<?xml version="1.0" encoding="iso-8859-1"?>
<!--
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>
<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 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>/usr/local</filename>,
the general layout sketched out by &man.hier.7; for
<filename>/usr</filename> should be used. Exceptions
are the man directory, which is directly under
<filename>/usr/local</filename> rather than under
<filename>/usr/local/share</filename>, and the ports
documentation is in
<filename>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>/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>foo</filename> contains a directory
<filename>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" format="EPS"/>
</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" format="EPS"/>
</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>/A1/B1</filename> or <filename>/A1/B2</filename> as
necessary. Any files that were in <filename>/A1</filename> have
been temporarily hidden. They will reappear if
<literal>B</literal> is <firstterm>unmounted</firstterm> from
A.</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" format="EPS"/>
</imageobject>
<textobject>
<literallayout class="monospaced"> /
|
+--- A1
|
`--- A2
|
+--- B1
|
`--- B2</literallayout>
</textobject>
</mediaobject>
<para>and the paths would be <filename>/A2/B1</filename> and
<filename>/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" format="EPS"/>
</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" format="EPS"/>
</imageobject>
<textobject>
<literallayout class="monospaced"> /
|
+--- A1
| |
| +--- C1
| |
| `--- C2
|
`--- A2
|
+--- B1
|
`--- B2</literallayout>
</textobject>
</mediaobject>
<para>This is similar, although not identical, to a &ms-dos;
<command>join</command>.</para>
<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>/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>Each partition-that-contains-a-file-system is stored in what
&os; calls a <firstterm>slice</firstterm>. Slice is
&os;'s term for what the common call partitions, and again,
this is because of &os;'s &unix; background. Slices are
numbered, starting at 1, through to 4.</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 a partition on
the first da 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 GB in
size, and contains two 2 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" format="EPS"/>
</imageobject>
<textobject>
<literallayout class="monospaced">.-----------------. --.
| | |
| DOS / Windows | |
: : > First slice, ad0s1
: : |
| | |
:=================: ==: --.
| | | Partition a, mounted as / |
| | > referred to as ad0s2a |
| | | |
:-----------------: ==: |
| | | Partition b, used as swap |
| | > referred to as ad0s2b |
| | | |
:-----------------: ==: | Partition c, no
| | | Partition e, used as /var > file system, all
| | > referred to as ad0s2e | of FreeBSD slice,
| | | | ad0s2c
:-----------------: ==: |
| | | |
: : | Partition f, used as /usr |
: : > 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)
48630 p0 S 2:50.89 /usr/local/lib/netscape-linux/navigator-linux-4.77.bi
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
48630 nik 2 0 29816K 9148K select 3:18 0.00% 0.00% navigator-linu
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>&netscape;</application> has
required almost 30 MB of RAM, but is currently only using
9 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>
<indexterm><primary>environment variables</primary></indexterm>
<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> >> /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>/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.
In time, support for <filename>a.out</filename> will be moved
out of the <filename>GENERIC</filename> kernel, and eventually
removed from the kernel once the need to run legacy
<filename>a.out</filename> programs is past.</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>/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>