes, but
only a week per new tape, which might be too expensive. A longer
backup history is useful, since deleted or corrupted files are
often not noticed for a long time. Even a version of a file that
is not very up to date is better than no file at all. </para>
<para> With multiple levels the backup history can be extended
more cheaply. For example, if we buy ten tapes, we could use
tapes 1 and 2 for monthly backups (first Friday each month),
tapes 3 to 6 for weekly backups (other Fridays; note that there
can be five Fridays in one month, so we need four more tapes),
and tapes 7 to 10 for daily backups (Monday to Thursday).
With only four more tapes, we've been able to extend the backup
history from two weeks (after all daily tapes have been used)
to two months. It is true that we can't restore every version
of each file during those two months, but what we can restore
is often good enough. </para>
<para><xref linkend="backup-history-timeline"> shows which backup
level is used each day, and which backups can be restored from
at the end of the month. </para>
<figure id="backup-history-timeline" float="1">
<title>A sample multilevel backup schedule.</title>
<graphic fileref="backup-timeline.png">
</figure>
<para> Backup levels can also be used to keep filesystem
restoration time to a minimum. If you have many incremental
backups with monotonously growing level numbers, you need to
restore all of them if you need to rebuild the whole filesystem.
Instead you can use level numbers that aren't monotonous, and
keep down the number of backups to restore. </para>
<para> To minimize the number of tapes needed to restore, you
could use a smaller level for each incremental tape. However,
then the time to make the backups increases (each backup copies
everything since the previous full backup). A better scheme is
suggested by the <command>dump</command> manual page and described
by the table XX (efficient-backup-levels). Use the following
succession of backup levels: 3, 2, 5, 4, 7, 6, 9, 8, 9, etc.
This keeps both the backup and restore times low. The most you
have to backup is two day's worth of work. The number of tapes
for a restore depends on how long you keep between full backups,
but it is less than in the simple schemes. </para>
<table id="efficient-backup-levels">
<title>Efficient backup scheme using many backup levels</title>
<tgroup cols=4>
<thead>
<row><entry>Tape</entry> <entry>Level</entry> <entry>Backup
(days)</entry> <entry>Restore
tapes</entry></row>
</thead>
<tbody>
<row><entry>1</entry> <entry>0</entry> <entry>n/a</entry>
<entry>1</entry></row>
<row><entry>2</entry> <entry>3</entry> <entry>1</entry> <entry>1,
2</entry></row>
<row><entry>3</entry> <entry>2</entry> <entry>2</entry> <entry>1,
3</entry></row>
<row><entry>4</entry> <entry>5</entry> <entry>1</entry> <entry>1, 2,
4</entry></row>
<row><entry>5</entry> <entry>4</entry> <entry>2</entry> <entry>1, 2,
5</entry></row>
<row><entry>6</entry> <entry>7</entry> <entry>1</entry> <entry>1, 2,
5, 6</entry></row>
<row><entry>7</entry> <entry>6</entry> <entry>2</entry> <entry>1, 2,
5, 7</entry></row>
<row><entry>8</entry> <entry>9</entry> <entry>1</entry> <entry>1, 2,
5, 7, 8</entry></row>
<row><entry>9</entry> <entry>8</entry> <entry>2</entry> <entry>1, 2,
5, 7, 9</entry></row>
<row><entry>10</entry> <entry>9</entry> <entry>1</entry> <entry>1, 2,
5, 7, 9, 10</entry></row>
<row><entry>11</entry> <entry>9</entry> <entry>1</entry> <entry>1, 2,
5, 7, 9, 10,
11</entry></row>
<row><entry>...</entry> <entry>9</entry> <entry>1</entry> <entry>1,
2, 5, 7, 9, 10, 11,
...</entry></row>
</tbody>
</tgroup>
</table>
<para> A fancy scheme can reduce the amount of labor needed, but
it does mean there are more things to keep track of. You must
decide if it is worth it. </para>
<para> <command>dump</command> has built-in support for backup
levels. For <command>tar</command> and <command>cpio</command>
it must be implemented with shell scripts. </para>
</sect1>
<sect1 id="what-to-backup">
<title>What to back up</title>
<para> You want to back up as much as possible. The major
exception is software that can be easily reinstalled,
but even they may have configuration files that it is
important to back up, lest you need to do all the work to
configure them all over again. Another major exception is
the <filename>/proc</filename> filesystem; since that only
contains data that the kernel always generates automatically,
it is never a good idea to back it up. Especially the
<filename>/proc/kcore</filename> file is unnecessary, since it
is just an image of your current physical memory; it's pretty
large as well. </para>
<para> Gray areas include the news spool, log files, and many
other things in <filename>/var</filename>. You must decide what
you consider important. </para>
<para> The obvious things to back up are user files
(<filename>/home</filename>) and system configuration files
(<filename>/etc</filename>, but possibly other things scattered
all over the filesystem). </para>
</sect1>
<sect1 id="compressed-backups">
<title>Compressed backups</title>
<para> Backups take a lot of space, which can cost quite
a lot of money. To reduce the space needed, the backups
can be compressed. There are several ways of doing this.
Some programs have support for for compression built in; for
example, the <option>--gzip</option> (<option>-z</option>)
option for GNU <command>tar</command> pipes the whole backup
through the <command>gzip</command> compression program, before
writing it to the backup medium. </para>
<para> Unfortunately, compressed backups can cause trouble.
Due to the nature of how compression works, if a single bit is
wrong, all the rest of the compressed data will be unusable.
Some backup programs have some built in error correction, but no
method can handle a large number of errors. This means that if
the backup is compressed the way GNU <command>tar</command> does
it, with the whole output compressed as a unit, a single error
makes all the rest of the backup lost. Backups must be reliable,
and this method of compression is not a good idea. </para>
<para> An alternative way is to compress each file separately.
This still means that the one file is lost, but all other files
are unharmed. The lost file would have been corrupted anyway,
so this situation is not much worse than not using compression
at all. The <command>afio</command> program (a variant of
<command>cpio</command>) can do this. </para>
<para>
Compression takes some time, which may make the backup program
unable to write data fast enough for a tape drive.
This can be avoided by buffering the output (either internally, if
the backup program if smart enough, or by using another program),
but even that might not work well enough. This should only be
a problem on slow computers. </para>
</sect1>
</chapter>

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<chapter id="task-automation">
<title>Task Automation --To Be Added</title>
<blockquote><para><quote>Never put off until tomorrow what you can do
the day after tomorrow.<quote>Mark Twain</para><blockquote>
<para>Basic discussion on scripting, cron & at - refer to
other HOWTO's for details. Discuss non-crontab cron jobs
such at those in the /etc directory.</para>
</chapter>

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<chapter id="keeping-time">
<title>Keeping Time</title>
<blockquote><para><quote>Time is an illusion. Lunchtime double
so.</quote> (Douglas Adams.)</para></blockquote>
<para> This chapter explains how a Linux system keeps time,
and what you need to do to avoid causing trouble. Usually,
you don't need to do anything about time, but it is good to
understand it.</para>
<sect1 id="localtime">
<title>The concept of localtime</title>
<para> Time measurement is based on mostly regular natural
phenomena, such as alternating light and dark periods caused
by the rotation of the planet. The total time taken by two
successive periods is constant, but the lengths of the light
and dark period vary. One simple constant is noon. </para>
<para> Noon is the time of the day when the Sun is at its
highest position. Since (according to recent research) the Earth is
round, noon happens at different times in different places. This
leads to the concept of <glossterm>local time</glossterm>. Humans
measure time in many units, most of which are tied to natural
phenomena like noon. As long as you stay in the same place,
it doesn't matter that local times differ. </para>
<para> As soon as you need to communicate with distant places,
you'll notice the need for a common time. In modern times,
most of the places in the world communicate with most other
places in the world, so a global standard for measuring time
has been defined. This time is called <glossterm>universal
time</glossterm> (UT or UTC, formerly known as Greenwich Mean Time
or GMT, since it used to be local time in Greenwich, England).
When people with different local times need to communicate,
they can express times in universal time, so that there is no
confusion about when things should happen. </para>
<para> Each local time is called a time zone. While geography
would allow all places that have noon at the same time have the
same time zone, politics makes it difficult. For various reasons,
many countries use <glossterm>daylight savings time</glossterm>,
that is, they move their clocks to have more natural light
while they work, and then move the clocks back during winter.
Other countries do not do this. Those that do, do not agree when
the clocks should be moved, and they change the rules from year
to year. This makes time zone conversions definitely non-trivial.
</para>
<para> Time zones are best named by the location or by telling
the difference between local and universal time. In the US
and some other countries, the local time zones have a name and
a three letter abbreviation. The abbreviations are not unique,
however, and should not be used unless the country is also named.
It is better to talk about the local time in, say, Helsinki,
than about East European time, since not all countries in Eastern
Europe follow the same rules. </para>
<para> Linux has a time zone package that knows about all
existing time zones, and that can easily be updated when the
rules change. All the system administrator needs to do is to
select the appropriate time zone. Also, each user can set his
own time zone; this is important since many people work with
computers in different countries over the Internet. When the
rules for daylight savings time change in your local time zone,
make sure you'll upgrade at least that part of your Linux system.
Other than setting the system time zone and upgrading the time
zone data files, there is little need to bother about time.
</para>
</sect1>
<sect1 id="hw-sw-clocks">
<title>The hardware and software clocks</title>
<para> A personal computer has a battery driven hardware clock.
The battery ensures that the clock will work even if the rest of
the computer is without electricity. The hardware clock can be
set from the BIOS setup screen or from whatever operating system
is running. </para>
<para> The Linux kernel keeps track of time independently from
the hardware clock. During the boot, Linux sets its own clock
to the same time as the hardware clock. After this, both clocks
run independently. Linux maintains its own clock because looking
at the hardware is slow and complicated. </para>
<para> The kernel clock always shows universal time. This way,
the kernel does not need to know about time zones at all. The
simplicity results in higher reliability and makes it easier
to update the time zone information. Each process handles time
zone conversions itself (using standard tools that are part of
the time zone package). </para>
<para> The hardware clock can be in local time or in universal
time. It is usually better to have it in universal time,
because then you don't need to change the hardware clock when
daylight savings time begins or ends (UTC does not have DST).
Unfortunately, some PC operating systems, including MS-DOS,
Windows, and OS/2, assume the hardware clock shows local time.
Linux can handle either, but if the hardware clock shows local
time, then it must be modified when daylight savings time begins
or ends (otherwise it wouldn't show local time). </para>
</sect1>
<sect1 id="showing-setting-time">
<title>Showing and setting time</title>
<para> In Linux, the system time zone is determined
by the symbolic link <filename>/etc/localtime</filename>.
This link points to a time zone data file that describes
the local time zone. The time zone data files are located at
either <filename>/usr/lib/zoneinfo</filename> or
<filename>/usr/share/zoneinfo</filename> depending on what distribution
of Linux you use.</para>
<para> For example, on a SuSE system located in New Jersey the
<filename>/etc/localtime</filename> link would point to
<filename>/usr/share/zoneinfo/US/Eastern</filename>. On a Debian system
the <filename>/etc/localtime</filename> link would point to
<filename>/usr/lib/zoneinfo/US/Eastern</filename>.</para>
<para> If you fail to find the <filename>zoneinfo</filename>
directory in either the <filename>/usr/lib</filename> or
<filename>/usr/share</filename> directories, either do a
<command>find /usr -print | grep zoneinfo</command> or consult
your distribution's documentation.
</para>
<para> What happens when you have a users located in a different
timezone? A user can change his private time zone by setting the
TZ environment variable. If it is unset, the system time zone
is assumed. The syntax of the TZ variable is described in the
<function>tzset</function> manual page. </para>
<para>
The <command>date</command> command shows the current date and
time.
For example:
<screen>
<prompt>$</prompt> <userinput>date</userinput>
<computeroutput>Sun Jul 14 21:53:41 EET DST 1996</computeroutput>
<prompt>$</prompt>
</screen>
That time is Sunday, 14th of July, 1996, at about ten before
ten at the evening, in the time zone called ``EET DST''
(which might be East European Daylight Savings Time).
<command>date</command> can also show the universal time:
<screen>
<prompt>$</prompt> <userinput>date -u</userinput>
<computeroutput>Sun Jul 14 18:53:42 UTC 1996</computeroutput>
<prompt>$</prompt>
</screen>
<command>date</command> is also used to set the kernel's software
clock:
<screen>
<prompt>#</prompt> <userinput>date 07142157</userinput>
<computeroutput>Sun Jul 14 21:57:00 EET DST 1996</computeroutput>
<prompt>#</prompt> <userinput>date</userinput>
<computeroutput>Sun Jul 14 21:57:02 EET DST 1996</computeroutput>
<prompt>#</prompt>
</screen>
See the <command>date</command> manual page for more details;
the syntax is a bit arcane. Only root can set the time.
While each user can have his own time zone, the clock is the
same for everyone. </para>
<para>Beware of the <command>time</command> command. This is not
used to get the system time. Instead it's used to time how long
something takes. Refer the the time man page.</para>
<para> <command>date</command> only shows or sets the software
clock. The <command>clock</command> commands synchronizes
the hardware and software clocks. It is used when the system
boots, to read the hardware clock and set the software clock.
If you need to set both clocks, you first set the software clock
with <command>date</command>, and then the hardware clock with
<command>clock -w</command>. </para>
<para> The <option>-u</option> option to <command>clock</command>
tells it that the hardware clock is in universal time.
You <emphasis>must</emphasis> use the <option>-u</option>
option correctly. If you don't, your computer will be quite
confused about what the time is. </para>
<para> The clocks should be changed with care. Many parts of a
Unix system require the clocks to work correctly. For example,
the <command>cron</command> daemon runs commands periodically.
If you change the clock, it can be confused of whether
it needs to run the commands or not. On one early Unix
system, someone set the clock twenty years into the future,
and <command>cron</command> wanted to run all the periodic
commands for twenty years all at once. Current versions of
<command>cron</command> can handle this correctly, but you should
still be careful. Big jumps or backward jumps are more dangerous
than smaller or forward ones. </para>
</sect1>
<sect1 id="clock-wrong">
<title>When the clock is wrong</title>
<para> The Linux software clock is not always accurate. It is
kept running by a periodic <glossterm>timer interrupt</glossterm>
generated by PC hardware. If the system has too many processes
running, it may take too long to service the timer interrupt, and
the software clock starts slipping behind. The hardware clock
runs independently and is usually more accurate. If you boot
your computer often (as is the case for most systems that aren't
servers), it will usually keep fairly accurate time. </para>
<para> If you need to adjust the hardware clock, it is usually
simplest to reboot, go into the BIOS setup screen, and do it
from there. This avoids all trouble that changing system time
might cause. If doing it via BIOS is not an option, set the new
time with <command>date</command> and <command>clock</command>
(in that order), but be prepared to reboot, if some part of the
system starts acting funny. </para>
<para> Another method would be to use either <command>hwclock -w</command>
or <command>hwclock --systohc</command> to sync the hardware clock
to the software clock. If you want to sync your software clock to your
hardware clock then you would use <command>hwclock -s</command> or
<command>hwclock --hwtosys</command>. For more information on this
command read <command>man hwclock</command>.</para>
<!-- Commented out to add NTP info.
<para> A networked computer (even if just over the modem) can
check its own clock automatically, by comparing it to some other
computer's time. If the other computer is known to keep very
accurate time, then both computers will keep accurate time.
This can be done by using the <command>rdate</command> and
<command>netdate</command> commands. Both check the time of a
remote computer (<command>netdate</command> can handle several
remote computers), and set the local computer's time to that.
By running one these commands regularly, your computer will keep
as accurate time as the remote computer. </para>
<para> XXX say something intelligent about NTP </para> -->
</sect1>
<sect1 id="ntp">
<title>NTP - Network Time Protocol</title>
<para> A networked computer (even if just over a modem) can
check its own clock automatically by comparing it to the time
on another computer known to keep accurate time. Network Time
Protocol (or NTP) does exactly that. It is a method of verifying
and correcting your computer's time by synchronizing it with a
another system. With NTP your system's time can be maintained
to within milliseconds of Coordinated Universal Time. Visit
<ulink url="http://www.time.gov/about.html/">
http://www.time.gov/about.html</ulink> for more info.
</para>
<para> For more casual Linux users, this is just a nice luxury.
At my home all our clocks are set based upon what my Linux system
says the time is. For larger organizations this "luxury" can
become essential. Being able to search log files for events based
upon time can make life a lot easier and take a lot of the "guess work"
out of debugging.
</para>
<para> Another example of how important NTP can be is with a SAN.
Some SAN's require NTP be configured and running properly to allow
for proper synchronization over filesystem usage, and proper
timestamp control. Some SANs (and some applications) can become
confused when dealing with files that have timestamps that are in
the future.
</para>
<para> Most Linux distributions come with a NTP package of some kind,
either a .deb or .rpm package. You can use that to install NTP, or you
can download the source files from <ulink url="http://www.ntp.org/downloads.html">
http://www.ntp.org/downloads.html</ulink> and compile it yourself. Either way,
the basic configuration is the same.</para>
</sect1>
<sect1 id="basic-ntp-config">
<title>Basic NTP configuration</title>
<para>The NTP program is configured using either the
<filename>/etc/ntp.conf </filename> or <filename>/etc/xntp.conf</filename>
file depending on what distribution of Linux you have. I won't go
into too much detail on how to configure NTP. Instead I'll just
cover the basics.</para>
<para>An example of a basic ntp.conf file would look like:
<screen>
<computeroutput># --- GENERAL CONFIGURATION ---
server aaa.bbb.ccc.ddd
server 127.127.1.0
fudge 127.127.1.0 stratum 10
# Drift file.
driftfile /etc/ntp/drift
</computeroutput>
</para>
<para>The most basic ntp.conf file will simply list 2 servers, one
that it wishes to synchronize with, and a pseudo IP address for
itself (in this case 127.127.1.0). The pseudo IP is used in case of
network problems or if the remote NTP server goes down. NTP will
synchronize against itself until the it can start synchronizing with
the remote server again. It is recommended that you list at
least 2 remote servers that you can synchronize against. One will
act as a primary server and the other as a backup.</para>
<para>You should also list a location for a drift file. Over time
NTP will "learn" the system clock's error rate and automatically
adjust for it.</para>
<para>The restrict option can be used to provide better control and
security over what NTP can do, and who can effect it. For example:
<screen>
<computeroutput># Prohibit general access to this service.
restrict default ignore
# Permit systems on this network to synchronize with this
# time service. But not modify our time.
restrict aaa.bbb.ccc.ddd nomodify
# Allow the following unrestricted access to ntpd
restrict aaa.bbb.ccc.ddd
restrict 127.0.0.1
</computeroutput>
</screen>
It is advised that you wait until you have NTP working properly before
adding the restrict option. You can accidental restrict yourself from
synchronizing and waste time debugging why.
</para>
<para>NTP slowly corrects your systems time. Be patient! A simple test
is to change your system clock by 10 minutes before you go to bed and then
check it when you get up. The time should be correct.</para>
<para>Many people get the idea that instead of running the NTP
daemon, they should just setup a <command>cron</command> job
job to periodically run the <command>ntpdate</command> command.
There are 2 main disadvantages of using using this method.</para>
<para>The first is that <command>ntpdate</command> does a "brute force"
method of changing the time. So if your computer's time is off my 5
minutes, it immediately corrects it. In some environments, this can
cause problems if time drastically changes. For example, if you are
using time sensitive security software, you can inadvertently kill
someones access. The NTP daemon slowly changes the time to avoid
causing this kind of disruption.</para>
<para>The other reason is that the NTP daemon can be configured to
try to learn your systems <glossterm>time drift</glossterm> and
then automatically adjust for it.</para>
</sect1>
<sect1 id="ntp-toolkit">
<title>NTP Toolkit</title>
<para>There are a number of utilities available to check if
NTP is doing it's job. The <command>ntpq -p</command> command
will print out your system's current time status.
<screen>
<prompt>#</prompt> <userinput>ntpq -p</userinput>
<computeroutput> remote refid st t when poll reach delay offset jitter
==============================================================================
*cudns.cit.corne ntp0.usno.navy. 2 u 832 1024 377 43.208 0.361 2.646
LOCAL(0) LOCAL(0) 10 l 13 64 377 0.000 0.000 0.008
</computeroutput>
</screen>
</para>
<para> The <command>ntpdc -c loopinfo</command> will display
how far off the system time is in seconds, based upon the last time
the remote server was contacted.
<screen>
<prompt>#</prompt> <userinput>ntpdc -c loopinfo</userinput>
<computeroutput>offset: -0.004479 s
frequency: 133.625 ppm
poll adjust: 30
watchdog timer: 404 s
</computeroutput>
</para>
<para><command>ntpdc -c kerninfo</command> will display
the current remaining correction.
<screen>
<prompt>#</prompt> <userinput>ntpdc -c kerninfo</userinput>
<computeroutput>pll offset: -0.003917 s
pll frequency: 133.625 ppm
maximum error: 0.391414 s
estimated error: 0.003676 s
status: 0001 pll
pll time constant: 6
precision: 1e-06 s
frequency tolerance: 512 ppm
pps frequency: 0.000 ppm
pps stability: 512.000 ppm
pps jitter: 0.0002 s
calibration interval: 4 s
calibration cycles: 0
jitter exceeded: 0
stability exceeded: 0
calibration errors: 0
</computeroutput>
</para>
<para> A slightly more different version of
<command>ntpdc -c kerninfo</command> is <command>ntptime</command>
<screen>
<prompt>#</prompt> <userinput>ntptime</userinput>
<computeroutput>ntp_gettime() returns code 0 (OK)
time c35e2cc7.879ba000 Thu, Nov 13 2003 11:16:07.529, (.529718),
maximum error 425206 us, estimated error 3676 us
ntp_adjtime() returns code 0 (OK)
modes 0x0 (),
offset -3854.000 us, frequency 133.625 ppm, interval 4 s,
maximum error 425206 us, estimated error 3676 us,
status 0x1 (PLL),
time constant 6, precision 1.000 us, tolerance 512 ppm,
pps frequency 0.000 ppm, stability 512.000 ppm, jitter 200.000 us,
intervals 0, jitter exceeded 0, stability exceeded 0, errors 0.
</computeroutput>
</screen>
</para>
<para> Yet another way to see how well NTP is working is
with the <command>ntpdate -d</command> command. This will
contact an NTP server and determine the time difference
but not change your system's time.
<screen>
<prompt>#</prompt> <userinput>ntpdate -d 132.236.56.250</userinput>
<computeroutput>13 Nov 14:43:17 ntpdate[29631]: ntpdate 4.1.1c-rc1@1.836 Thu Feb 13 12:17:20 EST 2003 (1)
transmit(132.236.56.250)
receive(132.236.56.250)
transmit(132.236.56.250)
receive(132.236.56.250)
transmit(132.236.56.250)
receive(132.236.56.250)
transmit(132.236.56.250)
receive(132.236.56.250)
transmit(132.236.56.250)
server 132.236.56.250, port 123
stratum 2, precision -17, leap 00, trust 000
refid [192.5.41.209], delay 0.06372, dispersion 0.00044
transmitted 4, in filter 4
reference time: c35e5998.4a46cfc8 Thu, Nov 13 2003 14:27:20.290
originate timestamp: c35e5d55.d69a6f82 Thu, Nov 13 2003 14:43:17.838
transmit timestamp: c35e5d55.d16fc9bc Thu, Nov 13 2003 14:43:17.818
filter delay: 0.06522 0.06372 0.06442 0.06442
0.00000 0.00000 0.00000 0.00000
filter offset: 0.000036 0.001020 0.000527 0.000684
0.000000 0.000000 0.000000 0.000000
delay 0.06372, dispersion 0.00044
offset 0.001020
13 Nov 14:43:17 ntpdate[29631]: adjust time server 132.236.56.250 offset 0.001020 sec
</computeroutput>
</screen>
</para>
<para> If you want actually watch the system
synchronize you can use <command>ntptrace</command>.
<screen>
<prompt>#</prompt> <userinput>ntptrace 132.236.56.250</userinput>
<computeroutput>cudns.cit.cornell.edu: stratum 2, offset -0.003278, synch distance 0.02779
truetime.ntp.com: stratum 1, offset -0.014363, synch distance 0.00000, refid 'ACTS'</computeroutput>
</screen>
</para>
<!-- for reference
<screen>
<prompt>#</prompt> <userinput></userinput>
<computeroutput></computeroutput>
</screen>
-->
<para>If you need your system time synchronized immediately
you can use the <command>ntpdate remote-servername</command>
to force a synchronization. No waiting!
<screen>
<prompt>#</prompt> <userinput>ntpdate 132.236.56.250</userinput>
13 Nov 14:56:28 ntpdate[29676]: adjust time server 132.236.56.250 offset -0.003151 sec
<computeroutput></computeroutput>
</screen>
</para>
</sect1>
<sect1 id="ntp-servers">
<title>Some known NTP servers</title>
<para>A list of public NTP servers can be obtained from:
<ulink url="http://www.eecis.udel.edu/~mills/ntp/servers.html/">
http://www.eecis.udel.edu/~mills/ntp/servers.html</ulink>. Please read
the usage information on the page prior so using a server. Not all
servers have the available bandwidth to allow a large number of systems
synchronizing against them. Therefore it is a good idea to contact
a system's administrator prior to using his/her server for NTP services.
</para>
</sect1>
<sect1 id="ntp-links">
<title>NTP Links</title>
<para>More detailed information on NTP can be obtained from the
NTP homepage:<ulink url="http://www.ntp.org/">http://www.ntp.org</ulink>.
</para>
<para>Or from <ulink url="http://www.ntp.org/ntpfaq/NTP-a-faq.htm">
http://www.ntp.org/ntpfaq/NTP-a-faq.htm</ulink></para>
</sect1>
</chapter>

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<chapter id="system-logs">
<title>System Logs --To Be Added</title>
<para>Log info, rotation, monitoring, etc..</para>
</chapter>

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<chapter id="system-updates">
<title>System Updates --To Be Added</title>
<blockquote><para><quote>A lie gets halfway around the world before
the truth has a chance to get its pants on.</quote> Winston Churchill
</para></blockquote>
<para>Discussion on how and when to update the system.</para>
</chapter>

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<chapter id="kernel">
<title>The Linux Kernel Source</title>
<blockquote><para><quote>Black holes are where God divided by zero.
</quote> Steven Wright</para></blockquote>
<para>BASIC info on the kernel source and compiling it. It will
also provide some info on kdb debugger. Refer
to other kernel HOWTO's for more info.</para>
</chapter>

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<!-- Open Source Chapter. Do not release until more work has been done to
it. The goal of this chapter will be to provide an introduction to
the concepts of the "Open Source Community" and encourage others to
participate in their own ways.
<chapter id="open-source">
<title>Open Source Software</title>
<blockquote><para><quote>If you have an apple and I have an apple and we
exchange these apples then you and I will still each have one apple.
But if you have an idea and I have an idea and we exchange these
ideas, then each of us will have two ideas.</quote> G. Bernard Shaw</para>
</blockquote>
<blockquote><para><quote>The basic idea behind open source is very simple:
When programmers can read, redistribute, and modify the source code for a
piece of software, the software evolves. People improve it, people adapt it,
people fix bugs. And this can happen at a speed that, if one is used to the
slow pace of conventional software development, seems astonishing.</quote>
Taken from the Open Source Initiative website <ulink url="www.opensource.com">
http://www.opensource.com/</ulink></para></blockquote>
<para> What does it mean to be Open Source? Open Source means many things,
but primarily it means the sharing of ideas. Creating something and
letting the world learn from it, use it, and change it. Almost everything
created for Linux, including applications, documentation, and the Linux
Operating System itself are released as Open Source. Please remember to
share what you know and learn with others, so that we all can benefit.</para>
</chapter>
-->

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<chapter id="finding-help">
<title>Finding Help</title>
<blockquote><para><quote>Help me if you can I'm feeling down. And I do
appreciate you being 'round.</quote> - The
Beatles</para></blockquote>
<para>Help is out there. You just have to know where to look. With
Linux there are an amazing number of places you can go. There are
mailing lists, IRC channels, web pages with public forums, and many
other resources available. This chapter will try to help you get the
most out of your quest for help.</para>
<sect1 id="newsgroups-mailling-lists">
<title>Newsgroups and Mailing Lists</title>
<para>
This guide cannot teach you everything about Linux. There
just isn't enough space. It is almost inevitable that at some point
you will find something you need to do, that isn't covered in
this (or any other) document at the LDP.
</para>
<para>
One of the nicest things about Linux is the large number of forums
devoted to it. There are forums relating to almost all facets of
Linux ranging from newbie FAQs to in depth kernel development issues.
To receive the most from them, there are a few things you can do.
</para>
<sect2 id="right-forum">
<title>Finding The Right Forum</title>
<para>
The first thing to do is to find an appropriate forum. There are many
newsgroups and mailing lists devoted to Linux, so try to find and use
the one which most closely matches your question. For example, there
isn't much point in you asking a question about sendmail in a forum
devoted to Linux kernel development. At best the people there will
think you are stupid and you will get few responses, at worst you may
receive lots of highly insulting replies (flames). A quick look
through the newsgroups available finds comp.mail.sendmail, which
looks like an appropriate place to ask a sendmail question. Your news
client probably has a list of the newsgroups available to you, but if
not then a full list of newsgroups is available at <ulink
url="http://groups.google.com/groups?group=*">
http://groups.google.com/groups?group=*</ulink>.
</para>
</sect2>
<sect2 id="before-you-post">
<title>Before You Post</title>
<para>
Now that you have found your appropriate forum, you may think you are
ready to post your question. Stop. You aren't ready yet. Have you already
looked for the answer yourself? There are a huge number of HOWTOs and
FAQs available, if any of them relate to the thing you are having a
problem with then <emphasis>read them first</emphasis>. Even if they
don't contain the answer to your problem, what they will do is give you a
better understanding of the subject area, and that understanding will
allow you to ask a more informed and sensible question. There are also archives
of newsgroups and mailing lists and it is entirely possible that your
question has been asked and answered previously. <ulink
url="http://www.google.com">http://www.google.com</ulink> or a similar
search engine should be something you try <emphasis>before</emphasis>
posting a question.
</para>
</sect2>
<sect2 id="writing-your-post">
<title>Writing Your Post</title>
<para>Okay, you have found your appropriate forum, you have read the
relevant HOWTOs and FAQs, you have searched the web, but you still
have not found the answer you need. Now you can start writing your post.
It is always a good idea to make it clear that you already have read
up on the subject by saying something like ``I have read the
Winmodem-HOWTO and the PPP FAQ, but neither contained what I was looking for,
searching for `Winmodem Linux PPP Setup' on google didn't return
anything of use either''. This shows you to be someone who is willing to make
an effort rather than a lazy idiot who requires spoonfeeding. The former
is likely to receive help if anyone knows the answer, the latter
is likely to meet with either stony silence or outright
derision.</para>
<para>Write in clear, grammatical and correctly spelt English. This
is incredibly important. It marks you as a precise and considered
thinker. There are no such words as ``u'' or ``b4.'' Try to make yourself look
like an educated and intelligent person rather than an idiot. It will
help. I promise.</para>
<para>Similarly do not type in all capitals LIKE THIS. That is
considered shouting and looks very rude.</para>
<para>Provide clear details stating what the problem is and what you
have already tried to do to fix it. A question like ``My linux has stopped
working, what can I do?'' is totally useless. Where has it stopped
working? In what way has it stopped working? You need to be as
precise as possible. There are limits however. Try not to include irrelevant
information either. If you are having problems with your mail client
it is unlikely that a dump of your kernel boot log
(<command>dmesg</command>) would be of help.<para>
<para>Don't ask for replies by private email. The point of most Linux
forums is that everybody can learn something from each other. Asking
for private replies simply removes value from the newsgroup or mailing
list.</para>
</sect2>
<sect2 id="formatting-your-post">
<title>Formatting Your Post</title>
<para> Do not post in HTML. Many Linux users have mail clients which
can't easily read HTML email. Whilst with some effort, they
<emphasis>can</emphasis> read HTML email, they usually don't. If you
send them HTML mail it often gets deleted unread. Send plain text
emails, they will reach a wider audience that way.</para>
</sect2>
<sect2 id="follow-up">
<title>Follow Up</title>
<para>After your problem has been solved, post a short followup
explaining what the problem was and how you solved it. People will
appreciate this as it not only gives a sense of closure about the
problem but also helps the next time someone has a similar question. When they
look at the archives of the newsgroup or mailing list, they will see
you had the same problem, the discussion that followed your question and
your final solution.</para>
</sect2>
<sect2 id="getting-help-more-info">
<title>More Information</title>
<para>This short guide is simply a paraphrase
and summary of the excellent (and more detailed) document ``How To
Ask Questions The Smart Way'' by Eric S Raymond. <ulink
url="http://www.catb.org/~esr/faqs/smart-questions.html">
http://www.catb.org/~esr/faqs/smart-questions.html</ulink>. It is
recommend that you read it before you post anything. It will help
you formulate your question to maximize your
chances of getting the answer you are looking for.</para>
</sect2>
</sect1>
<sect1 id="irc">
<title>IRC</title>
<para>IRC (Internet Relay Chat) is not covered in the Eric Raymond
document, but IRC can also be an excellent way of finding the answers you need.
However it does require some practice in asking questions in the right way.
Most IRC networks have busy #linux channels and if the answer to your question
is contained in the man pages, or in the HOWTOs then expect to be told
to go read them. The rule about typing in clear and grammatical English
still applies.</para>
<para>Most of what has been said about newsgroups and mailing lists
is still relevant for IRC, with a the following additions</para>
<sect2 id="colours">
<title>Colours</title>
<para>Do not use colours, bold, underline or strange (non ASCII)
characters. This breaks some older terminals and is just plain ugly
to look at. If you arrive in a channel and start spewing colour or bold
then expect to be kicked out.</para>
</sect2>
<sect2 id="be-polite">
<title>Be Polite</title>
<para>Remember you are not entitled to an answer. If you ask the
question in the right way then you will probably get one, but you have
no right to get one. The people in Linux IRC channels are all there
on their own time, nobody is paying them, especially not you.</para>
<para>Be polite. Treat others as you would like to be
treated. If you think people are not being polite to you then don't
start calling them names or getting annoyed, become even politer.
This makes them look foolish rather than dragging you down to their level.</para>
<para>Don't go slapping anyone with large trouts. Would you believe
this has been done before once or twice? And that we it wasn't
funny the first time?</para>
</sect2>
<sect2 id="type-properly">
<title>Type Properly, in English</title>
<para>Most #linux channels are English channels. Speak English whilst
in them. Most of the larger IRC networks also have #linux channel in
other languages, for example the French language channel might be
called #linuxfr, the Spanish one might be #linuxes or #linuxlatino.
If you can't find the right channel then asking in the main #linux
channel (preferably in English) should help you find the one you are looking
for.</para>
<para>Do not type like a ``1337 H4X0R d00d!!!''. Even if other people
are. It looks silly and thereby makes you look silly. At best you
will only look like an idiot, at worst you will be derided then kicked
out.</para>
</sect2>
<sect2 id="port-scanning">
<title>Port scanning</title>
<para>Never <emphasis>ever</emphasis> ask anyone to port scan you, or
try to ``hack'' you. This is inviolable. There is no way of knowing that
you are who you say you are, or that the IP that you are connected
from belongs to you. Don't put people in the position where they have to
say no to a request like this.</para>
<para><emphasis>Don't ever port scan anyone</emphasis>, even if they
ask you to. You have no way to tell
that they are who they say they are or that the IP they are connected
from is their own IP. In some jurisdictions port scanning may be illegal
and it is certainly against the Terms of Service of most ISPs.
Most people log TCP connections, they will notice they are being
scanned. Most people <emphasis>will</emphasis> report you to your ISP
for this (it is trivial to find out who that is).</para>
</sect2>
<sect2 id="keep-in-channel">
<title>Keep it in the Channel</title>
<para>Don't /msg anyone unless they ask you to. It diminishes the
usefulness of the channel and some people just prefer that
you not do it.</para>
</sect2>
<sect2><title>Stay On Topic</title>
<para>Stay on topic. The channel is a ``Linux'' channel, not a ``What
Uncle Bob Got Up To Last Weekend'' channel. Even if you see other
people being off topic, this does not mean that you should be. They
are probably channel regulars and different conventions apply to
them.</para>
</sect2>
<sect2 id="mass-ctcp">
<title>CTCPs</title>
<para>If you are thinking of mass CTCP pinging the channel or CTCP
version or CTCP anything, then think again. It is liable to get you
kicked out very quickly.</para>
<para>If you are not familiar with IRC, CTCP stands for Client To
Client Protocol. It is a method whereby you can find out things
about other peoples' clients. See the documentation for your IRC
for more details.</para>
</sect2>
<sect2 id="hacking">
<title>Hacking, Cracking, Phreaking, Warezing</title>
<para>Don't ask about exploits, unless you are looking for a further
way to be unceremoniously kicked out.</para>
<para>Don't be in hacker/cracker/phreaker/warezer channels whilst in a
#linux channel. For some reason the people in charge of #linux
channels seem to hate people who like causing destruction to people's machines
or who like to steal software. Can't imagine why.</para>
</sect2>
<sect2 id="round-up">
<title>Round Up</title>
<para>Apologies if that seems like a lot of DON'Ts, and very few DOs.
The DOs were already pretty much covered in the section on newsgroups and
mailing lists.</para>
<para>Probably the best thing you can do is to go into a #linux
channel, sit there and watch, getting the feel for a half hour before
you say anything. This can help you to recognize the correct tone you
should be using.</para>
</sect2>
<sect2 id="further-reading">
<title>Further Reading</title>
<para>There are excellent FAQs about how to get the most of IRC #linux
channels. Most #linux channels have an FAQ and/or set or channel
rules. How to find this will usually be in the channel topic (which you can
see at any time using the <command>/topic</command> command. Make sure
you read the rules if there are any and follow them. One fairly generic
set of rules and advice is the ``Undernet #linux FAQ'' which can be found
at <ulink url="http://linuxfaq.quartz.net.nz">http://linuxfaq.quartz.net.nz
</ulink>.</para>
</sect2>
</sect1>
</chapter>

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<glossary id="glossary">
<title>Glossary (DRAFT, but not for long hopefully)</title>
<blockquote><para><quote>The Librarian of the Unseen University
had unilaterally decided to aid comprehension
by producing an Orang-utan/Human Dictionary.
He'd been working on it for three months.
It wasn't easy. He'd got as far as `Oook.'</quote>
(Terry Pratchett, ``Men At Arms'')</para></blockquote>
<para> This is a short list of word definitions for concepts
relating to Linux and system administration. </para>
<glossentry>
<glossterm>CMOS RAM</glossterm>
<glossdef><para>
CMOS stands for "Complementary Metal Oxide Semiconductor".
It is a complex technology, but put very simply it is a type
of transistor which maintains its state even if computer is
powered off. This is due to a small battery on the motherboard.
As a result, it does not lose what was stored on it when the
power is switched off.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>account</glossterm>
<glossdef><para>
A Unix system gives users <glossterm>accounts</glossterm>. It
gives them a username and a password with which to log on to the
account. A home directory in which to store files is usually
provided, and permissions to access hardware and software. These
things taken as a whole are an <glossterm>account</glossterm>.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>application program</glossterm>
<glossdef><para>
Software that does something useful. The results of using an
application program is what the computer was bought for.
See also system program, operating system.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>bad block</glossterm>
<glossdef><para>
A block (usually one sector on a disk) that cannot reliably hold
data.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>bad sector</glossterm>
<glossdef><para>
Similar to <glossterm>bad block</glossterm> but more precise in
the case where a block and a sector may be of differing sizes.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>boot sector</glossterm>
<glossdef><para>
Usually the first sector on any given partition. It contains
a very short program (on the order of a few hundred bytes) which
will load and start running the operating system proper.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>booting</glossterm>
<glossdef><para>
Everything that happens between the time the computer is
switched on and it is ready to accept commands/input from
the user is known as <glossterm>booting</glossterm>.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>bootstrap loader</glossterm>
<glossdef><para>
A very small program (usually residing in ROM) which reads
a fixed location on a disk (eg. the <glossterm>MBR</glossterm>)
and passes control over to it. The data residing on that fixed
location is, in general, slightly bigger and more sophisticated,
and it then takes responsibility for loading the actual operating
system and passing control to it.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>cylinder</glossterm>
<glossdef><para>
The set of <glossterm>tracks</glossterm> on a multi-headed disk
that may be accessed without head movement. In other words the
tracks which are the same distance from the spindle about which
the disk <glossterm>platters</glossterm> rotate. Placing data
that is more likely to be accessed at the same time on the same
cylinder can reduce the access time significantly as moving the
read-write heads is slow compared to the speed with which the
disks rotate.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>daemon</glossterm>
<glossdef><para>
A process lurking in the background, usually unnoticed, until
something triggers it into action. For example, the
<command>update</command>
daemon wakes up every thirty seconds or so to flush the buffer
cache, and the <command>sendmail</command> daemon awakes whenever
someone sends mail.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>daylight savings time</glossterm>
<glossdef><para>
A time of the year during which clocks are set forward one hour.
Widely used around the world in summer so that evenings have more
daylight than they would otherwise.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>disk controller</glossterm>
<glossdef><para>
A hardware circuit which translates instructions about disk access
from the operating system to the physical disk. This provides a
layer of abstraction that means that an operating system does not
need to know how to talk to the many different types of disks, but
only needs to know about the (comparatively low) number of types of
disk controller. Common disk controller types are IDE and SCSI.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>emergency boot floppy</glossterm>
<glossdef><para>
A floppy disk which can be used to boot the system even
if the hard disk has suffered damage on its filesystem.
Most linux distributions offer to make one of these during
installation, this is highly recommended. If your Linux
distribution does not offer this facility then read the
Boot floppy HOWTO, available at the LDP (**Find URL to cite**).
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>fibre channel</glossterm>
<glossdef><para>
A high speed networking protocol primarily used in Storage
Area Networks. Unlike it's name suggests, fibre channel can be
ran over fiber optic, or copper cables.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>filesystem</glossterm>
<glossdef><para>
A term which is used for two purposes and which can have two
subtly different meanings. It is either the collection of
files and directories on a drive (whether hard drive, floppy,
Cd-ROM, etc). Or it is the markers put onto the disk media
which the OS uses to decide where to write files to (inodes,
blocks, superblocks etc). The actual meaning can almost
always be inferred from context.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>formatting</glossterm>
<glossdef><para>
Strictly, formatting is organizing and marking the surface of
a disk into <glossterm>tracks</glossterm>, <glossterm>sectors
</glossterm>, and <glossterm>cylinders</glossterm>. It is also
sometimes (incorrectly) a term used to signify the action of
writing a <glossterm>filesystem</glossterm> to a disk (especially
in the MS Windows/MS DOS world).
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>fragmented</glossterm>
<glossdef><para>
When a file is not written to a disk in contiguous <glossterm>
blocks</glossterm>. If there is not enough free space to write
a full file to a disk in one continuous stream of <glossterm>
blocks</glossterm> then the file gets split up between two or
more parts of the disk surface. This is known as <glossterm>
fragmenting</glossterm> and can make the time for loading a
file longer as the disk has to seek for the rest of the file.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>full backup</glossterm>
<glossdef><para>
Taking a copy of the whole filesystem to a backup media
(eg tape, floppy, or CD).
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>geometry</glossterm>
<glossdef><para>
How many cylinders, sectors per cylinder and heads a disk
drive has.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>high level formatting</glossterm>
<glossdef><para>
An incorrect term for writing a filesystem to a disk. Often
used in the MS Windows and MS DOS world.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>incremental backups</glossterm>
<glossdef><para>
A backup of what has changed in a filesystem since the last
<glossterm>full backup</glossterm>. <glossterm>Incremental
backups</glossterm> if used sensibly as part of a backup regime,
can save a lot of time and effort in maintaining a backup of data.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>inode</glossterm>
<glossdef><para>
A data structure holding information about files in a Unix
file system. There is an inode for each file and a file is
uniquely identified by the file system on which it resides
and its inode number on that system. Each inode contains
the following information: the device where the inode resides,
locking information, mode and type of file, the number of links
to the file, the owner's user and group ids, the number of bytes
in the file, access and modification times, the time the inode
itself was last modified and the addresses of the file's
blocks on disk. A Unix directory is an association between
file leafnames and inode numbers. A file's inode number
can be found using the "-i" switch to ls.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>iSCSI</glossterm>
<glossdef><para>
A network storage protocol that enables the sending of SCSI commands
over a TCP/IP network. Primarily used in Storage Area Networks.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>kernel</glossterm>
<glossdef><para>
Part of an operating system that implements the interaction with
hardware and the sharing of resources. See also system program.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>local time</glossterm>
<glossdef><para>
The official time in a local region (adjusted for location around
the Earth); established by law or custom.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>logical partition</glossterm>
<glossdef><para>
A partition inside an <glossterm>extended partition</glossterm>,
which is ``logical'' in that it does not exist in reality,
but only inside the logical structure of the software.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>logical volume manager (LVM)</glossterm>
<glossdef><para>
A collection of programs that allow larger physical
disks to be reassembled into "logical" disks that can be shrunk or
expanded as data needs change.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>low level formatting</glossterm>
<glossdef><para>
Synonymous with <glossterm>formatting</glossterm> and used in
the MS DOS world so differentiate from creating a filesystem
which is also known as formatting sometimes.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>mail transfer agent</glossterm>
<glossdef><para>
(MTA) The program responsible for delivering e-mail messages.
Upon receiving a message from a <glossterm>mail user agent
</glossterm> or another MTA it stores it temporarily locally
and analyzes the recipients and either delivers it (local
addressee) or forwards it to another MTA. In either case
it may edit and/or add to the message headers. A widely used
MTA for Unix is sendmail.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>mail user agent</glossterm>
<glossdef><para>
(MUA) The program that allows the user to compose and read
electronic mail messages. The MUA provides the interface
between the user and the <glossterm>mail transfer agent
</glossterm>. Outgoing mail is eventually handed over to an
MTA for delivery while the incoming messages are picked up
from where the MTA left it (although MUAs running on
single-user machines may pick up mail using POP).
Examples of MUAs are pine, elm and mutt.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>master boot record</glossterm>
<glossdef><para>
(MBR) The first logical sector on a disk, this is (usually)
where the BIOS looks to load a small program that will boot
the computer.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>network file system</glossterm>
<glossdef><para>
(NFS) A protocol developed by Sun Microsystems, and defined in
RFC 1094 (FIND URL), which allows a computer to access files
over a network as if they were on its local disks.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>operating system</glossterm>
<glossdef><para>
Software that shares a computer system's resources (processor,
memory, disk space, network bandwidth, and so on) between
users and the application programs they run. Controls access
to the system to provide security. See also kernel, system program,
application program.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>partition</glossterm>
<glossdef><para>
A logical section of a disk. Each partition normally has its
own file system. Unix tends to treat partitions as though
they were separate physical entities.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>password file</glossterm>
<glossdef><para>
A file that holds usernames and information about their accounts
like their password. On Unix systems this file is usually
<filename>/etc/passwd</filename>. On most modern Linux systems
the <filename>/etc/passwd</filename> file does not actually hold
password data. That tends to be held in a different file <filename>
/etc/shadow</filename> for security reasons. See manual pages
passwd(5) and shadow(5) for more information.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>physical extents</glossterm>
<glossdef><para>
A term used to describe a the chunks a physical volume is broken
down into when using the Logical Volume Manager.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>physical volume</glossterm>
<glossdef><para>
A term used an actual disk partition, usually in reference to the
logical volume manager.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>platters</glossterm>
<glossdef><para>
A physical disk inside a hard drive. Usually a hard drive is
made up of multiple physical disks stacked up on top of each
other. One individual disk is known as a <glossterm>platter
</glossterm>.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>power on self test</glossterm>
<glossdef><para>
(POST) A series of diagnostic tests which are run when a computer
is powered on. Typically this might include testing the memory,
testing that the hardware configuration is the same as the last
saved configuration, checking that any floppy drives, or hard
drives which are known about by the BIOS are installed and working.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>print queue</glossterm>
<glossdef><para>
A file (or set of files) which the print <glossterm>daemon
<glossterm> uses so that applications which wish to use the
printer do not have to wait until the print job they have sent
is finished before they can continue. It also allows multiple
users to share a printer.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>read-write head</glossterm>
<glossdef><para>
A tiny electromagnetic coil and metal pole used to write and read
magnetic patterns on a disk. These coils move laterally against
the rotary motion on the <glossterm>platters</glossterm>.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>root filesystem</glossterm>
<glossdef><para>
The parent of all the other filesystems mounted in a Unix filesystem
tree. Mounted as / it might have other filesystems mounted on it
(/usr for example). If the root filesystem cannot be mounted then the
<glossterm>kernel</glossterm> will panic and the system will not be
able to continue <glossterm>booting</glossterm>
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>run level</glossterm>
<glossdef><para>
Linux has up to 10 runlevels (0-9) available (of which usually only
the first 7 are defined). Each runlevel may start a different set
of services, giving multiple different configurations in the same
system. Runlevel 0 is defined as ``system halt'', runlevel 1 is
defined as ``<glossterm>single user mode</glossterm>'', and runlevel
6 is defined as ``reboot system''. The remaining runlevels can,
theoretically, be defined by the system administrator in any way.
However most distributions provide some other predefined runlevels.
For example, runlevel 2 might be defined as ``multi-user console'',
and runlevel 5 as ``multi-user X-Window system''. These definitions
vary considerably from distribution to distribution, so please check
the documentation for your own distribution.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>sectors</glossterm>
<glossdef><para>
The minimum <glossterm>track</glossterm> length that can be
allocated
to store data. This is usually (but not always) 512 bytes.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>shadow passwords</glossterm>
<glossdef><para>
Because the <glossterm>password file</glossterm> on Unix systems
often
needs to be world readable it usually does not actually contain the
encrypted passwords for users' accounts. Instead a shadow file is
employed (which is not world readable) which holds the encrypted
passwords for users' accounts.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>single user mode</glossterm>
<glossdef><para>
Usually runlevel 1. A runlevel where logins are not allowed except
by the root account. Used either for system repairs (if the
filesystem is partially damaged it may still be possible to boot into
runlevel 1 and repair it), or for moving filesystems around between
partitions. These are just two examples. Any task that requires a
system where only one person can write to a disk at a time is a
candidate for requiring runlevel 1.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>spool</glossterm>
<glossdef><para>
To send a file (or other data) to a queue. Generally used in
conjunction with printers, but might also be used for other
things (mail for example). The term is reported to be an acronym
for ``Simultaneous Peripheral Operation On-Line'', but according
to the <ulink url="http://www.tuxedo.org/~esr/jargon">Jargon File
</ulink> it may have been a backronym (something made up later
for effect).
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>system call</glossterm>
<glossdef><para>
The services provided by the kernel to application programs,
and the way in which they are invoked. See section 2 of the
manual pages.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>swap space</glossterm>
<glossdef><para>
Space on a disk in which the system can write portions of memory
to. Usually this is a dedicated partition, but it may also be
a swapfile.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>system program</glossterm>
<glossdef><para>
Programs that implement high level functionality of an operating
system, i.e., things that aren't directly dependent on the
hardware. May sometimes require special privileges to run
(e.g., for delivering electronic mail), but often just commonly
thought of as part of the system (e.g., a compiler). See also
application program, kernel, operating system.
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>time drift</glossterm>
<glossdef><para>
This is a term for a computers inaccuracy at keeping track of time.
All computers have some rate of error when keeping time. With newer
computers this rate of error is extremely small.</para></glossdef>
</glossentry>
<glossentry>
<glossterm>track</glossterm>
<glossdef><para>
The part of a disk <glossterm>platter</glossterm> which passes
under one <glossterm>read-write head</glossterm> while the head
is stationary but the disk is spinning. Each track is divided
into <glossterm>sectors</glossterm>, and a vertical collection of
tracks is a <glossterm>cylinder</glossterm>
</para></glossdef>
</glossentry>
<glossentry>
<glossterm>volume group</glossterm>
<glossdef><para>
A collection of physical volumes broken down into physical
extents, and available for use in logical partitions.
</para></glossdef>
</glossentry
</glossary>

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<para>Copyright 2001--2003 Stephen Stafford.</para>
<para>Copyright 2003--2004 Stephen Stafford & Alex Weeks.</para>
<para>Copyright 2004--Present Alex Weeks.</para>
<para>Trademarks are owned by their owners.</para>
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