path: root/en_US.ISO8859-1/books/handbook/basics/chapter.xml

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<!--
     The FreeBSD Documentation Project

     $FreeBSD$
-->

<chapter id="basics">
  <chapterinfo>
    <authorgroup>
      <author>
	<firstname>Chris</firstname>
	<surname>Shumway</surname>
	<contrib>Rewritten by </contrib>
      </author>
    </authorgroup>
    <!-- 10 Mar 2000 -->
  </chapterinfo>

  <title>UNIX Basics</title>

  <sect1 id="basics-synopsis">
    <title>Synopsis</title>

    <para>This chapter covers the basic commands and functionality of
      the &os; operating system.  Much of this material is relevant
      for any &unix;-like operating system.  New &os; users are
      encouraged to read through this chapter carefully.</para>

    <para>After reading this chapter, you will know:</para>

    <itemizedlist>
      <listitem>
	<para>How to use the <quote>virtual consoles</quote> of
	  &os;.</para>
      </listitem>

      <listitem>
	<para>How &unix; file permissions and &os; file flags
	  work.</para>
      </listitem>

      <listitem>
	<para>The default &os; file system layout.</para>
      </listitem>

      <listitem>
	<para>The &os; disk organization.</para>
      </listitem>

      <listitem>
	<para>How to mount and unmount file systems.</para>
      </listitem>

      <listitem>
	<para>What processes, daemons, and signals are.</para>
      </listitem>

      <listitem>
	<para>What a shell is, and how to change your default login
	  environment.</para>
      </listitem>

      <listitem>
	<para>How to use basic text editors.</para>
      </listitem>

      <listitem>
	<para>What devices and device nodes are.</para>
      </listitem>

      <listitem>
	<para>What binary format is used under &os;.</para>
      </listitem>

      <listitem>
	<para>How to read manual pages for more information.</para>
      </listitem>
    </itemizedlist>
  </sect1>

  <sect1 id="consoles">
    <title>Virtual Consoles and Terminals</title>

    <indexterm><primary>virtual consoles</primary></indexterm>
    <indexterm><primary>terminals</primary></indexterm>

    <para>&os; can be used in various ways.  One of them is typing
      commands to a text terminal.  A lot of the flexibility and power
      of a &unix; operating system is readily available at your hands
      when using &os; this way.  This section describes what
      <quote>terminals</quote> and <quote>consoles</quote> are, and
      how you can use them in &os;.</para>

    <sect2 id="consoles-intro">
      <title>The Console</title>

      <indexterm><primary>console</primary></indexterm>

      <para>Unless &os; has been configured to automatically start
	a graphical environment during startup, the system will boot
	into a command line login prompt, as seen in this
	example:</para>

      <screen>FreeBSD/amd64 (pc3.example.org) (ttyv0)

login:</screen>

      <para>The first line contains some information about the
	system.  The <literal>amd64</literal> indicates that the
	system in this example is running a 64-bit version of &os;.
	The hostname is <hostid>pc3.example.org</hostid>, and
	<devicename>ttyv0</devicename> indicates that this is the
	system console.</para>

      <para>The second line is the login prompt.  The next section
	describes how to log into &os; at this prompt.</para>
    </sect2>

    <sect2 id="consoles-login">
      <title>Logging into &os;</title>

      <para>&os; is a multiuser, multiprocessing system.  This is
	the formal description that is usually given to a system that
	can be used by many different people, who simultaneously run a
	lot of programs on a single machine.</para>

      <para>Every multiuser system needs some way to distinguish one
	<quote>user</quote> from the rest.  In &os; (and all the
	&unix;-like operating systems), this is accomplished by
	requiring that every user must <quote>log into</quote> the
	system before being able to run programs.  Every user has a
	unique name (the <quote>username</quote>) and a personal,
	secret key (the <quote>password</quote>).  &os; will ask
	for these two before allowing a user to run any
	programs.</para>

      <indexterm><primary>startup scripts</primary></indexterm>
      <para>When a &os; system boots, startup scripts are
	automatically executed in order to prepare the system and to
	start any services which have been configured to start at
	system boot.  Once the system finishes running its startup
	scripts, it will present a login prompt:</para>

      <screen>login:</screen>

      <para>Type the username that was configured during <link
	  linkend="bsdinstall-addusers">system installation</link> and
	press <keycap>Enter</keycap>.  Then enter the password
	associated with the username and press <keycap>Enter</keycap>.
	The password is <emphasis>not echoed</emphasis> for security
	reasons.</para>

      <para>Once the correct password is input, the message of
	the day (<acronym>MOTD</acronym>) will be displayed followed
	by a command prompt (a <literal>#</literal>,
	<literal>$</literal>, or <literal>%</literal> character).  You
	are now logged into the &os; console and ready to try the
	available commands.</para>
    </sect2>

    <sect2 id="consoles-virtual">
      <title>Virtual Consoles</title>

      <para>&os; can be configured to provide many virtual consoles
	for inputting commands.  Each virtual console has its own
	login prompt and output channel, and &os; takes care of
	properly redirecting keyboard input and monitor output as you
	switch between virtual consoles.</para>

      <para>Special key combinations have been reserved by &os; for
	switching consoles.<footnote>
	  <para>Refer to &man.syscons.4;, &man.atkbd.4;,
	    &man.vidcontrol.1; and &man.kbdcontrol.1; for a more
	    technical description of the &os; console and its keyboard
	    drivers.</para></footnote>.  Use
	<keycombo><keycap>Alt</keycap><keycap>F1</keycap></keycombo>,
	<keycombo><keycap>Alt</keycap><keycap>F2</keycap></keycombo>,
	through
	<keycombo><keycap>Alt</keycap><keycap>F8</keycap></keycombo>
	to switch to a different virtual console in &os;.</para>

      <para>When switching from one console to the next, &os; takes
	care of saving and restoring the screen output.  The result is
	an <quote>illusion</quote> of having multiple
	<quote>virtual</quote> screens and keyboards that can be used
	to type commands for &os; to run.  The programs that are
	launched in one virtual console do not stop running when that
	console is not visible because the user has switched to a
	different virtual console.</para>
    </sect2>

    <sect2 id="consoles-ttys">
      <title>The <filename>/etc/ttys</filename> File</title>

      <para>By default, &os; is configured to start eight virtual
	consoles.  The configuration can be customized to start
	more or fewer virtual consoles.  To change the number of and
	the settings of the virtual consoles, edit
	<filename>/etc/ttys</filename>.</para>

      <para>Each uncommented line in <filename>/etc/ttys</filename>
	(lines that do not start with a <literal>#</literal>
	character) contains settings for a single terminal or virtual
	console.  The default version configures nine virtual
	consoles, and enables eight of them.  They are the lines that
	start with <literal>ttyv</literal>:</para>

      <programlisting># name    getty                         type  status comments
#
ttyv0   "/usr/libexec/getty Pc"         cons25  on  secure
# Virtual terminals
ttyv1   "/usr/libexec/getty Pc"         cons25  on  secure
ttyv2   "/usr/libexec/getty Pc"         cons25  on  secure
ttyv3   "/usr/libexec/getty Pc"         cons25  on  secure
ttyv4   "/usr/libexec/getty Pc"         cons25  on  secure
ttyv5   "/usr/libexec/getty Pc"         cons25  on  secure
ttyv6   "/usr/libexec/getty Pc"         cons25  on  secure
ttyv7   "/usr/libexec/getty Pc"         cons25  on  secure
ttyv8   "/usr/X11R6/bin/xdm -nodaemon"  xterm   off secure</programlisting>

      <para>For a detailed description of every column in this file
	and the available options for the virtual consoles, refer to
	&man.ttys.5;.</para>
    </sect2>

    <sect2 id="consoles-singleuser">
      <title>Single User Mode Console</title>

      <para>A detailed description of <quote>single user mode</quote>
	can be found <link linkend="boot-singleuser">here</link>.
	There is only one console when &os; is in single user mode as
	no other virtual consoles are available in this mode.  The
	settings for single user mode are found in this section of
	<filename>/etc/ttys</filename>:</para>

      <programlisting># name  getty                           type  status  comments
#
# If console is marked "insecure", then init will ask for the root password
# when going to single-user mode.
console none                            unknown  off  secure</programlisting>

      <note>
	<para>As the comments above the <literal>console</literal>
	  line indicate, editing <literal>secure</literal> to
	  <literal>insecure</literal> will prompt for the
	  <username>root</username> password when booting into single
	  user mode.  The default setting enters single user mode
	  without prompting for a password.</para>

	<para><emphasis>Be careful when changing this setting to
	    <literal>insecure</literal></emphasis>.  If you ever
	  forget the <username>root</username> password, booting into
	  single user mode is still possible, but may be difficult for
	  someone who is not comfortable with the &os; booting
	  process.</para>
      </note>
    </sect2>

    <sect2 id="consoles-vidcontrol">
      <title>Changing Console Video Modes</title>

      <para>The &os; console default video mode may be adjusted to
	1024x768, 1280x1024, or any other size supported by the
	graphics chip and monitor.  To use a different video mode
	load the <literal>VESA</literal> module:</para>

      <screen>&prompt.root; <userinput>kldload vesa</userinput></screen>

      <para>To determine which video modes are supported by the
	hardware, use &man.vidcontrol.1;.  To get a list of supported
	video modes issue the following:</para>

      <screen>&prompt.root; <userinput>vidcontrol -i mode</userinput></screen>

      <para>The output of this command lists the video modes that
	are supported by the hardware.  To select a new video mode,
	specify the mode using &man.vidcontrol.1; as the
	<username>root</username> user:</para>

      <screen>&prompt.root; <userinput>vidcontrol MODE_279</userinput></screen>

      <para>If the new video mode is acceptable, it can be permanently
	set on boot by adding it to
	<filename>/etc/rc.conf</filename>:</para>

      <programlisting>allscreens_flags="MODE_279"</programlisting>
    </sect2>
  </sect1>

  <sect1 id="permissions">
    <title>Permissions</title>

    <indexterm><primary>UNIX</primary></indexterm>

    <para>&os;, being a direct descendant of BSD &unix;, is based
      on several key &unix; concepts.  The first and most pronounced
      is that &os; is a multi-user operating system that can handle
      several users working simultaneously on completely unrelated
      tasks.  The system is responsible for properly sharing and
      managing requests for hardware devices, peripherals, memory, and
      CPU time fairly to each user.</para>

    <para>Because the system is capable of supporting multiple users,
      everything the system manages has a set of permissions governing
      who can read, write, and execute the resource.  These
      permissions are stored as three octets broken into three pieces,
      one for the owner of the file, one for the group that the file
      belongs to, and one for everyone else.  This numerical
      representation works like this:</para>

    <note>
      <para>This section will discuss the traditional &unix;
	permissions.  For finer grained file system access
	control, see the <link linkend="fs-acl">File System
	Access Control Lists</link> section.</para>
    </note>

    <indexterm><primary>permissions</primary></indexterm>
    <indexterm>
      <primary>file permissions</primary>
    </indexterm>
    <informaltable frame="none" pgwide="1">
      <tgroup cols="3">
	<thead>
	  <row>
	    <entry>Value</entry>
	    <entry>Permission</entry>
	    <entry>Directory Listing</entry>
	  </row>
	</thead>

	<tbody>
	  <row>
	    <entry>0</entry>
	    <entry>No read, no write, no execute</entry>
	    <entry><literal>---</literal></entry>
	  </row>

	  <row>
	    <entry>1</entry>
	    <entry>No read, no write, execute</entry>
	    <entry><literal>--x</literal></entry>
	  </row>

	  <row>
	    <entry>2</entry>
	    <entry>No read, write, no execute</entry>
	    <entry><literal>-w-</literal></entry>
	  </row>

	  <row>
	    <entry>3</entry>
	    <entry>No read, write, execute</entry>
	    <entry><literal>-wx</literal></entry>
	  </row>

	  <row>
	    <entry>4</entry>
	    <entry>Read, no write, no execute</entry>
	    <entry><literal>r--</literal></entry>
	  </row>

	  <row>
	    <entry>5</entry>
	    <entry>Read, no write, execute</entry>
	    <entry><literal>r-x</literal></entry>
	  </row>

	  <row>
	    <entry>6</entry>
	    <entry>Read, write, no execute</entry>
	    <entry><literal>rw-</literal></entry>
	  </row>

	  <row>
	    <entry>7</entry>
	    <entry>Read, write, execute</entry>
	    <entry><literal>rwx</literal></entry>
	  </row>
	</tbody>
      </tgroup>
    </informaltable>
    <indexterm>
      <primary><command>ls</command></primary>
    </indexterm>
    <indexterm><primary>directories</primary></indexterm>

    <para>Use the <option>-l</option> argument to &man.ls.1; to view a
      long directory listing that includes a column of information
      about a file's permissions for the owner, group, and everyone
      else.  For example, a <command>ls -l</command> in an arbitrary
      directory may show:</para>

    <screen>&prompt.user; <userinput>ls -l</userinput>
total 530
-rw-r--r--  1 root  wheel     512 Sep  5 12:31 myfile
-rw-r--r--  1 root  wheel     512 Sep  5 12:31 otherfile
-rw-r--r--  1 root  wheel    7680 Sep  5 12:31 email.txt</screen>

    <para>The first (leftmost) character in the first column indicates
      whether this file is a regular file, a directory, a special
      character device, a socket, or any other special pseudo-file
      device.  In this example, the <literal>-</literal> indicates a
      regular file.  The next three characters, <literal>rw-</literal>
      in this example, give the permissions for the owner of the file.
      The next three characters, <literal>r--</literal>, give the
      permissions for the group that the file belongs to.  The final
      three characters, <literal>r--</literal>, give the permissions
      for the rest of the world.  A dash means that the permission is
      turned off.  In this example, the permissions are set so the
      owner can read and write to the file, the group can read the
      file, and the rest of the world can only read the file.
      According to the table above, the permissions for this file
      would be <literal>644</literal>, where each digit represents the
      three parts of the file's permission.</para>

    <para>How does the system control permissions on devices? &os;
      treats most hardware devices as a file that programs can open,
      read, and write data to.  These special device files are
      stored in <filename class="directory">/dev/</filename>.</para>

    <para>Directories are also treated as files.  They have read,
      write, and execute permissions.  The executable bit for a
      directory has a slightly different meaning than that of files.
      When a directory is marked executable, it means it is possible
      to change into that directory using
      <application>cd</application>.  This also means that it is
      possible to access the files within that directory, subject to
      the permissions on the files themselves.</para>

    <para>In order to perform a directory listing, the read permission
      must be set on the directory.  In order to delete a file that
      one knows the name of, it is necessary to have write
      <emphasis>and</emphasis> execute permissions to the directory
      containing the file.</para>

    <para>There are more permission bits, but they are primarily used
      in special circumstances such as setuid binaries and sticky
      directories.  For more information on file permissions and how
      to set them, refer to &man.chmod.1;.</para>

    <sect2>
      <sect2info>
	<authorgroup>
	  <author>
	    <firstname>Tom</firstname>
	    <surname>Rhodes</surname>
	    <contrib>Contributed by </contrib>
	  </author>
	</authorgroup>
      </sect2info>

      <title>Symbolic Permissions</title>

      <indexterm>
	<primary>permissions</primary>
	<secondary>symbolic</secondary>
      </indexterm>

      <para>Symbolic permissions use characters instead of octal
	values to assign permissions to files or directories.
	Symbolic permissions use the syntax of (who) (action)
	(permissions), where the following values are
	available:</para>

      <informaltable frame="none" pgwide="1">
	<tgroup cols="3">
	  <thead>
	    <row>
	      <entry>Option</entry>
	      <entry>Letter</entry>
	      <entry>Represents</entry>
	    </row>
	  </thead>

	  <tbody>
	    <row>
	      <entry>(who)</entry>
	      <entry>u</entry>
	      <entry>User</entry>
	    </row>

	    <row>
	      <entry>(who)</entry>
	      <entry>g</entry>
	      <entry>Group owner</entry>
	    </row>

	    <row>
	      <entry>(who)</entry>
	      <entry>o</entry>
	      <entry>Other</entry>
	    </row>

	    <row>
	      <entry>(who)</entry>
	      <entry>a</entry>
	      <entry>All (<quote>world</quote>)</entry>
	    </row>

	    <row>
	      <entry>(action)</entry>
	      <entry>+</entry>
	      <entry>Adding permissions</entry>
	    </row>

	    <row>
	      <entry>(action)</entry>
	      <entry>-</entry>
	      <entry>Removing permissions</entry>
	    </row>

	    <row>
	      <entry>(action)</entry>
	      <entry>=</entry>
	      <entry>Explicitly set permissions</entry>
	    </row>

	    <row>
	      <entry>(permissions)</entry>
	      <entry>r</entry>
	      <entry>Read</entry>
	    </row>

	    <row>
	      <entry>(permissions)</entry>
	      <entry>w</entry>
	      <entry>Write</entry>
	    </row>

	    <row>
	      <entry>(permissions)</entry>
	      <entry>x</entry>
	      <entry>Execute</entry>
	    </row>

	    <row>
	      <entry>(permissions)</entry>
	      <entry>t</entry>
	      <entry>Sticky bit</entry>
	    </row>

	    <row>
	      <entry>(permissions)</entry>
	      <entry>s</entry>
	      <entry>Set UID or GID</entry>
	    </row>
	  </tbody>
	</tgroup>
      </informaltable>

      <para>These values are used with &man.chmod.1;, but with
	letters instead of numbers.  For example, the following
	command would block other users from accessing
	<replaceable>FILE</replaceable>:</para>

      <screen>&prompt.user; <userinput>chmod go= FILE</userinput></screen>

      <para>A comma separated list can be provided when more than one
	set of changes to a file must be made.  For example, the
	following command removes the group and
	<quote>world</quote> write permission on
	<replaceable>FILE</replaceable>, and adds the execute
	permissions for everyone:</para>

      <screen>&prompt.user; <userinput>chmod go-w,a+x <replaceable>FILE</replaceable></userinput></screen>

<!--
      <para>Most users will not notice this, but it should be pointed
	out that using the octal method will only set or assign
	permissions to a file; it does not add or delete them.</para>
-->
    </sect2>

    <sect2>
      <sect2info>
	<authorgroup>
	  <author>
	    <firstname>Tom</firstname>
	    <surname>Rhodes</surname>
	    <contrib>Contributed by </contrib>
	  </author>
	</authorgroup>
      </sect2info>

      <title>&os; File Flags</title>

      <para>In addition to file permissions, &os; supports the use of
	<quote>file flags</quote>.  These flags add an additional
	level of security and control over files, but not
	directories.  With file flags, even
	<username>root</username> can be prevented from removing or
	altering files.</para>

      <para>File flags are modified using &man.chflags.1;.  For
	example, to enable the system undeletable flag on the file
	<filename>file1</filename>, issue the following
	command:</para>

      <screen>&prompt.root; <userinput>chflags sunlink <filename>file1</filename></userinput></screen>

      <para>To disable the system undeletable flag, put a
	<quote>no</quote> in front of the
	<option>sunlink</option>:</para>

      <screen>&prompt.root; <userinput>chflags nosunlink <filename>file1</filename></userinput></screen>

      <para>To view the flags of a file, use <option>-lo</option> with
	&man.ls.1;:</para>

      <screen>&prompt.root; <userinput>ls -lo <filename>file1</filename></userinput></screen>

      <programlisting>-rw-r--r--  1 trhodes  trhodes  sunlnk 0 Mar  1 05:54 file1</programlisting>

      <para>Several file flags may only be added or removed by the
	<username>root</username> user.  In other cases, the file
	owner may set its file flags.  Refer to &man.chflags.1; and
	&man.chflags.2; for more information.</para>
    </sect2>

    <sect2>
      <sect2info>
	<authorgroup>
	  <author>
	    <firstname>Tom</firstname>
	    <surname>Rhodes</surname>
	    <contrib>Contributed by </contrib>
	  </author>
	</authorgroup>
      </sect2info>

      <title>The <literal>setuid</literal>, <literal>setgid</literal>,
	and <literal>sticky</literal> Permissions</title>

      <para>Other than the permissions already discussed, there are
	three other specific settings that all administrators should
	know about.  They are the <literal>setuid</literal>,
	<literal>setgid</literal>, and <literal>sticky</literal>
	permissions.</para>

      <para>These settings are important for some &unix; operations
	as they provide functionality not normally granted to normal
	users.  To understand them, the difference between the real
	user ID and effective user ID must be noted.</para>

      <para>The real user ID is the <acronym>UID</acronym> who owns
	or starts the process.  The effective <acronym>UID</acronym>
	is the user ID the process runs as.  As an example,
	&man.passwd.1; runs with the real user ID when a user changes
	their password.  However, in order to update the password
	database, the command runs as the effective ID of the
	<username>root</username> user.  This allows users to change
	their passwords without seeing a
	<errorname>Permission Denied</errorname> error.</para>

      <para>The setuid permission may be set by prefixing a permission
	set with the number four (4) as shown in the following
	example:</para>

      <screen>&prompt.root; <userinput>chmod 4755 suidexample.sh</userinput></screen>

      <para>The permissions on
	<filename><replaceable>suidexample.sh</replaceable></filename>
	now look like the following:</para>

      <programlisting>-rwsr-xr-x   1 trhodes  trhodes    63 Aug 29 06:36 suidexample.sh</programlisting>

      <para>Note that a <literal>s</literal> is now part of the
	permission set designated for the file owner, replacing the
	executable bit.  This allows utilities which need elevated
	permissions, such as <command>passwd</command>.</para>

      <note>
	<para>The <literal>nosuid</literal> &man.mount.8; option will
	  cause such binaries to silently fail without alerting
	  the user.  That option is not completely reliable as a
	  <literal>nosuid</literal> wrapper may be able to circumvent
	  it.</para>
      </note>

      <para>To view this in real time, open two terminals.  On
	one, start the <command>passwd</command> process as a normal
	user.  While it waits for a new password, check the process
	table and look at the user information for
	<command>passwd</command>:</para>

      <para>In terminal A:</para>

      <screen>Changing local password for trhodes
Old Password:</screen>

      <para>In terminal B:</para>

      <screen>&prompt.root; <userinput>ps aux | grep passwd</userinput></screen>

      <screen>trhodes  5232  0.0  0.2  3420  1608   0  R+    2:10AM   0:00.00 grep passwd
root     5211  0.0  0.2  3620  1724   2  I+    2:09AM   0:00.01 passwd</screen>

      <para>As stated above, the <command>passwd</command> is run
	by a normal user, but is using the effective
	<acronym>UID</acronym> of <username>root</username>.</para>

      <para>The <literal>setgid</literal> permission performs the
	same function as the <literal>setuid</literal> permission;
	except that it alters the group settings.  When an application
	or utility executes with this setting, it will be granted the
	permissions based on the group that owns the file, not the
	user who started the process.</para>

      <para>To set the <literal>setgid</literal> permission on a
	file, provide <command>chmod</command> with a leading two
	(2):</para>

      <screen>&prompt.root; <userinput>chmod 2755 sgidexample.sh</userinput></screen>

      <para>In the following listing, notice that the
	<literal>s</literal> is now in the field designated for the
	group permission settings:</para>

      <screen>-rwxr-sr-x   1 trhodes  trhodes    44 Aug 31 01:49 sgidexample.sh</screen>

      <note>
	<para>In these examples, even though the shell script in
	  question is an executable file, it will not run with
	  a different <acronym>EUID</acronym> or effective user ID.
	  This is because shell scripts may not access the
	  &man.setuid.2; system calls.</para>
      </note>

      <para>The <literal>setuid</literal> and
	<literal>setgid</literal> permission bits may lower system
	security, by allowing for elevated permissions.  The third
	special permission, the <literal>sticky bit</literal>, can
	strengthen the security of a system.</para>

      <para>When the <literal>sticky bit</literal> is set on a
	directory, it allows file deletion only by the file owner.
	This is useful to prevent file deletion in public directories,
	such as <filename class="directory">/tmp</filename>, by users
	who do not own the file.  To utilize this permission, prefix
	the permission set with a one (1):</para>

      <screen>&prompt.root; <userinput>chmod 1777 /tmp</userinput></screen>

      <para>The <literal>sticky bit</literal> permission will display
	as a <literal>t</literal> at the very end of the permission
	set:</para>

      <screen>&prompt.root; <userinput>ls -al / | grep tmp</userinput></screen>

      <screen>drwxrwxrwt  10 root  wheel         512 Aug 31 01:49 tmp</screen>

    </sect2>
  </sect1>

  <sect1 id="dirstructure">
    <title>Directory Structure</title>

    <indexterm><primary>directory hierarchy</primary></indexterm>

    <para>The &os; directory hierarchy is fundamental to obtaining
      an overall understanding of the system.  The most important
      directory is root or, <quote>/</quote>.  This directory is the
      first one mounted at boot time and it contains the base system
      necessary to prepare the operating system for multi-user
      operation.  The root directory also contains mount points for
      other file systems that are mounted during the transition to
      multi-user operation.</para>

    <para>A mount point is a directory where additional file systems
      can be grafted onto a parent file system (usually the root file
      system).  This is further described in <xref
	linkend="disk-organization"/>.  Standard mount points
      include <filename class="directory">/usr/</filename>,
      <filename class="directory">/var/</filename>,
      <filename class="directory">/tmp/</filename>,
      <filename class="directory">/mnt/</filename>, and
      <filename class="directory">/cdrom/</filename>.  These
      directories are usually referenced to entries in
      <filename>/etc/fstab</filename>.  This file is a table of
      various file systems and mount points and is read by the system.
      Most of the file systems in <filename>/etc/fstab</filename> are
      mounted automatically at boot time from the script &man.rc.8;
      unless their entry includes <option>noauto</option>.  Details
      can be found in <xref linkend="disks-fstab"/>.</para>

    <para>A complete description of the file system hierarchy is
      available in &man.hier.7;.  The following table provides a brief
      overview of the most common directories.</para>

    <para>
      <informaltable frame="none" pgwide="1">
	<tgroup cols="2">
	  <thead>
	    <row>
	      <entry>Directory</entry>
	      <entry>Description</entry>
	    </row>
	  </thead>
	  <tbody valign="top">
	    <row>
	      <entry><filename class="directory">/</filename></entry>
	      <entry>Root directory of the file system.</entry>
	    </row>

	    <row>
	      <entry><filename
		  class="directory">/bin/</filename></entry>
	      <entry>User utilities fundamental to both single-user
		and multi-user environments.</entry>
	    </row>

	    <row>
	      <entry><filename
		  class="directory">/boot/</filename></entry>
	      <entry>Programs and configuration files used during
		operating system bootstrap.</entry>
	    </row>

	    <row>
	      <entry><filename
		  class="directory">/boot/defaults/</filename></entry>
	      <entry>Default boot configuration files.  Refer to
		&man.loader.conf.5; for details.</entry>
	    </row>

	    <row>
	      <entry><filename
		  class="directory">/dev/</filename></entry>
	      <entry>Device nodes.  Refer to &man.intro.4; for
		details.</entry>
	    </row>

	    <row>
	      <entry><filename
		  class="directory">/etc/</filename></entry>
	      <entry>System configuration files and scripts.</entry>
	    </row>

	    <row>
	      <entry><filename
		  class="directory">/etc/defaults/</filename></entry>
	      <entry>Default system configuration files.  Refer to
		&man.rc.8; for details.</entry>
	    </row>

	    <row>
	      <entry><filename
		  class="directory">/etc/mail/</filename></entry>
	      <entry>Configuration files for mail transport agents
		such as &man.sendmail.8;.</entry>
	    </row>

	    <row>
	      <entry><filename
		  class="directory">/etc/namedb/</filename></entry>
	      <entry><command>named</command> configuration files.
		Refer to &man.named.8; for details.</entry>
	    </row>

	    <row>
	      <entry><filename
		  class="directory">/etc/periodic/</filename></entry>
	      <entry>Scripts that run daily, weekly, and monthly,
		via &man.cron.8;.  Refer to &man.periodic.8; for
		details.</entry>
	    </row>

	    <row>
	      <entry><filename
		  class="directory">/etc/ppp/</filename></entry>
	      <entry><command>ppp</command> configuration files as
		described in &man.ppp.8;.</entry>
	    </row>

	    <row>
	      <entry><filename
		  class="directory">/mnt/</filename></entry>
	      <entry>Empty directory commonly used by system
		administrators as a temporary mount point.</entry>
	    </row>

	    <row>
	      <entry><filename
		  class="directory">/proc/</filename></entry>
	      <entry>Process file system.  Refer to &man.procfs.5;,
		&man.mount.procfs.8; for details.</entry>
	    </row>

	    <row>
	      <entry><filename
		  class="directory">/rescue/</filename></entry>
	      <entry>Statically linked programs for emergency
		recovery as described in &man.rescue.8;.</entry>
	    </row>

	    <row>
	      <entry><filename
		  class="directory">/root/</filename></entry>
	      <entry>Home directory for the <username>root</username>
		account.</entry>
	    </row>

	    <row>
	      <entry><filename
		  class="directory">/sbin/</filename></entry>
	      <entry>System programs and administration utilities
		fundamental to both single-user and multi-user
		environments.</entry>
	    </row>

	    <row>
	      <entry><filename
		  class="directory">/tmp/</filename></entry>
	      <entry>Temporary files which are usually
		<emphasis>not</emphasis> preserved across a system
		reboot.  A memory-based file system is often mounted
		at <filename class="directory">/tmp</filename>.  This
		can be automated using the tmpmfs-related variables of
		&man.rc.conf.5; or with an entry in
		<filename>/etc/fstab</filename>; refer to
		&man.mdmfs.8; for details.</entry>
	    </row>

	    <row>
	      <entry><filename
		  class="directory">/usr/</filename></entry>
	      <entry>The majority of user utilities and
		applications.</entry>
	    </row>

	    <row>
	      <entry><filename
		  class="directory">/usr/bin/</filename></entry>
	      <entry>Common utilities, programming tools, and
		applications.</entry>
	    </row>

	    <row>
	      <entry><filename
		  class="directory">/usr/include/</filename></entry>
	      <entry>Standard C include files.</entry>
	    </row>

	    <row>
	      <entry><filename
		  class="directory">/usr/lib/</filename></entry>
	      <entry>Archive libraries.</entry>
	    </row>


	    <row>
	      <entry><filename
		  class="directory">/usr/libdata/</filename></entry>
	      <entry>Miscellaneous utility data files.</entry>
	    </row>

	    <row>
	      <entry><filename
		  class="directory">/usr/libexec/</filename></entry>
	      <entry>System daemons and system utilities executed
		by other programs.</entry>
	    </row>

	    <row>
	      <entry><filename
		  class="directory">/usr/local/</filename></entry>
	      <entry>Local executables and libraries.  Also used as
		the default destination for the &os; ports
		framework.  Within
		<filename class="directory">/usr/local</filename>, the
		general layout sketched out by &man.hier.7; for
		<filename class="directory">/usr</filename> should be
		used.  Exceptions are the man directory, which is
		directly under
		<filename class="directory">/usr/local</filename>
		rather than under
		<filename class="directory">/usr/local/share</filename>,
		and the ports documentation is in
		<filename class="directory">share/doc/<replaceable>port</replaceable></filename>.</entry>
	    </row>

	    <row>
	      <entry><filename
		  class="directory">/usr/obj/</filename></entry>
	      <entry>Architecture-specific target tree produced by
		building the
		<filename class="directory">/usr/src</filename>
		tree.</entry>
	    </row>

	    <row>
	      <entry><filename
		  class="directory">/usr/ports/</filename></entry>
	      <entry>The &os; Ports Collection (optional).</entry>
	    </row>

	    <row>
	      <entry><filename
		  class="directory">/usr/sbin/</filename></entry>
	      <entry>System daemons and system utilities executed
		by users.</entry>
	    </row>

	    <row>
	      <entry><filename
		  class="directory">/usr/share/</filename></entry>
	      <entry>Architecture-independent files.</entry>
	    </row>

	    <row>
	      <entry><filename
		  class="directory">/usr/src/</filename></entry>
	      <entry>BSD and/or local source files.</entry>
	    </row>

	    <row>
	      <entry><filename
		  class="directory">/var/</filename></entry>
	      <entry>Multi-purpose log, temporary, transient, and
		spool files.  A memory-based file system is sometimes
		mounted at <filename
		  class="directory">/var</filename>. This can be
		automated using the varmfs-related variables in
		&man.rc.conf.5; or with an entry in
		<filename>/etc/fstab</filename>; refer to
		&man.mdmfs.8; for details.</entry>
	    </row>

	    <row>
	      <entry><filename
		  class="directory">/var/log/</filename></entry>
	      <entry>Miscellaneous system log files.</entry>
	    </row>

	    <row>
	      <entry><filename
		  class="directory">/var/mail/</filename></entry>
	      <entry>User mailbox files.</entry>
	    </row>

	    <row>
	      <entry><filename
		  class="directory">/var/spool/</filename></entry>
	      <entry>Miscellaneous printer and mail system spooling
		directories.</entry>
	    </row>

	    <row>
	      <entry><filename
		  class="directory">/var/tmp/</filename></entry>
	      <entry>Temporary files which are usually preserved
		across a system reboot, unless
		<filename class="directory">/var</filename> is a
		memory-based file system.</entry>
	    </row>

	    <row>
	      <entry><filename
		  class="directory">/var/yp/</filename></entry>
	      <entry>NIS maps.</entry>
	    </row>
	  </tbody>
	</tgroup>
      </informaltable></para>
  </sect1>

  <sect1 id="disk-organization">
    <title>Disk Organization</title>

    <para>The smallest unit of organization that &os; uses to find
      files is the filename.  Filenames are case-sensitive, which
      means that <filename>readme.txt</filename> and
      <filename>README.TXT</filename> are two separate files.  &os;
      does not use the extension of a file to determine whether the
      file is a program, document, or some other form of data.</para>

    <para>Files are stored in directories.  A directory may contain no
      files, or it may contain many hundreds of files.  A directory
      can also contain other directories, allowing you to build up a
      hierarchy of directories within one another in order to organize
      data.</para>

    <para>Files and directories are referenced by giving the file or
      directory name, followed by a forward slash,
      <literal>/</literal>, followed by any other directory names that
      are necessary.  For example, if the directory
      <filename class="directory">foo</filename> contains a directory
      <filename class="directory">bar</filename> which contains the
      file <filename>readme.txt</filename>, the full name, or
      <firstterm>path</firstterm>, to the file is
      <filename>foo/bar/readme.txt</filename>.  Note that this is
      different from &windows; which uses
      <literal>\</literal> to separate file and directory
      names.  &os; does not use drive letters, or other drive names in
      the path.  For example, you would not type
      <filename>c:/foo/bar/readme.txt</filename> on &os;.</para>

    <para>Directories and files are stored in a file system.  Each
      file system contains exactly one directory at the very top
      level, called the <firstterm>root directory</firstterm> for that
      file system.  This root directory can contain other
      directories.  One file system is designated the
      <firstterm>root file system</firstterm> or <literal>/</literal>.
      Every other file system is <firstterm>mounted</firstterm> under
      the root file system.  No matter how many disks you have on your
      &os; system, every directory appears to be part of the same
      disk.</para>

    <para>Suppose you have three file systems, called
      <literal>A</literal>, <literal>B</literal>, and
      <literal>C</literal>.  Each file system has one root directory,
      which contains two other directories, called
      <literal>A1</literal>, <literal>A2</literal> (and likewise
      <literal>B1</literal>, <literal>B2</literal> and
      <literal>C1</literal>, <literal>C2</literal>).</para>

    <para>Call <literal>A</literal> the root file system.  If you used
      <command>ls</command> to view the contents of this directory you
      would see two subdirectories, <literal>A1</literal> and
      <literal>A2</literal>.  The directory tree looks like
      this:</para>

    <mediaobject>
      <imageobject>
	<imagedata fileref="install/example-dir1"/>
      </imageobject>

      <textobject>
	<literallayout class="monospaced"> /
 |
 +--- A1
 |
 `--- A2</literallayout>
      </textobject>
    </mediaobject>

    <para>A file system must be mounted on to a directory in another
      file system.  When mounting file system
      <literal>B</literal> on to the directory <literal>A1</literal>,
      the root directory of <literal>B</literal> replaces
      <literal>A1</literal>, and the directories in
      <literal>B</literal> appear accordingly:</para>

    <mediaobject>
      <imageobject>
	<imagedata fileref="install/example-dir2"/>
      </imageobject>

      <textobject>
	<literallayout class="monospaced"> /
 |
 +--- A1
 |     |
 |     +--- B1
 |     |
 |     `--- B2
 |
 `--- A2</literallayout>
      </textobject>
    </mediaobject>

    <para>Any files that are in the <literal>B1</literal> or
      <literal>B2</literal> directories can be reached with the path
      <filename class="directory">/A1/B1</filename> or
      <filename class="directory">/A1/B2</filename> as
      necessary.  Any files that were in
      <filename class="directory">/A1</filename> have
      been temporarily hidden.  They will reappear if
      <literal>B</literal> is <firstterm>unmounted</firstterm> from
      <literal>A</literal>.</para>

    <para>If <literal>B</literal> had been mounted on
      <literal>A2</literal> then the diagram would look like
      this:</para>

    <mediaobject>
      <imageobject>
	<imagedata fileref="install/example-dir3"/>
      </imageobject>

      <textobject>
	<literallayout class="monospaced"> /
 |
 +--- A1
 |
 `--- A2
       |
       +--- B1
       |
       `--- B2</literallayout>
      </textobject>
    </mediaobject>

    <para>and the paths would be
      <filename class="directory">/A2/B1</filename> and
      <filename class="directory">/A2/B2</filename>
      respectively.</para>

    <para>File systems can be mounted on top of one another.
      Continuing the last example, the <literal>C</literal> file
      system could be mounted on top of the <literal>B1</literal>
      directory in the <literal>B</literal> file system, leading to
      this arrangement:</para>

    <mediaobject>
      <imageobject>
	<imagedata fileref="install/example-dir4"/>
      </imageobject>

      <textobject>
	<literallayout class="monospaced"> /
 |
 +--- A1
 |
 `--- A2
       |
       +--- B1
       |     |
       |     +--- C1
       |     |
       |     `--- C2
       |
       `--- B2</literallayout>
      </textobject>
    </mediaobject>

    <para>Or <literal>C</literal> could be mounted directly on to the
      <literal>A</literal> file system, under the
      <literal>A1</literal> directory:</para>

    <mediaobject>
      <imageobject>
	<imagedata fileref="install/example-dir5"/>
      </imageobject>

      <textobject>
	<literallayout class="monospaced"> /
 |
 +--- A1
 |     |
 |     +--- C1
 |     |
 |     `--- C2
 |
 `--- A2
       |
       +--- B1
       |
       `--- B2</literallayout>
      </textobject>
    </mediaobject>

    <para>Typically you create file systems when installing &os;
      and decide where to mount them, and then never change them
      unless you add a new disk.</para>

    <para>It is entirely possible to have one large root file system,
      and not need to create any others.  There are some drawbacks to
      this approach, and one advantage.</para>

    <itemizedlist>
      <title>Benefits of Multiple File Systems</title>

      <listitem>
	<para>Different file systems can have different
	  <firstterm>mount options</firstterm>.  For example, the root
	  file system can be mounted read-only, making it impossible
	  for users to inadvertently delete or edit a critical file.
	  Separating user-writable file systems, such as
	  <filename class="directory">/home</filename>, from other
	  file systems allows them to be mounted
	  <firstterm>nosuid</firstterm>.  This option prevents the
	  <firstterm>suid</firstterm>/<firstterm>guid</firstterm> bits
	  on executables stored on the file system from taking effect,
	  possibly improving security.</para>
      </listitem>

      <listitem>
	<para>&os; automatically optimizes the layout of files on a
	  file system, depending on how the file system is being used.
	  So a file system that contains many small files that are
	  written frequently will have a different optimization to one
	  that contains fewer, larger files.  By having one big file
	  system this optimization breaks down.</para>
      </listitem>

      <listitem>
	<para>&os;'s file systems are very robust should you lose
	  power.  However, a power loss at a critical point could
	  still damage the structure of the file system.  By splitting
	  data over multiple file systems it is more likely that the
	  system will still come up, making it easier to restore from
	  backup as necessary.</para>
      </listitem>
    </itemizedlist>

    <itemizedlist>
      <title>Benefit of a Single File System</title>

      <listitem>
	<para>File systems are a fixed size.  If you create a file
	  system when you install &os; and give it a specific size,
	  you may later discover that you need to make the partition
	  bigger.  This is not easily accomplished without backing up,
	  recreating the file system with the new size, and then
	  restoring the backed up data.</para>

	<important>
	  <para>&os; features the &man.growfs.8; command, which
	    makes it possible to increase the size of file system on
	    the fly, removing this limitation.</para>
	</important>
      </listitem>
    </itemizedlist>

    <para>File systems are contained in partitions.  This does not
      have the same meaning as the common usage of the term partition
      (for example, &ms-dos; partition), because of &os;'s &unix;
      heritage.  Each partition is identified by a letter from
      <literal>a</literal> through to <literal>h</literal>.  Each
      partition can contain only one file system, which means that
      file systems are often described by either their typical mount
      point in the file system hierarchy, or the letter of the
      partition they are contained in.</para>

    <para>&os; also uses disk space for <firstterm>swap
	space</firstterm> to provide
      <firstterm>virtual memory</firstterm>.  This allows your
      computer to behave as though it has much more memory than it
      actually does.  When &os; runs out of memory, it moves some of
      the data that is not currently being used to the swap space, and
      moves it back in (moving something else out) when it needs
      it.</para>

    <para>Some partitions have certain conventions associated with
      them.</para>

    <informaltable frame="none" pgwide="1">
      <tgroup cols="2">
	<colspec colwidth="1*"/>
	<colspec colwidth="5*"/>

	<thead>
	  <row>
	    <entry>Partition</entry>
	    <entry>Convention</entry>
	  </row>
	</thead>

	<tbody valign="top">
	  <row>
	    <entry><literal>a</literal></entry>
	    <entry>Normally contains the root file system.</entry>
	  </row>

	  <row>
	    <entry><literal>b</literal></entry>
	    <entry>Normally contains swap space.</entry>
	  </row>

	  <row>
	    <entry><literal>c</literal></entry>
	    <entry>Normally the same size as the enclosing slice.
	      This allows utilities that need to work on the entire
	      slice, such as a bad block scanner, to work on the
	      <literal>c</literal> partition.  You would not normally
	      create a file system on this partition.</entry>
	  </row>

	  <row>
	    <entry><literal>d</literal></entry>
	    <entry>Partition <literal>d</literal> used to have a
	      special meaning associated with it, although that is now
	      gone and <literal>d</literal> may work as any normal
	      partition.</entry>
	  </row>
	</tbody>
      </tgroup>
    </informaltable>

    <para>Disks in &os; are divided into slices, referred to in
      &windows; as partitions, which are numbered from 1 to 4.  These
      are then then divided into partitions, which contain file
      systems, and are labeled using letters.</para>

    <indexterm><primary>slices</primary></indexterm>
    <indexterm><primary>partitions</primary></indexterm>
    <indexterm><primary>dangerously dedicated</primary></indexterm>

    <para>Slice numbers follow the device name, prefixed with an
      <literal>s</literal>, starting at 1.  So
      <quote>da0<emphasis>s1</emphasis></quote> is the first slice on
      the first SCSI drive.  There can only be four physical slices on
      a disk, but you can have logical slices inside physical slices
      of the appropriate type.  These extended slices are numbered
      starting at 5, so <quote>ad0<emphasis>s5</emphasis></quote> is
      the first extended slice on the first IDE disk.  These devices
      are used by file systems that expect to occupy a slice.</para>

    <para>Slices, <quote>dangerously dedicated</quote> physical
      drives, and other drives contain
      <firstterm>partitions</firstterm>, which are represented as
      letters from <literal>a</literal> to <literal>h</literal>.  This
      letter is appended to the device name, so
      <quote>da0<emphasis>a</emphasis></quote> is the <literal>a</literal> partition on
      the first <literal>da</literal> drive, which is <quote>dangerously
      dedicated</quote>. <quote>ad1s3<emphasis>e</emphasis></quote> is
      the fifth partition in the third slice of the second IDE disk
      drive.</para>

    <para>Finally, each disk on the system is identified.  A disk name
      starts with a code that indicates the type of disk, and then a
      number, indicating which disk it is.  Unlike slices, disk
      numbering starts at 0.  Common codes that you will see are
      listed in <xref linkend="basics-dev-codes"/>.</para>

    <para>When referring to a partition, include the disk name,
      <literal>s</literal>, the slice number, and then the partition
      letter.  Examples are shown in <xref
	linkend="basics-disk-slice-part"/>.</para>

    <para><xref linkend="basics-concept-disk-model"/> shows a
      conceptual model of a disk layout.</para>

    <para>When installing &os;, configure the disk slices, create
      partitions within the slice to be used for &os;, create a file
      system or swap space in each partition, and decide where each
      file system will be mounted.</para>

    <table frame="none" pgwide="1" id="basics-dev-codes">
      <title>Disk Device Codes</title>

      <tgroup cols="2">
	<colspec colwidth="1*"/>
	<colspec colwidth="5*"/>

	<thead>
	  <row>
	    <entry>Code</entry>
	    <entry>Meaning</entry>
	  </row>
	</thead>

	<tbody>
	  <row>
	    <entry><devicename>ad</devicename></entry>
	    <entry>ATAPI (IDE) disk</entry>
	  </row>

	  <row>
	    <entry><devicename>da</devicename></entry>
	    <entry>SCSI direct access disk</entry>
	  </row>

	  <row>
	    <entry><devicename>acd</devicename></entry>
	    <entry>ATAPI (IDE) CDROM</entry>
	  </row>

	  <row>
	    <entry><devicename>cd</devicename></entry>
	    <entry>SCSI CDROM</entry>
	  </row>

	  <row>
	    <entry><devicename>fd</devicename></entry>
	    <entry>Floppy disk</entry>
	  </row>
	</tbody>
      </tgroup>
    </table>

    <example id="basics-disk-slice-part">
      <title>Sample Disk, Slice, and Partition Names</title>

      <informaltable frame="none" pgwide="1">
	<tgroup cols="2">
	  <colspec colwidth="1*"/>
	  <colspec colwidth="5*"/>

	  <thead>
	    <row>
	      <entry>Name</entry>
	      <entry>Meaning</entry>
	    </row>
	  </thead>

	  <tbody>
	    <row>
	      <entry><literal>ad0s1a</literal></entry>
	      <entry>The first partition (<literal>a</literal>) on the
		first slice (<literal>s1</literal>) on the first IDE
		disk (<literal>ad0</literal>).</entry>
	    </row>

	    <row>
	      <entry><literal>da1s2e</literal></entry>

	      <entry>The fifth partition (<literal>e</literal>) on the
		second slice (<literal>s2</literal>) on the second
		SCSI disk (<literal>da1</literal>).</entry>
	    </row>
	  </tbody>
	</tgroup>
      </informaltable>
    </example>

    <example id="basics-concept-disk-model">
      <title>Conceptual Model of a Disk</title>

      <para>This diagram shows &os;'s view of the first IDE disk
	attached to the system.  Assume that the disk is 4&nbsp;GB in
	size, and contains two 2&nbsp;GB slices (&ms-dos; partitions).
	The first slice contains a &ms-dos; disk,
	<devicename>C:</devicename>, and the second slice contains a
	&os; installation.  This example &os; installation has
	three data partitions, and a swap partition.</para>

      <para>The three partitions will each hold a file system.
	Partition <literal>a</literal> will be used for the root file
	system, <literal>e</literal> for the <filename
	  class="directory">/var/</filename> directory hierarchy, and
	<literal>f</literal> for the <filename
	  class="directory">/usr/</filename> directory
	hierarchy.</para>

      <mediaobject>
	<imageobject>
	  <imagedata fileref="install/disk-layout"/>
	</imageobject>

	<textobject>
	  <literallayout class="monospaced">.-----------------.  --.
|                 |    |
|  DOS / Windows  |    |
:                 :     &gt;  First slice, ad0s1
:                 :    |
|                 |    |
:=================:  ==:                               --.
|                 |    |  Partition a, mounted as /      |
|                 |     &gt; referred to as ad0s2a          |
|                 |    |                                 |
:-----------------:  ==:                                 |
|                 |    |  Partition b, used as swap      |
|                 |     &gt; referred to as ad0s2b          |
|                 |    |                                 |
:-----------------:  ==:                                 |  Partition c, no
|                 |    |  Partition e, used as /var       &gt; file system, all
|                 |     &gt; referred to as ad0s2e          |  of FreeBSD slice,
|                 |    |                                 |  ad0s2c
:-----------------:  ==:                                 |
|                 |    |                                 |
:                 :    |  Partition f, used as /usr      |
:                 :     &gt; referred to as ad0s2f          |
:                 :    |                                 |
|                 |    |                                 |
|                 |  --'                                 |
`-----------------'                                    --'</literallayout>
	</textobject>
      </mediaobject>
    </example>
  </sect1>

  <sect1 id="mount-unmount">
    <title>Mounting and Unmounting File Systems</title>

    <para>The file system is best visualized as a tree,
      rooted, as it were, at <filename class="directory">/</filename>.
      <filename class="directory">/dev</filename>,
      <filename class="directory">/usr</filename>, and the
      other directories in the root directory are branches, which may
      have their own branches, such as
      <filename class="directory">/usr/local</filename>, and so
      on.</para>

    <indexterm><primary>root file system</primary></indexterm>
    <para>There are various reasons to house some of these
      directories on separate file systems.  <filename
	class="directory">/var</filename> contains the directories
      <filename class="directory">log/</filename>,
      <filename class="directory">spool/</filename>, and various types
      of temporary files, and as such, may get filled up.  Filling up
      the root file system is not a good idea, so splitting <filename
	class="directory">/var</filename> from
      <filename class="directory">/</filename> is often
      favorable.</para>

    <para>Another common reason to contain certain directory trees on
      other file systems is if they are to be housed on separate
      physical disks, or are separate virtual disks, such as
      <link linkend="network-nfs">Network File System</link> mounts,
      or CDROM drives.</para>

    <sect2 id="disks-fstab">
      <title>The <filename>fstab</filename> File</title>

      <indexterm>
	<primary>file systems</primary>
	<secondary>mounted with fstab</secondary>
      </indexterm>

      <para>During the <link linkend="boot">boot process</link>,
	file systems listed in <filename>/etc/fstab</filename> are
	automatically mounted except for the entries containing
	<option>noauto</option>.  This file contains entries in the
	following format:</para>

      <programlisting><replaceable>device</replaceable>       <replaceable>/mount-point</replaceable> <replaceable>fstype</replaceable>     <replaceable>options</replaceable>      <replaceable>dumpfreq</replaceable>     <replaceable>passno</replaceable></programlisting>

      <variablelist>
	<varlistentry>
	  <term><literal>device</literal></term>
	  <listitem>
	    <para>An existing device name as explained in
	      <xref linkend="disks-naming"/>.</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><literal>mount-point</literal></term>

	  <listitem>
	    <para>An existing directory on which to mount the file
	      system.</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><literal>fstype</literal></term>

	  <listitem>
	    <para>The file system type to pass to &man.mount.8;.  The
	      default &os; file system is
	      <literal>ufs</literal>.</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><literal>options</literal></term>

	  <listitem>
	    <para>Either <option>rw</option> for read-write
	      file systems, or <option>ro</option> for read-only file
	      systems, followed by any other options that may be
	      needed.  A common option is <option>noauto</option> for
	      file systems not normally mounted during the boot
	      sequence.  Other options are listed in
	      &man.mount.8;.</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><literal>dumpfreq</literal></term>

	  <listitem>
	    <para>Used by &man.dump.8; to determine which file systems
	      require dumping.  If the field is missing, a value of
	      zero is assumed.</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><literal>passno</literal></term>

	  <listitem>
	    <para>Determines the order in which file systems should be
	      checked.  File systems that should be skipped should
	      have their <literal>passno</literal> set to zero.  The
	      root file system needs to be checked before everything
	      else and should have its <literal>passno</literal> set
	      to one.  The other file systems should be set to
	      values greater than one.  If more than one file system
	      has the same <literal>passno</literal>, &man.fsck.8;
	      will attempt to check file systems in parallel if
	      possible.</para>
	  </listitem>
	</varlistentry>
      </variablelist>

      <para>Refer to &man.fstab.5; for more information on the format
	of <filename>/etc/fstab</filename> and its options.</para>
    </sect2>

    <sect2 id="disks-mount">
      <title>The <command>mount</command> Command</title>

      <indexterm>
	<primary>file systems</primary>
	<secondary>mounting</secondary>
      </indexterm>

      <para>File systems are mounted using &man.mount.8;.  The most
	basic syntax is as follows:</para>

      <informalexample>
	<screen>&prompt.root; <userinput>mount <replaceable>device</replaceable> <replaceable>mountpoint</replaceable></userinput></screen>
      </informalexample>

      <para>This command provides many options which are described in
	&man.mount.8;, The most commonly used options include:</para>

      <variablelist>
	<title>Mount Options</title>

	<varlistentry>
	  <term><option>-a</option></term>

	  <listitem>
	    <para>Mount all the file systems listed in
	      <filename>/etc/fstab</filename>, except those marked as
	      <quote>noauto</quote>, excluded by the
	      <option>-t</option> flag, or those that are already
	      mounted.</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><option>-d</option></term>

	  <listitem>

	    <para>Do everything except for the actual mount system
	      call.  This option is useful in conjunction with the
	      <option>-v</option> flag to determine what &man.mount.8;
	      is actually trying to do.</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><option>-f</option></term>

	  <listitem>
	    <para>Force the mount of an unclean file system
	      (dangerous), or the revocation of write access when
	      downgrading a file system's mount status from read-write
	      to read-only.</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><option>-r</option></term>

	  <listitem>
	    <para>Mount the file system read-only.  This is identical
	      to using <option>-o ro</option>.</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><option>-t</option>
	    <replaceable>fstype</replaceable></term>

	  <listitem>
	    <para>Mount the specified file system type or mount only
	      file systems of the given type, if <option>-a</option>
	      is included.  <quote>ufs</quote> is the default file
	      system type.</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><option>-u</option></term>

	  <listitem>
	    <para>Update mount options on the file system.</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><option>-v</option></term>

	  <listitem>
	    <para>Be verbose.</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><option>-w</option></term>

	  <listitem>
	    <para>Mount the file system read-write.</para>
	  </listitem>
	</varlistentry>
      </variablelist>

      <para>The following options can be passed to <option>-o</option>
	as a comma-separated list:</para>

      <variablelist>
	<varlistentry>
	  <term>noexec</term>

	  <listitem>
	    <para>Do not allow execution of binaries on this file
	      system.  This is also a useful security option.</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term>nosuid</term>

	  <listitem>
	    <para>Do not interpret setuid or setgid flags on the
	      file system.  This is also a useful security
	      option.</para>
	  </listitem>
	</varlistentry>
      </variablelist>
    </sect2>

    <sect2 id="disks-umount">
      <title>The <command>umount</command> Command</title>

      <indexterm>
	<primary>file systems</primary>
	<secondary>unmounting</secondary>
      </indexterm>

      <para>To unmount a filesystem use &man.umount.8;.  This command
	takes one parameter which can be a mountpoint, device name,
	<option>-a</option> or <option>-A</option>.</para>

      <para>All forms take <option>-f</option> to force unmounting,
	and <option>-v</option> for verbosity.  Be warned that
	<option>-f</option> is not generally a good idea as it might
	crash the computer or damage data on the file system.</para>

      <para>To unmount all mounted file systems, or just the file
	system types listed after <option>-t</option>, use
	<option>-a</option> or <option>-A</option>.  Note that
	<option>-A</option> does not attempt to unmount the root file
	system.</para>
    </sect2>
  </sect1>

  <sect1 id="basics-processes">
    <title>Processes</title>

    <para>&os; is a multi-tasking operating system.  Each program
      running at any one time is called a
      <firstterm>process</firstterm>.  Every running command starts
      at least one new process and there are a number of system
      processes that are run by &os;.</para>

    <para>Each process is uniquely identified by a number called a
      <firstterm>process ID</firstterm>
      (<firstterm>PID</firstterm>).  Similar to files, each process
      has one owner and group, and the owner and group permissions are
      used to determine which files and devices the process can open.
      Most processes also have a parent process that started them.
      For example, the shell is a process, and any command started in
      the shell is a process which has the shell as its parent
      process.  The exception is a special process called
      &man.init.8; which is always the first process to start at boot
      time and which always has a PID of 1.</para>

    <para>To see the processes on the system, use &man.ps.1; and
      &man.top.1;.   To display a static list of the currently running
      processes, their PIDs, how much memory they are using, and the
      command they were started with, use <command>ps</command>.  To
      display all the running processes and update the display every
      few seconds so that you can interactively see what the computer
      is doing, use <command>top</command>.</para>

    <para>By default, <command>ps</command> only shows the commands
      that are running and owned by the user.  For example:</para>

    <screen>&prompt.user; <userinput>ps</userinput>
  PID  TT  STAT      TIME COMMAND
  298  p0  Ss     0:01.10 tcsh
 7078  p0  S      2:40.88 xemacs mdoc.xsl (xemacs-21.1.14)
37393  p0  I      0:03.11 xemacs freebsd.dsl (xemacs-21.1.14)
72210  p0  R+     0:00.00 ps
  390  p1  Is     0:01.14 tcsh
 7059  p2  Is+    1:36.18 /usr/local/bin/mutt -y
 6688  p3  IWs    0:00.00 tcsh
10735  p4  IWs    0:00.00 tcsh
20256  p5  IWs    0:00.00 tcsh
  262  v0  IWs    0:00.00 -tcsh (tcsh)
  270  v0  IW+    0:00.00 /bin/sh /usr/X11R6/bin/startx -- -bpp 16
  280  v0  IW+    0:00.00 xinit /home/nik/.xinitrc -- -bpp 16
  284  v0  IW     0:00.00 /bin/sh /home/nik/.xinitrc
  285  v0  S      0:38.45 /usr/X11R6/bin/sawfish</screen>

    <para>The output from &man.ps.1; is organized into a number of
      columns.  The <literal>PID</literal> column displays the process
      ID.  PIDs are assigned starting at 1, go up to 99999, then wrap
      around back to the beginning.  However, a PID is not reassigned
      if it is already in use.  The <literal>TT</literal> column shows
      the tty the program is running on and <literal>STAT</literal>
      shows the program's state.  <literal>TIME</literal> is the
      amount of time the program has been running on the CPU.  This is
      usually not the elapsed time since the program was started, as
      most programs spend a lot of time waiting for things to happen
      before they need to spend time on the CPU.  Finally,
      <literal>COMMAND</literal> is the command that was used to start
      the program.</para>

    <para>&man.ps.1; supports a number of different options to change
      the information that is displayed.  One of the most useful sets
      is <literal>auxww</literal>.  <option>a</option> displays
      information about all the running processes of all users.
      <option>u</option> displays the username of the process' owner,
      as well as memory usage.  <option>x</option> displays
      information about daemon processes, and <option>ww</option>
      causes &man.ps.1; to display the full command line for each
      process, rather than truncating it once it gets too long to fit
      on the screen.</para>

    <para>The output from &man.top.1; is similar.  A sample session
      looks like this:</para>

    <screen>&prompt.user; <userinput>top</userinput>
last pid: 72257;  load averages:  0.13,  0.09,  0.03    up 0+13:38:33  22:39:10
47 processes:  1 running, 46 sleeping
CPU states: 12.6% user,  0.0% nice,  7.8% system,  0.0% interrupt, 79.7% idle
Mem: 36M Active, 5256K Inact, 13M Wired, 6312K Cache, 15M Buf, 408K Free
Swap: 256M Total, 38M Used, 217M Free, 15% Inuse

  PID USERNAME PRI NICE  SIZE    RES STATE    TIME   WCPU    CPU COMMAND
72257 nik       28   0  1960K  1044K RUN      0:00 14.86%  1.42% top
 7078 nik        2   0 15280K 10960K select   2:54  0.88%  0.88% xemacs-21.1.14
  281 nik        2   0 18636K  7112K select   5:36  0.73%  0.73% XF86_SVGA
  296 nik        2   0  3240K  1644K select   0:12  0.05%  0.05% xterm
  175 root       2   0   924K   252K select   1:41  0.00%  0.00% syslogd
 7059 nik        2   0  7260K  4644K poll     1:38  0.00%  0.00% mutt
...</screen>

    <para>The output is split into two sections.  The header (the
      first five lines) shows the PID of the last process to run, the
      system load averages (which are a measure of how busy the system
      is), the system uptime (time since the last reboot) and the
      current time.  The other figures in the header relate to how
      many processes are running (47 in this case), how much memory
      and swap space has been used, and how much time the system is
      spending in different CPU states.</para>

    <para>Below the header is a series of columns containing similar
      information to the output from &man.ps.1;, such as the PID,
      username, amount of CPU time, and the command that started the
      process.  By default, &man.top.1; also displays the amount of
      memory space taken by the process.  This is split into two
      columns: one for total size and one for resident size.  Total
      size is how much memory the application has needed and the
      resident size is how much it is actually using at the moment.
      In this example, <application>mutt</application> has
      required almost 8&nbsp;MB of RAM, but is currently only using
      5&nbsp;MB.</para>

    <para>&man.top.1; automatically updates the display every two
      seconds.  A different interval can be specified with
      <option>-s</option>.</para>
  </sect1>

  <sect1 id="basics-daemons">
    <title>Daemons, Signals, and Killing Processes</title>

    <para>When using an editor, it is easy to control the editor and
      load files because the editor provides facilities to do so, and
      because the editor is attached to a
      <firstterm>terminal</firstterm>.  Some programs are not designed
      to be run with continuous user input and disconnect from the
      terminal at the first opportunity.  For example, a web server
      responds to web requests, rather than user input.  Mail servers
      are another example of this type of application.</para>

    <para>These programs are known as <firstterm>daemons</firstterm>.
      The term daemon comes from Greek mythology and represents an
      entity that is neither good or evil, and which invisibly
      performs useful tasks.  This is why the BSD mascot is the
      cheerful-looking daemon with sneakers and a pitchfork.</para>

    <para>There is a convention to name programs that normally run as
      daemons with a trailing <quote>d</quote>.
      <application>BIND</application> is the Berkeley Internet Name
      Domain, but the actual program that executes is
      <command>named</command>.  The <application>Apache</application>
      web server program is <command>httpd</command> and the
      line printer spooling daemon is <command>lpd</command>.  This is
      only a naming convention.  For example, the main mail daemon for
      the <application>Sendmail</application> application is
      <command>sendmail</command>, and not
      <command>maild</command>.</para>

    <para>One way to communicate with a daemon, or any running
      process, is to send a <firstterm>signal</firstterm> using
      &man.kill.1;.  There are a number of different signals; some
      have a specific meaning while others are described in the
      application's documentation.  A user can only send a signal to a
      process they own and sending a signal to someone else's process
      will result in a permission denied error.  The exception is the
      <username>root</username> user, who can send signals to anyone's
      processes.</para>

    <para>&os; can also send a signal to a process.  If an application
      is badly written and tries to access memory that it is not
      supposed to, &os; will send the process the
      <firstterm>Segmentation Violation</firstterm> signal
      (<literal>SIGSEGV</literal>).  If an application has used the
      &man.alarm.3; system call to be alerted after a period of time
      has elapsed, it will be sent the Alarm signal
      (<literal>SIGALRM</literal>).</para>

    <para>Two signals can be used to stop a process:
      <literal>SIGTERM</literal> and <literal>SIGKILL</literal>.
      <literal>SIGTERM</literal> is the polite way to kill a process
      as the process can read the signal, close any log files it may
      have open, and attempt to finish what it is doing before
      shutting down.  In some cases, a process may ignore
      <literal>SIGTERM</literal> if it is in the middle of some task
      that can not be interrupted.</para>

    <para><literal>SIGKILL</literal> can not be ignored by a process.
      This is the <quote>I do not care what you are doing, stop right
      now</quote> signal.  Sending a <literal>SIGKILL</literal> to a
      process will usually stop that process there and then.<footnote>
	<para>There are a few tasks that can not be interrupted.  For
	  example, if the process is trying to read from a file that
	  is on another computer on the network, and the other
	  computer is unavailable, the process is said to be
	  <quote>uninterruptible</quote>.  Eventually the process will
	  time out, typically after two minutes.  As soon as this time
	  out occurs the process will be killed.</para>
	</footnote>.</para>

    <para>Other commonly used signals are <literal>SIGHUP</literal>,
      <literal>SIGUSR1</literal>, and <literal>SIGUSR2</literal>.
      These are general purpose signals and different applications
      will respond differently.</para>

    <para>For example, after changing a web server's configuration
      file, the web server needs to be told to re-read its
      configuration.  Restarting <command>httpd</command> would result
      in a brief outage period on the web server.  Instead, send the
      daemon the <literal>SIGHUP</literal> signal.  Be aware that
      different daemons will have different behavior, so refer to the
      documentation for the daemon to determine if
      <literal>SIGHUP</literal> will achieve the desired
      results.</para>

    <procedure>
      <title>Sending a Signal to a Process</title>

      <para>This example shows how to send a signal to &man.inetd.8;.
	The <command>inetd</command> configuration file is
	<filename>/etc/inetd.conf</filename>, and
	<command>inetd</command> will re-read this configuration file
	when it is sent a <literal>SIGHUP</literal>.</para>

      <step>
	<para>Find the PID of the process you want to send the signal
	  to using &man.pgrep.1;.  In this example, the PID for
	  &man.inetd.8; is 198:</para>

	<screen>&prompt.user; <userinput>pgrep -l inetd</userinput>
198  inetd -wW</screen>

      </step>

      <step>
	<para>Use &man.kill.1; to send the signal.  Because
	  &man.inetd.8; is owned by <username>root</username>, use
	  &man.su.1; to become <username>root</username> first.</para>

	<screen>&prompt.user; <userinput>su</userinput>
<prompt>Password:</prompt>
&prompt.root; <userinput>/bin/kill -s HUP 198</userinput></screen>

	<para>Like most &unix; commands, &man.kill.1; will not print
	  any output if it is successful.  If you send a signal to a
	  process that you do not own, you will instead see
	  <errorname>kill: <replaceable>PID</replaceable>: Operation
	    not permitted</errorname>.  Mistyping the PID will either
	  send the signal to the wrong process, which could have
	  negative results, or will send the signal to a PID that is
	  not currently in use, resulting in the error
	  <errorname>kill: <replaceable>PID</replaceable>: No such
	    process</errorname>.</para>

	<note>
	  <title>Why Use <command>/bin/kill</command>?</title>

	  <para>Many shells provide <command>kill</command> as a built
	    in command, meaning that the shell will send the signal
	    directly, rather than running
	    <filename>/bin/kill</filename>.  Be aware that different
	    shells have a different syntax for specifying the name of
	    the signal to send.  Rather than try to learn all of them,
	    it can be simpler to use <command>/bin/kill
	      <replaceable>...</replaceable></command>
	    directly.</para>
	</note>
      </step>
    </procedure>

    <para>When sending other signals, substitute
      <literal>TERM</literal> or <literal>KILL</literal> in the
      command line as necessary.</para>

    <important>
      <para>Killing a random process on the system can be a bad idea.
	In particular, &man.init.8;, PID 1, is special.  Running
	<command>/bin/kill -s KILL 1</command> is a quick, and
	unrecommended, way to shutdown the system.
	<emphasis>Always</emphasis> double check the arguments to
	&man.kill.1; <emphasis>before</emphasis> pressing
	<keycap>Return</keycap>.</para>
    </important>
  </sect1>

  <sect1 id="shells">
    <title>Shells</title>

    <indexterm><primary>shells</primary></indexterm>
    <indexterm><primary>command line</primary></indexterm>

    <para>&os; provides a command line interface called a shell.  A
      shell receives commands from the input channel and executes
      them.  Many shells provide built in functions to help with
      everyday tasks such as file management, file globbing, command
      line editing, command macros, and environment variables.  &os;
      comes with several shells, including <command>sh</command>, the
      Bourne Shell, and <command>tcsh</command>, the improved C-shell.
      Other shells are available from the &os; Ports Collection, such
      as <command>zsh</command> and <command>bash</command>.</para>

    <para>The shell that is used is really a matter of taste.  A C
      programmer might feel more comfortable with a C-like shell such
      as <command>tcsh</command>.  A Linux user might prefer
      <command>bash</command>.  Each shell has unique properties that
      may or may not work with a user's preferred working environment,
      which is why there is a choice of which shell to use.</para>

    <para>One common shell feature is filename completion.  After a
      user types the first few letters of a command or filename and
      presses <keycap>Tab</keycap>, the shell will automatically
      complete the rest of the command or filename.  Consider two
      files called <filename>foobar</filename> and
      <filename>foo.bar</filename>.  To delete
      <filename>foo.bar</filename>, type <command>rm
	fo[<keycap>Tab</keycap>].[<keycap>Tab</keycap>]</command>.</para>

    <para>The shell should print out <command>rm
	foo[BEEP].bar</command>.</para>

    <para>The [BEEP] is the console bell, which the shell used to
      indicate it was unable to complete the filename because there
      is more than one match.  Both <filename>foobar</filename> and
      <filename>foo.bar</filename> start with <literal>fo</literal>.
      By typing <literal>.</literal>, then pressing
      <keycap>Tab</keycap> again, the shell would be able to fill in
      the rest of the filename.</para>

    <indexterm><primary>environment variables</primary></indexterm>

    <para>Another feature of the shell is the use of environment
      variables.  Environment variables are a variable/key pair stored
      in the shell's environment.  This environment can be read by any
      program invoked by the shell, and thus contains a lot of program
      configuration.  Here is a list of common environment variables
      and their meanings:</para>

    <informaltable frame="none" pgwide="1">
      <tgroup cols="2">
	<thead>
	  <row>
	    <entry>Variable</entry>
	    <entry>Description</entry>
	  </row>
	</thead>

	<tbody>
	  <row>
	    <entry><envar>USER</envar></entry>
	    <entry>Current logged in user's name.</entry>
	  </row>

	  <row>
	    <entry><envar>PATH</envar></entry>
	    <entry>Colon-separated list of directories to search for
	      binaries.</entry>
	  </row>

	  <row>
	    <entry><envar>DISPLAY</envar></entry>
	    <entry>Network name of the <application>Xorg</application>
	      display to connect to, if available.</entry>
	  </row>

	  <row>
	    <entry><envar>SHELL</envar></entry>
	    <entry>The current shell.</entry>
	  </row>

	  <row>
	    <entry><envar>TERM</envar></entry>

	    <entry>The name of the user's type of terminal.  Used to
	      determine the capabilities of the terminal.</entry>
	  </row>

	  <row>
	    <entry><envar>TERMCAP</envar></entry>

	    <entry>Database entry of the terminal escape codes to
	      perform various terminal functions.</entry>
	  </row>

	  <row>
	    <entry><envar>OSTYPE</envar></entry>
	    <entry>Type of operating system.</entry>
	  </row>

	  <row>
	    <entry><envar>MACHTYPE</envar></entry>
	    <entry>The system's CPU architecture.</entry>
	  </row>

	  <row>
	    <entry><envar>EDITOR</envar></entry>
	    <entry>The user's preferred text editor.</entry>
	  </row>

	  <row>
	    <entry><envar>PAGER</envar></entry>
	    <entry>The user's preferred text pager.</entry>
	  </row>

	  <row>
	    <entry><envar>MANPATH</envar></entry>
	    <entry>Colon-separated list of directories to search for
	      manual pages.</entry>
	  </row>
	</tbody>
      </tgroup>
    </informaltable>

    <indexterm><primary>Bourne shells</primary></indexterm>

    <para>How to set an environment variable differs between shells.
      In <command>tcsh</command> and <command>csh</command>, use
      <command>setenv</command> to set environment variables.  In
      <command>sh</command> and <command>bash</command>, use
      <command>export</command> to set the current environment
      variables.  This example sets the default <envar>EDITOR</envar>
      to <filename>/usr/local/bin/emacs</filename> for the
      <command>tcsh</command> shell:</para>

    <screen>&prompt.user; <userinput>setenv EDITOR /usr/local/bin/emacs</userinput></screen>

    <para>The equivalent command for <command>bash</command>
      would be:</para>

    <screen>&prompt.user; <userinput>export EDITOR="/usr/local/bin/emacs"</userinput></screen>

    <para>To expand an environment variable in order to see its
      current setting, type a <literal>$</literal> character in front
      of its name on the command line.  For example,
      <command>echo $TERM</command> displays the current
      <envar>$TERM</envar> setting.</para>

    <para>Shells treat special characters, known as meta-characters,
      as special representations of data.  The most common
      meta-character is <literal>*</literal>, which
      represents any number of characters in a filename.
      Meta-characters can be used to perform filename globbing.  For
      example, <command>echo *</command> is equivalent to
      <command>ls</command> because the shell takes all the files that
      match <literal>*</literal> and <command>echo</command> lists
      them on the command line.</para>

    <para>To prevent the shell from interpreting a special character,
      escape it from the shell by starting it with a backslash
      (<literal>\</literal>).  For example,
      <command>echo $TERM</command> prints the terminal setting
      whereas <command>echo \$TERM</command> literally prints the
      string <literal>$TERM</literal>.</para>

    <sect2 id="changing-shells">
      <title>Changing Your Shell</title>

      <para>The easiest way to permanently change the default shell is
	to use <command>chsh</command>.  Running this command will
	open the editor that is configured in the
	<envar>EDITOR</envar> environment variable, which by default
	is set to <command>vi</command>.  Change
	the <quote>Shell:</quote> line to the full path of the
	new shell.</para>

      <para>Alternately, use <command>chsh -s</command> which will set
	the specified shell without opening an editor.  For example,
	to change the shell to <command>bash</command>:</para>

      <screen>&prompt.user; <userinput>chsh -s /usr/local/bin/bash</userinput></screen>

      <note>
	<para>The new shell <emphasis>must</emphasis> be present in
	  <filename>/etc/shells</filename>.  If the shell was
	  installed from the &os; <link linkend="ports">Ports
	    Collection</link>, it should be automatically added to
	  this file.  If it is missing, add it using this
	  command, replacing the path with the path of the
	  shell:</para>

	<screen>&prompt.root; <userinput>echo <replaceable>/usr/local/bin/bash</replaceable> &gt;&gt; /etc/shells</userinput></screen>

	<para>Then rerun <command>chsh</command>.</para>
      </note>
    </sect2>
  </sect1>

  <sect1 id="editors">
    <title>Text Editors</title>

    <indexterm><primary>text editors</primary></indexterm>
    <indexterm><primary>editors</primary></indexterm>

    <para>Most &os; configuration is done by editing text files.
      Because of this, it is a good idea to become familiar with a
      text editor.  &os; comes with a few as part of the base system,
      and many more are available in the Ports Collection.</para>

    <indexterm>
      <primary><command>ee</command></primary>
    </indexterm>
    <indexterm>
      <primary>editors</primary>
      <secondary><command>ee</command></secondary>
    </indexterm>

    <para>A simple editor to learn is <application>ee</application>,
      which stands for easy editor.  To start this editor, type
      <command>ee <replaceable>filename</replaceable></command> where
      <replaceable>filename</replaceable> is the name of the file to
      be edited.  Once inside the editor, all of the commands for
      manipulating the editor's functions are listed at the top of the
      display.  The caret <literal>^</literal> represents
      <keycap>Ctrl</keycap>, so <literal>^e</literal> expands to
      <keycombo
	action="simul"><keycap>Ctrl</keycap><keycap>e</keycap></keycombo>.
      To leave <application>ee</application>, press
      <keycap>Esc</keycap>, then choose the <quote>leave
	editor</quote> option from the main menu.  The editor will
      prompt you to save any changes if the file has been
      modified.</para>

    <indexterm>
      <primary><command>vi</command></primary>
    </indexterm>
    <indexterm>
      <primary>editors</primary>
      <secondary><command>vi</command></secondary>
    </indexterm>
    <indexterm>
      <primary><command>emacs</command></primary>
    </indexterm>
    <indexterm>
      <primary>editors</primary>
      <secondary><command>emacs</command></secondary>
    </indexterm>

    <para>&os; also comes with more powerful text editors such as
      <application>vi</application> as part of the base system.
      Other editors, like <filename
	role="package">editors/emacs</filename> and
      <filename role="package">editors/vim</filename>, are part of the
      &os; Ports Collection.  These editors offer more functionality
      at the expense of being a more complicated to learn.  Learning a
      more powerful editor such as <application>vim</application> or
      <application>Emacs</application> can save more time in the long
      run.</para>

    <para>Many applications which modify files or require typed input
      will automatically open a text editor.  To alter the default
      editor used, set the <envar>EDITOR</envar> environment
      variable as described in the <link
	linkend="shells">shells</link> section.</para>
  </sect1>

  <sect1 id="basics-devices">
    <title>Devices and Device Nodes</title>

    <para>A device is a term used mostly for hardware-related
      activities in a system, including disks, printers, graphics
      cards, and keyboards.  When &os; boots, the majority of the boot
      messages refer to devices being detected.  A copy of the boot
      messages are saved to
      <filename>/var/run/dmesg.boot</filename>.</para>

    <para>Each device has a device name and number.  For example,
      <devicename>acd0</devicename> is the first IDE CD-ROM drive,
      while <devicename>kbd0</devicename> represents the
      keyboard.</para>

    <para>Most devices in a &os; must be accessed through special
      files called device nodes, which are located in
      <filename class="directory">/dev</filename>.</para>

    <sect2>
      <title>Creating Device Nodes</title>

      <para>When adding a new device to your system, or compiling
	in support for additional devices, new device nodes must
	be created.</para>

      <sect3>
	<title><literal>DEVFS</literal> (DEVice File System)</title>

	<para> The device file system, <literal>DEVFS</literal>,
	  provides access to the kernel's device namespace in the
	  global file system namespace.  Instead of having to
	  manually create and modify device nodes,
	  <literal>DEVFS</literal> automatically maintains this
	  particular file system.  Refer to &man.devfs.5; for
	  more information.</para>
      </sect3>
    </sect2>
  </sect1>

  <sect1 id="binary-formats">
    <title>Binary Formats</title>

    <para>To understand why &os; uses the &man.elf.5; format,the three
      currently <quote>dominant</quote> executable formats for &unix;
      must be described:</para>

    <itemizedlist>
      <listitem>
	<para>&man.a.out.5;</para>

	<para>The oldest and <quote>classic</quote> &unix; object
	  format.  It uses a short and compact header with a
	  &man.magic.5; number at the beginning that is often used to
	  characterize the format.  It contains three loaded segments:
	  .text, .data, and .bss, plus a symbol table and a string
	  table.</para>
      </listitem>

      <listitem>
	<para><acronym>COFF</acronym></para>

	<para>The SVR3 object format.  The header comprises a section
	  table which can contain more than just .text, .data, and
	  .bss sections.</para>
      </listitem>

      <listitem>
	<para>&man.elf.5;</para>

	<para>The successor to <acronym>COFF</acronym>, featuring
	  multiple sections and 32-bit or 64-bit possible values.  One
	  major drawback is that <acronym>ELF</acronym> was designed
	  with the assumption that there would be only one ABI per
	  system architecture.  That assumption is actually incorrect,
	  and not even in the commercial SYSV world (which has at
	  least three ABIs: SVR4, Solaris, SCO) does it hold
	  true.</para>

	<para>&os; tries to work around this problem somewhat by
	  providing a utility for <emphasis>branding</emphasis> a
	  known <acronym>ELF</acronym> executable with information
	  about its compliant ABI.  Refer to &man.brandelf.1; for more
	  information.</para>
      </listitem>
    </itemizedlist>

    <para>&os; comes from the <quote>classic</quote> camp and used
      the &man.a.out.5; format, a technology tried and proven through
      many generations of BSD releases, until the beginning of the 3.X
      branch.  Though it was possible to build and run native
      <acronym>ELF</acronym> binaries and kernels on a &os;
      system for some time before that, &os; initially resisted the
      <quote>push</quote> to switch to <acronym>ELF</acronym> as the
      default format.  Why?  When Linux made its painful transition to
      <acronym>ELF</acronym>, it was due to their inflexible
      jump-table based shared library mechanism, which made the
      construction of shared libraries difficult for vendors and
      developers.  Since <acronym>ELF</acronym> tools offered a
      solution to the shared library problem and were generally seen
      as <quote>the way forward</quote>, the migration cost was
      accepted as necessary and the transition made.  &os;'s shared
      library mechanism is based more closely on the &sunos; style
      shared library mechanism and is easy to use.</para>

    <para>So, why are there so many different formats?  Back in the
      PDP-11 days when simple hardware supported a simple, small
      system, <filename>a.out</filename> was adequate for the job of
      representing binaries.  As &unix; was ported, the
      <filename>a.out</filename> format was retained because it was
      sufficient for the early ports of &unix; to architectures like
      the Motorola 68k or VAXen.</para>

    <para>Then some hardware engineer decided that if he could force
      software to do some sleazy tricks, a few gates could be shaved
      off the design and the CPU core could run faster.
      <filename>a.out</filename> was ill-suited for this new kind of
      hardware, known as <acronym>RISC</acronym>.  Many formats were
      developed to get better performance from this hardware than the
      limited, simple <filename>a.out</filename> format could offer.
      <acronym>COFF</acronym>, <acronym>ECOFF</acronym>, and a few
      others were invented and their limitations explored before
      settling on <acronym>ELF</acronym>.</para>

    <para>In addition, program sizes were getting huge while disks
      and physical memory were still relatively small, so the concept
      of a shared library was born.  The virtual memory system became
      more sophisticated. While each advancement was done using the
      <filename>a.out</filename> format, its usefulness was stretched
      with each new feature.  In addition, people wanted to
      dynamically load things at run time, or to junk parts of their
      program after the init code had run to save in core memory and
      swap space.  Languages became more sophisticated and people
      wanted code called before the main() function automatically.
      Lots of hacks were done to the <filename>a.out</filename> format
      to allow all of these things to happen, and they basically
      worked for a time.  In time, <filename>a.out</filename> was not
      up to handling all these problems without an ever increasing
      overhead in code and complexity.  While <acronym>ELF</acronym>
      solved many of these problems, it would be painful to switch
      from the system that basically worked.  So
      <acronym>ELF</acronym> had to wait until it was more painful to
      remain with <filename>a.out</filename> than it was to migrate to
      <acronym>ELF</acronym>.</para>

    <para>As time passed, the build tools that &os; derived their
      build tools from, especially the assembler and loader, evolved
      in two parallel trees.  The &os; tree added shared libraries and
      fixed some bugs.  The GNU folks that originally wrote these
      programs rewrote them and added simpler support for building
      cross compilers and plugging in different formats.  Those who
      wanted to build cross compilers targeting &os; were out of luck
      since the older sources that &os; had for
      <application>as</application> and <application>ld</application>
      were not up to the task.  The new GNU tools chain
      (<application>binutils</application>) supports cross
      compiling, <acronym>ELF</acronym>, shared libraries, and C++
      extensions.  In addition, many vendors release
      <acronym>ELF</acronym> binaries, and &os; should be able to run
      them.</para>

    <para><acronym>ELF</acronym> is more expressive than
      <filename>a.out</filename> and allows more extensibility in the
      base system.  The <acronym>ELF</acronym> tools are better
      maintained and offer cross compilation support.
      <acronym>ELF</acronym> may be a little slower than
      <filename>a.out</filename>, but trying to measure it can be
      difficult.  There are also numerous details that are different
      between the two such as how they map pages and handle init
      code.</para>
  </sect1>

  <sect1 id="basics-more-information">
    <title>For More Information</title>

    <sect2 id="basics-man">
      <title>Manual Pages</title>

      <indexterm><primary>manual pages</primary></indexterm>

      <para>The most comprehensive documentation on &os; is in the
	form of manual pages.  Nearly every program on the system
	comes with a short reference manual explaining the basic
	operation and available arguments.  These manuals can be
	viewed using <command>man</command>:</para>

      <screen>&prompt.user; <userinput>man <replaceable>command</replaceable></userinput></screen>

      <para>where <replaceable>command</replaceable> is the name of
	the command you wish to learn about.  For example, to learn
	more about <command>ls</command>, type:</para>

      <screen>&prompt.user; <userinput>man ls</userinput></screen>

      <para>The online manual is divided into numbered
	sections:</para>

      <orderedlist>
	<listitem>
	  <para>User commands.</para>
	</listitem>

	<listitem>
	  <para>System calls and error numbers.</para>
	</listitem>

	<listitem>
	  <para>Functions in the C libraries.</para>
	</listitem>

	<listitem>
	  <para>Device drivers.</para>
	</listitem>

	<listitem>
	  <para>File formats.</para>
	</listitem>

	<listitem>
	  <para>Games and other diversions.</para>
	</listitem>

	<listitem>
	  <para>Miscellaneous information.</para>
	</listitem>

	<listitem>
	  <para>System maintenance and operation commands.</para>
	</listitem>

	<listitem>
	  <para>Kernel developers.</para>
	</listitem>
      </orderedlist>

      <para>In some cases, the same topic may appear in more than one
	section of the online manual.  For example, there is a
	<command>chmod</command> user command and a
	<function>chmod()</function> system call.  To tell
	<command>man</command> which section to display, specify the
	section number:</para>

      <screen>&prompt.user; <userinput>man 1 chmod</userinput></screen>

      <para>This will display the manual page for the user command
	<command>chmod</command>.  References to a particular section
	of the online manual are traditionally placed in parenthesis
	in written documentation, so &man.chmod.1; refers to the
	<command>chmod</command> user command and &man.chmod.2; refers
	to the system call.</para>

      <para>If you do not know the command name, use <command>man
	  -k</command> to search for keywords in the command
	descriptions:</para>

      <screen>&prompt.user; <userinput>man -k <replaceable>mail</replaceable></userinput></screen>

      <para>This command displays a list of commands that have the
	keyword <quote>mail</quote> in their descriptions.  This is
	equivalent to using &man.apropos.1;.</para>

      <para>To determine what the commands in
	<filename class="directory">/usr/bin</filename> do,
	type:</para>

      <screen>&prompt.user; <userinput>cd /usr/bin</userinput>
&prompt.user; <userinput>man -f *</userinput></screen>

      <para>or</para>

      <screen>&prompt.user; <userinput>cd /usr/bin</userinput>
&prompt.user; <userinput>whatis *</userinput></screen>

    </sect2>

    <sect2 id="basics-info">
      <title>GNU Info Files</title>

      <indexterm>
	<primary>Free Software Foundation</primary>
      </indexterm>

      <para>&os; includes many applications and utilities produced
	by the Free Software Foundation (FSF).  In addition to manual
	pages, these programs may include hypertext documents called
	<literal>info</literal> files.  These can be viewed using
	<command>info</command> or, if <filename
	  role="package">editors/emacs</filename> is installed, the
	info mode of <application>emacs</application>.</para>

      <para>To use &man.info.1;, type:</para>

      <screen>&prompt.user; <userinput>info</userinput></screen>

      <para>For a brief introduction, type <literal>h</literal>.  For
	a quick command reference, type <literal>?</literal>.</para>
    </sect2>
  </sect1>
</chapter>