root-filesystem.xml

<chapter id='theRootFilesystem'><title>The Root Filesystem</title>

<section id='purpose2'>

<title>Purpose</title>

<para>The contents of the root filesystem must be adequate to boot,
restore, recover, and/or repair the system.</para>

<itemizedlist spacing="normal" mark="bullet">

<listitem>
<para>To boot a system, enough software and data must be present on the root partition
to mount other filesystems.  This includes utilities, configuration,
boot loader information, and other essential start-up data.
<filename>/usr</filename>, <filename>/opt</filename>, and
<filename>/var</filename> are designed such that they may be located
on other partitions or filesystems.</para>
</listitem>

<listitem>
<para>To enable recovery and/or repair of a system, those utilities
needed by an experienced maintainer to diagnose and reconstruct a
damaged system must be present on the root filesystem.</para>
</listitem>

<listitem>
<para>To restore a system, those utilities needed to restore from
system backups (on floppy, tape, etc.) must be present on the root
filesystem.</para>
</listitem>

</itemizedlist>

<tip><title>Rationale</title>

<para>The minimum requirements for the root filesystem should be as
small as reasonably possible, but no smaller.  While many users may
not want the extra complexity of a partitioned system, the option to
keep the root small should be preserved for several reasons:</para>

<itemizedlist>

<listitem>
<para>It is occasionally mounted from very small media.</para>
</listitem>

<listitem>
<para>The root filesystem contains many system-specific configuration
files.  Possible examples include a kernel that is specific to the
system, a specific hostname, etc.  This means that the root filesystem
isn't always shareable between networked systems.  Keeping it small on
servers in networked systems minimizes the amount of lost space for
areas of unshareable files.  It also allows workstations with smaller
local hard drives.</para>
</listitem>

<listitem>
<para>While you may have the root filesystem on a large partition, and
may be able to fill it to your heart's content, there will be people
with smaller partitions.  If you have more files installed, you may
find incompatibilities with other systems using root filesystems on
smaller partitions.  If you are a developer then you may be turning
your assumption into a problem for a large number of users.</para>
</listitem>

<listitem>
<para>Disk errors that corrupt data on the root filesystem are a
greater problem than errors on any other partition.  A small root
filesystem is less prone to corruption as the result of a system
crash.</para>
</listitem>

</itemizedlist>

<para>These considerations must be balanced against the need for a
minimally useful operating environment, for the sake of the boot
process as well as in failure recovery situations.</para>
</tip>

<para>Applications must never create or require special files or
subdirectories in the root directory.  Other locations in the FHS
hierarchy provide more than enough flexibility for any package.</para>

<tip><title>Rationale</title>

<para>There are several reasons why creating a new subdirectory of
the root filesystem is prohibited:</para>

<itemizedlist spacing="normal" mark="bullet">

<listitem>
<para>It demands space on a root partition which the system
administrator may want kept small and simple for either performance or
security reasons.</para>
</listitem>

<listitem>
<para>It evades whatever discipline the system administrator may have
set up for distributing standard file hierarchies across mountable
volumes.</para>
</listitem>

</itemizedlist>

<para>Distributions should not create new directories in the root
hierarchy without extremely careful consideration of the consequences
including for application portability.</para>

</tip>
</section>

<section id='requirements'><title>Requirements</title>

<para>The following directories, or symbolic links to directories, are
required in <filename>/</filename>.</para>

<informaltable frame='none'>
  <tgroup cols='2' align='left'>
  <thead>
     <row>
       <entry>Directory</entry>
       <entry>Description</entry>
     </row>
  </thead>

  <tbody>
     <row>
       <entry><filename>bin</filename></entry>
       <entry>Essential command binaries</entry>
     </row>

     <row>
       <entry><filename>boot</filename></entry>
       <entry>Static files of the boot loader</entry>
     </row>

     <row>
       <entry><filename>dev</filename></entry>
       <entry>Device files</entry>
     </row>

     <row>
       <entry><filename>etc</filename></entry>
       <entry>Host-specific system configuration</entry>
     </row>

     <row>
       <entry><filename>lib</filename></entry>
       <entry>Essential shared libraries and kernel modules</entry>
     </row>

     <row>
       <entry><filename>media</filename></entry>
       <entry>Mount point for removable media</entry>
     </row>

     <row>
       <entry><filename>mnt</filename></entry>
       <entry>Mount point for mounting a filesystem temporarily</entry>
     </row>

     <row>
       <entry><filename>opt</filename></entry>
       <entry>Add-on application software packages</entry>
     </row>

     <row>
       <entry><filename>run</filename></entry>
       <entry>Data relevant to running processes</entry>
     </row>

     <row>
       <entry><filename>sbin</filename></entry>
       <entry>Essential system binaries</entry>
     </row>

     <row>
       <entry><filename>srv</filename></entry>
       <entry>Data for services provided by this system</entry>
     </row>

     <row>
       <entry><filename>tmp</filename></entry>
       <entry>Temporary files</entry>
     </row>

     <row>
       <entry><filename>usr</filename></entry>
       <entry>Secondary hierarchy</entry>
     </row>

     <row>
       <entry><filename>var</filename></entry>
       <entry>Variable data</entry>
     </row>
   </tbody>
  </tgroup>
</informaltable>

<para>Each directory listed above is specified in detail in separate
subsections below.  <filename>/usr</filename> and
<filename>/var</filename> each has a complete section in this
document due to the complexity of those directories.</para>

</section>

<section id='specificOptions'><title>Specific Options</title>

<para>The following directories, or symbolic links to directories,
must be in <filename>/</filename>, if the corresponding subsystem is
installed:</para>


<informaltable frame='none'>
  <tgroup cols='2' align='left'>
  <thead>
     <row>
       <entry>Directory</entry>
       <entry>Description</entry>
     </row>
  </thead>

  <tbody>

     <row>
       <entry><filename>home</filename></entry>
       <entry>User home directories (optional)</entry>
     </row>

     <row>
       <entry><filename>lib<replaceable>&lt;qual&gt;</replaceable></filename></entry>
       <entry>Alternate format essential shared libraries (optional)</entry>
     </row>

     <row>
       <entry><filename>root</filename></entry>
       <entry>Home directory for the root user (optional)</entry>
     </row>

   </tbody>
  </tgroup>
</informaltable>

<para>Each directory listed above is specified in detail in separate
subsections below.</para>

</section>

<section id='binEssentialUserCommandBinaries'>
<title>/bin : Essential user command binaries (for use by all users)</title>

<section id='purpose3'><title>Purpose</title>

<para><filename>/bin</filename> contains commands that may be used by
both the system administrator and by users, but which are required
when no other filesystems are mounted (e.g. in single user mode).  It
may also contain commands which are used indirectly by scripts.

<footnote>
<para>
Command binaries that are not essential enough to place into
<filename>/bin</filename> must be placed in
<filename>/usr/bin</filename>, instead.  Items that are required only
by non-root users (the X Window System, <filename>chsh</filename>,
etc.) are generally not essential enough to be placed into the root
partition.
</para>
</footnote>
</para>

</section>

<section id='requirements2'><title>Requirements</title>

<para>There must be no subdirectories in <filename>/bin</filename>.</para>

<para>The following commands, or symbolic links to commands, are
required in <filename>/bin</filename>:</para>

<informaltable pgwide='0' frame='none'>
  <tgroup cols='2' align='left'>
    <thead>
      <row><entry>Command</entry><entry>Description</entry></row>
    </thead>
    <tbody>
      <row>
        <entry><command>cat</command></entry>
        <entry>Utility to concatenate files to standard output</entry>
      </row>
      <row>
        <entry><command>chgrp</command></entry>
        <entry>Utility to change file group ownership</entry>
      </row>
      <row>
        <entry><command>chmod</command></entry>
        <entry>Utility to change file access permissions</entry>
      </row>
      <row>
        <entry><command>chown</command></entry>
        <entry>Utility to change file owner and group</entry>
      </row>
      <row>
        <entry><command>cp</command></entry>
        <entry>Utility to copy files and directories</entry>
      </row>
      <row>
        <entry><command>date</command></entry>
        <entry>Utility to print or set the system data and time</entry>
      </row>
      <row>
        <entry><command>dd</command></entry>
        <entry>Utility to convert and copy a file</entry>
      </row>
      <row>
        <entry><command>df</command></entry>
        <entry>Utility to report filesystem disk space usage</entry>
      </row>
      <row>
        <entry><command>dmesg</command></entry>
        <entry>Utility to print or control the kernel message buffer</entry>
      </row>
      <row>
        <entry><command>echo</command></entry>
        <entry>Utility to display a line of text</entry>
      </row>
      <row>
        <entry><command>false</command></entry>
        <entry>Utility to do nothing, unsuccessfully</entry>
      </row>
      <row>
        <entry><command>hostname</command></entry>
        <entry>Utility to show or set the system's host name</entry>
      </row>
      <row>
        <entry><command>kill</command></entry>
        <entry>Utility to send signals to processes</entry>
      </row>
      <row>
        <entry><command>ln</command></entry>
        <entry>Utility to make links between files</entry>
      </row>
      <row>
        <entry><command>login</command></entry>
        <entry>Utility to begin a session on the system</entry>
      </row>
      <row>
        <entry><command>ls</command></entry>
        <entry>Utility to list directory contents</entry>
      </row>
      <row>
        <entry><command>mkdir</command></entry>
        <entry>Utility to make directories</entry>
      </row>
      <row>
        <entry><command>mknod</command></entry>
        <entry>Utility to make block or character special files</entry>
      </row>
      <row>
        <entry><command>more</command></entry>
        <entry>Utility to page through text</entry>
      </row>
      <row>
        <entry><command>mount</command></entry>
        <entry>Utility to mount a filesystem</entry>
      </row>
      <row>
        <entry><command>mv</command></entry>
        <entry>Utility to move/rename files</entry>
      </row>
      <row>
        <entry><command>ps</command></entry>
        <entry>Utility to report process status</entry>
      </row>
      <row>
        <entry><command>pwd</command></entry>
        <entry>Utility to print name of current working directory</entry>
      </row>
      <row>
        <entry><command>rm</command></entry>
        <entry>Utility to remove files or directories</entry>
      </row>
      <row>
        <entry><command>rmdir</command></entry>
        <entry>Utility to remove empty directories</entry>
      </row>
      <row>
        <entry><command>sed</command></entry>
        <entry>The `sed' stream editor</entry>
      </row>
      <row>
        <entry><command>sh</command></entry>
        <entry>POSIX compatible command shell</entry>
      </row>
      <row>
        <entry><command>stty</command></entry>
        <entry>Utility to change and print terminal line settings</entry>
      </row>
      <row>
        <entry><command>su</command></entry>
        <entry>Utility to change user ID</entry>
      </row>
      <row>
        <entry><command>sync</command></entry>
        <entry>Utility to flush filesystem buffers</entry>
      </row>
      <row>
        <entry><command>true</command></entry>
        <entry>Utility to do nothing, successfully</entry>
      </row>
      <row>
        <entry><command>umount</command></entry>
        <entry>Utility to unmount file systems</entry>
      </row>
      <row>
        <entry><command>uname</command></entry>
        <entry>Utility to print system information</entry>
      </row>
    </tbody>
  </tgroup>
</informaltable>


<para>If <command>/bin/sh</command> is not the POSIX compatible shell
command itself, it must be a hard or symbolic link to the real shell
command.</para>

<para>The <command>[</command> and <command>test</command>
commands must be placed together in either <filename>/bin</filename>
or <filename>/usr/bin</filename>.</para>


<tip><title>Rationale</title>

<para>Various shells behave differently when called as
<command>sh</command>, so as to preserve POSIX compatibility while
allowing changes or extensions to POSIX when desired.</para>

<para>The requirement for the <command>[</command> and
<command>test</command> commands to be included as binaries (even if
implemented internally by the shell) is shared with the POSIX.1-2008
standard.</para>

</tip>

</section>

<section id='specificOptions2'><title>Specific Options</title>

<para>The following programs, or symbolic links to programs, must be
in <filename>/bin</filename> if the corresponding subsystem is
installed:</para>

<informaltable pgwide='0' frame='none'>
  <tgroup cols='2' align='left'>
    <thead>
      <row><entry>Command</entry><entry>Description</entry></row>
    </thead>
    <tbody>
      <row>
        <entry><command>csh</command></entry>
        <entry>The C shell (optional)</entry>
      </row>
      <row>
        <entry><command>ed</command></entry>
        <entry>The `ed' editor (optional)</entry>
      </row>
      <row>
        <entry><command>tar</command></entry>
        <entry>The tar archiving utility (optional)</entry>
      </row>
      <row>
        <entry><command>cpio</command></entry>
        <entry>The cpio archiving utility (optional)</entry>
      </row>
      <row>
        <entry><command>gzip</command></entry>
        <entry>The GNU compression utility (optional)</entry>
      </row>
      <row>
        <entry><command>gunzip</command></entry>
        <entry>The GNU uncompression utility (optional)</entry>
      </row>
      <row>
        <entry><command>zcat</command></entry>
        <entry>The GNU uncompression utility (optional)</entry>
      </row>
      <row>
        <entry><command>netstat</command></entry>
        <entry>The network statistics utility (optional)</entry>
      </row>
      <row>
        <entry><command>ping</command></entry>
        <entry>The ICMP network test utility (optional)</entry>
      </row>
    </tbody>
  </tgroup>
</informaltable>

<para><command>/bin/csh</command> may be a symbolic link to
<command>/bin/tcsh</command> or
<command>/usr/bin/tcsh</command>.</para>

<tip><title>Rationale</title>

<para>The <command>tar</command>, <command>gzip </command>
and <command>cpio</command>
commands have been added to make restoration of a
system possible (provided that <filename>/</filename> is intact).</para>

<para>Conversely, if no restoration from the root partition is ever
expected, then these binaries might be omitted (e.g., a ROM chip root,
mounting <filename>/usr</filename> through NFS).  If restoration of a
system is planned through the network, then <command>ftp</command>
or <command>tftp</command> (along with everything necessary to get
an ftp connection) must be available on the root partition.</para>

</tip>
</section>
</section>


<section id='bootStaticFilesOfTheBootLoader'>
<title>/boot : Static files of the boot loader</title>

<section id='purpose4'><title>Purpose</title>

<para>This directory contains everything required for the boot process
except configuration files not needed at boot time and the map
installer. Thus <filename>/boot</filename> stores data that is used
before the kernel begins executing user-mode programs.  This may
include saved master boot sectors and sector map files.</para>

<para> Programs necessary to arrange for the boot loader to be able to
boot a file must be placed in <filename>/sbin</filename>.
Configuration files for boot loaders that are not required at boot
time must be placed in <filename>/etc</filename>.</para>

</section>

<section id='specificOptions3'><title>Specific Options</title>

<para>The operating system kernel must be located in either
<filename>/</filename> or <filename>/boot</filename>.</para>

<para>Certain architectures may have other requirements for
<filename>/boot</filename> related to limitations or expectations
specific to that architecture.  These requirements are not enumerated
here; distributions are allowed to add requirements as needed to
enable system startup on these architectures.</para>

</section>
</section>

<section id='devDeviceFiles'><title>/dev : Device files</title>

<section id='purpose5'><title>Purpose</title>

<para>The <filename>/dev</filename> directory is the location of
special or device files.</para>

</section>

<section id='specificOptions4'><title>Specific Options</title>

<para>If it is possible that devices in <filename>/dev</filename> will
need to be manually created, <filename>/dev</filename> must contain a
command named <filename>MAKEDEV</filename>, which can create devices
as needed.  It may also contain a <filename>MAKEDEV.local</filename>
for any local devices.</para>

<para>If required, <filename>MAKEDEV</filename> must have provisions
for creating any device that may be found on the system, not just
those that a particular distribution installs.</para>

</section>
</section>

<section id='etcHostspecificSystemConfiguration'>
<title>/etc : Host-specific system configuration</title>

<section id='purpose6'>
<title>Purpose</title>

<para>The <filename>/etc</filename> hierarchy contains configuration
files.  A "configuration file" is a local file used to control the
operation of a program; it must be static and cannot be an executable
binary.

<footnote>
<para>
To be clear, <filename>/etc</filename> may contain executable scripts,
such as the command scripts commonly called by
<filename>init</filename> to start and shut down the system and start
daemon processes.  "Executable binary" in this context refers to
direct machine code or pseudocode not in a human-readable format, such
as native ELF executables.
</para>
</footnote>
</para>

<para>
It is recommended that files be stored in subdirectories of
<filename>/etc</filename> rather than directly in
<filename>/etc</filename>.
</para>

</section>

<section id='requirements3'><title>Requirements</title>

<para>No binaries may be located under
<filename>/etc</filename>.</para>

<para>The following directories, or symbolic links to directories are
required in <filename>/etc</filename>:</para>

<informaltable frame='none'>
  <tgroup cols='2' align='left'>
  <thead>
     <row>
       <entry>Directory</entry>
       <entry>Description</entry>
     </row>
  </thead>

  <tbody>

     <row>
       <entry>opt</entry>
       <entry>Configuration for /opt</entry>
     </row>

   </tbody>
  </tgroup>
</informaltable>

</section>

<section id='specificOptions5'><title>Specific Options</title>

<para>The following directories, or symbolic links to directories must
be in <filename>/etc</filename>, if the corresponding subsystem is
installed:</para>

<informaltable frame='none'>
  <tgroup cols='2' align='left'>
  <thead>
     <row>
       <entry>Directory</entry>
       <entry>Description</entry>
     </row>
  </thead>

  <tbody>

     <row>
       <entry>X11</entry>
       <entry>Configuration for the X Window system (optional)</entry>
     </row>
     <row>
       <entry>sgml</entry>
       <entry>Configuration for SGML (optional)</entry>
     </row>
     <row>
       <entry>xml</entry>
       <entry>Configuration for XML (optional)</entry>
     </row>

   </tbody>
  </tgroup>
</informaltable>


<para>The following files, or symbolic links to files, must be in
<filename>/etc</filename> if the corresponding subsystem is
installed:

<footnote>
<para>
Systems that use the shadow password suite will have additional
configuration files in <filename>/etc</filename>
(<filename>/etc/shadow</filename> and others) and programs in
<filename>/usr/sbin</filename> (<command>useradd</command>,
<command>usermod</command>, and others).
</para>
</footnote>
</para>


<informaltable frame='none'>
  <tgroup cols='2' align='left'>
    <thead>
      <row><entry>File</entry><entry>Description</entry></row>
    </thead>
    <tbody>
      <row>
        <entry><filename>csh.login</filename></entry>
        <entry>Systemwide initialization file for C shell logins (optional)</entry>
      </row>
      <row>
        <entry><filename>exports</filename></entry>
        <entry>NFS filesystem access control list (optional)</entry>
      </row>
      <row>
        <entry><filename>fstab</filename></entry>
        <entry>Static information about filesystems (optional)</entry>
      </row>
      <row>
        <entry><filename>ftpusers</filename></entry>
        <entry>FTP daemon user access control list (optional)</entry>
      </row>
      <row>
        <entry><filename>gateways</filename></entry>
        <entry>File which lists gateways for routed (optional)</entry>
      </row>
      <row>
        <entry><filename>gettydefs</filename></entry>
        <entry>Speed and terminal settings used by getty (optional)</entry>
      </row>
      <row>
        <entry><filename>group</filename></entry>
        <entry>User group file (optional)</entry>
      </row>
      <row>
        <entry><filename>host.conf</filename></entry>
        <entry>Resolver configuration file (optional)</entry>
      </row>
      <row>
        <entry><filename>hosts</filename></entry>
        <entry>Static information about host names (optional)</entry>
      </row>
      <row>
        <entry><filename>hosts.allow</filename></entry>
        <entry>Host access file for TCP wrappers (optional)</entry>
      </row>
      <row>
        <entry><filename>hosts.deny</filename></entry>
        <entry>Host access file for TCP wrappers (optional)</entry>
      </row>
      <row>
        <entry><filename>hosts.equiv</filename></entry>
        <entry>List of trusted hosts for rlogin, rsh, rcp (optional)</entry>
      </row>
      <row>
        <entry><filename>hosts.lpd</filename></entry>
        <entry>List of trusted hosts for lpd (optional)</entry>
      </row>
      <row>
        <entry><filename>inetd.conf</filename></entry>
        <entry>Configuration file for inetd (optional)</entry>
      </row>
      <row>
        <entry><filename>inittab</filename></entry>
        <entry>Configuration file for init (optional)</entry>
      </row>
      <row>
        <entry><filename>issue</filename></entry>
        <entry>Pre-login message and identification file (optional)</entry>
      </row>
      <row>
        <entry><filename>ld.so.conf</filename></entry>
        <entry>List of extra directories to search for shared libraries (optional)</entry>
      </row>
      <row>
        <entry><filename>motd</filename></entry>
        <entry>Post-login message of the day file (optional)</entry>
      </row>
      <row>
        <entry><filename>mtab</filename></entry>
        <entry>Dynamic information about filesystems (optional)</entry>
      </row>
      <row>
        <entry><filename>mtools.conf</filename></entry>
        <entry>Configuration file for mtools (optional)</entry>
      </row>
      <row>
        <entry><filename>networks</filename></entry>
        <entry>Static information about network names (optional)</entry>
      </row>
      <row>
        <entry><filename>passwd</filename></entry>
        <entry>The password file (optional)</entry>
      </row>
      <row>
        <entry><filename>printcap</filename></entry>
        <entry>The lpd printer capability database (optional)</entry>
      </row>
      <row>
        <entry><filename>profile</filename></entry>
        <entry>Systemwide initialization file for sh shell logins (optional)</entry>
      </row>
      <row>
        <entry><filename>protocols</filename></entry>
        <entry>IP protocol listing (optional)</entry>
      </row>
      <row>
        <entry><filename>resolv.conf</filename></entry>
        <entry>Resolver configuration file (optional)</entry>
      </row>
      <row>
        <entry><filename>rpc</filename></entry>
        <entry>RPC protocol listing (optional)</entry>
      </row>
      <row>
        <entry><filename>securetty</filename></entry>
        <entry>TTY access control for root login (optional)</entry>
      </row>
      <row>
        <entry><filename>services</filename></entry>
        <entry>Port names for network services (optional)</entry>
      </row>
      <row>
        <entry><filename>shells</filename></entry>
        <entry>Pathnames of valid login shells (optional)</entry>
      </row>
      <row>
        <entry><filename>syslog.conf</filename></entry>
        <entry>Configuration file for syslogd (optional)</entry>
      </row>
    </tbody>
  </tgroup>
</informaltable>



<para><filename>mtab</filename> does not fit the static nature of
<filename>/etc</filename>: it is excepted for historical reasons.

<footnote>
<para>
On some Linux systems, this may be a symbolic link to
<filename>/proc/mounts</filename>, in which case this exception is not
required.
</para>
</footnote>

</para>
</section>

<section id='etcoptConfigurationFilesForOpt'>
<title>/etc/opt : Configuration files for /opt</title>


<section id='purpose7'><title>Purpose</title>

<para>Host-specific configuration files for add-on application
software packages must be installed within the directory
<filename>/etc/opt/&lt;subdir&gt;</filename>, where
<filename>&lt;subdir&gt;</filename> is the name of the subtree in
<filename>/opt</filename> where the static data from that package is
stored.</para>

</section>

<section id='requirements4'><title>Requirements</title>

<para>No structure is imposed on the internal arrangement of
<filename>/etc/opt/&lt;subdir&gt;</filename>.</para>

<para>If a configuration file must reside in a different location in
order for the package or system to function properly, it may be placed
in a location other than
<filename>/etc/opt/&lt;subdir&gt;</filename>.</para>


<tip><title>Rationale</title>

<para>Refer to the rationale for <filename>/opt</filename>.</para>

</tip>
</section>
</section>

<section id='etcx11ConfigurationForTheXWindowS'>
<title>/etc/X11 : Configuration for the X Window System (optional)</title>

<section id='purpose8'><title>Purpose</title>

<para><emphasis>/etc/X11</emphasis> is the location for all X11
host-specific configuration.  This directory is necessary to allow
local control if <emphasis>/usr</emphasis> is mounted read
only.</para>

</section>

<section id='specificOptions6'><title>Specific Options</title>

<para>The following files, or symbolic links to files, must be in
<filename>/etc/X11</filename> if the corresponding subsystem is
installed:</para>


<informaltable frame='none'>
  <tgroup cols='2' align='left'>
    <colspec colname='c1'/>
    <colspec colname='c2'/>
    <thead>
     <row>
       <entry>File</entry>
       <entry>Description</entry>
     </row>
    </thead>
    <tbody>
      <row>
        <entry><filename>xorg.conf</filename></entry>
        <entry>The configuration file for X.org versions 7 and later (optional)</entry>
      </row>
      <row>
        <entry><filename>Xmodmap</filename></entry>
        <entry>Global X11 keyboard modification file (optional)</entry>
      </row>
    </tbody>
  </tgroup>
</informaltable>

<para>Subdirectories of <filename>/etc/X11</filename> may include
those for <filename>xdm</filename> and for any other programs (some
window managers, for example) that need them.

<footnote>
<para>
<filename>/etc/X11/xdm</filename> holds the configuration files for
<filename>xdm</filename>.  These are most of the files previously
found in <filename>/usr/lib/X11/xdm</filename>.  Some local variable
data for <filename>xdm</filename> is stored in
<filename>/var/lib/xdm</filename>.
</para>
</footnote>
</para>

</section>
</section>

<section id='etcsgmlConfigurationFilesForSgmlAn'>
<title>/etc/sgml : Configuration files for SGML (optional)</title>


<section id='purpose9'><title>Purpose</title>

<para>Generic configuration files defining high-level parameters of
the SGML systems are installed here.  Files with names
<filename>*.conf</filename> indicate generic configuration files.
File with names <filename>*.cat</filename> are the DTD-specific
centralized catalogs, containing references to all other catalogs
needed to use the given DTD.  The super catalog file
<filename>catalog</filename> references all the centralized
catalogs.</para>

</section>
</section>
<section>
<title>/etc/xml : Configuration files for XML (optional)</title>


<section><title>Purpose</title>

<para>Generic configuration files defining high-level parameters of
the XML systems are installed here.  Files with names
<filename>*.conf</filename> indicate generic configuration files.
The super catalog file
<filename>catalog</filename> references all the centralized
catalogs.</para>

</section>
</section>


</section>

<section id='homeUserHomeDirectories'>

<title>/home : User home directories (optional)</title>

<section id='purpose10'><title>Purpose</title>

<para><filename>/home</filename> is a fairly standard concept, but it
is clearly a site-specific filesystem.

<footnote>
<para>
Different people prefer to place user accounts in a variety of places.
This section describes only a suggested placement for user home
directories; nevertheless we recommend that all FHS-compliant
distributions use this as the default location for user home
directories. Non-login accounts created for administrative
purposes often have their home directories elsewhere.</para>

<para>On smaller systems, each user's home directory is typically implemented
as a subdirectory directly under <filename>/home</filename>, for example
<filename>/home/smith</filename>, <filename>/home/torvalds</filename>,
<filename>/home/operator</filename>, etc.  On large systems
(especially when the <filename>/home</filename> directories are shared
amongst many hosts using NFS) it is useful to subdivide user home
directories.  Subdivision may be accomplished by using subdirectories
such as <filename>/home/staff</filename>,
<filename>/home/guests</filename>,
<filename>/home/students</filename>, etc.
</para>
</footnote>
The setup will differ from host to host.  Therefore, no program should
assume any specific location for a home directory, rather it
should query for it.
<footnote>
<para>
To find a user's home directory, use a library function such
as <function>getpwent</function>,
<function>getpwent_r</function> of
<function>fgetpwent</function> rather than relying
on <filename>/etc/passwd</filename> because user information may be
stored remotely using systems such as NIS.
</para>
</footnote>

</para>

</section>

<section id='requirements4a'><title>Requirements</title>
<para>
User specific configuration files for applications are stored in the
user's home directory in a file that starts with the '.' character (a
"dot file").  If an application needs to create more than one dot file
then they should be placed in a subdirectory with a name starting with
a '.' character, (a "dot directory").  In this case the configuration
files should not start with the '.' character.
<footnote>
<para>
It is recommended that, apart from autosave and lock files, programs
should refrain from creating non dot files or directories in a home
directory without user consent.
</para>
</footnote>
</para>

</section>

<section id='homeReferences'><title>Home Directory Specifications and Conventions</title>
<para>
A number of efforts have been made in the past to standardize 
the layout of home directories, 
including the XDG Base Directories specification
<footnote>
<para>Found at
<ulink url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html" />
and
<ulink url="http://www.freedesktop.org/wiki/Software/xdg-user-dirs" />.
</para>
</footnote>
and the GLib conventions on user directory contents.
<footnote>
<para>A description of GLib's conventions can be found in the
documentation for GUserDirectory, at
<ulink url="http://developer.gnome.org/glib/unstable/glib-Miscellaneous-Utility-Functions.html#GUserDirectory" />.
</para>
</footnote>
Additional efforts in this direction are possible in the future.
To accomodate software which makes use of these specifications and conventions,
distributions may create directory hierarchies which follow
the specifications and conventions. Those directory hierarchies may be located
underneath home directories.
</para>
</section>
</section>

<section id='libEssentialSharedLibrariesAndKern'>
<title>/lib : Essential shared libraries and kernel modules</title>

<section id='purpose11'><title>Purpose</title>

<para>The <filename>/lib</filename> directory contains those shared
library images needed to boot the system and run the commands in the
root filesystem, ie. by binaries in <filename>/bin</filename> and
<filename>/sbin</filename>.

<footnote>
<para>
Shared libraries that are only necessary for binaries in
<filename>/usr</filename> (such as any X Window binaries) must not be
in <filename>/lib</filename>. Only the shared libraries required to
run binaries in <filename>/bin</filename> and
<filename>/sbin</filename> may be here.  In particular, the library
<filename>libm.so.*</filename> may also be placed in
<filename>/usr/lib</filename> if it is not required by anything in
<filename>/bin</filename> or <filename>/sbin</filename>.
</para>
</footnote>
</para>

</section>

<section id='requirements5'><title>Requirements</title>

<para>At least one of each of the following filename patterns are
required (they may be files, or symbolic links):</para>

<informaltable pgwide='0' frame='none'>
  <tgroup cols='2' align='left'>
    <thead>
     <row>
       <entry>File</entry>
       <entry>Description</entry>
     </row>
     </thead>
     <tbody>
      <row>
        <entry><filename>libc.so.*</filename></entry>
        <entry>The dynamically-linked C library (optional)</entry>
      </row>
      <row>
        <entry><filename>ld*</filename></entry>
        <entry>The execution time linker/loader (optional)</entry>
      </row>
    </tbody>
  </tgroup>
</informaltable>

<para>If a C preprocessor is installed, <emphasis>/lib/cpp</emphasis>
must be a reference to it, for historical reasons.

<footnote>
<para>
The usual placement of this binary is <filename>/usr/bin/cpp</filename>.
</para>
</footnote>
</para>

</section>

<section id='specificOptions7'><title>Specific Options</title>

<para>The following directories, or symbolic links to directories,
must be in <filename>/lib</filename>, if the corresponding subsystem
is installed:</para>

<informaltable frame='none'>
  <tgroup cols='2' align='left'>
  <thead>
     <row>
       <entry>Directory</entry>
       <entry>Description</entry>
     </row>
  </thead>

  <tbody>

     <row>
       <entry><filename>modules</filename></entry>
       <entry>Loadable kernel modules (optional)</entry>
     </row>

   </tbody>
  </tgroup>
</informaltable>


</section>
</section>

<section id='libltqualgtAlternateFormatEssential'>
<title>/lib<replaceable>&lt;qual&gt;</replaceable> : Alternate format essential shared libraries (optional)</title>

<section id='purpose12'><title>Purpose</title>

<para>There may be one or more variants of the
<filename>/lib</filename> directory on systems which support more than
one binary format requiring separate libraries.

<footnote>
<para>
This is commonly used for 64-bit or 32-bit support on
systems which support multiple binary formats, but require libraries
of the same name.  In this case, <filename>/lib32</filename> and
<filename>/lib64</filename> might be the library directories, and
<filename>/lib</filename> a symlink to one of them.
</para>
</footnote>
</para>

</section>

<section id='requirements6'><title>Requirements</title>

<para>If one or more of these directories exist, the requirements for
their contents are the same as the normal <filename>/lib</filename>
directory, except that <filename>/lib<replaceable>&lt;qual&gt;</replaceable>/cpp</filename> is
not required.

<footnote>
<para>
<filename>/lib<replaceable>&lt;qual&gt;</replaceable>/cpp</filename> is still permitted: this
allows the case where <filename>/lib</filename> and
<filename>/lib<replaceable>&lt;qual&gt;</replaceable></filename> are the same (one is a symbolic
link to the other). </para> </footnote></para>

</section>
</section>

<section id='mediaMountPoint'>
<title>/media : Mount point for removable media</title>

<section id='purposeMediaMountPoint'><title>Purpose</title>

<para>This directory contains subdirectories which are used as mount
points for removable media such as floppy disks, cdroms and zip
disks.</para>

<tip><title>Rationale</title> 

<para>Historically there have been a number of other different places
used to mount removable media such as <filename>/cdrom</filename>,
<filename>/mnt</filename> or <filename>/mnt/cdrom</filename>. Placing
the mount points for all removable media directly in the root
directory would potentially result in a large number of extra
directories in <filename>/</filename>. Although the use of
subdirectories in <filename>/mnt</filename> as a mount point has
recently been common, it conflicts with a much older tradition of
using <filename>/mnt</filename> directly as a temporary mount point.
</para>
</tip>

</section>

<section id='specificOptionsMediaMount'><title>Specific Options</title>

<para>The following directories, or symbolic links to directories,
must be in <filename>/media</filename>, if the corresponding subsystem
is installed:</para>

<informaltable frame='none'>
  <tgroup cols='2' align='left'>
  <thead>
     <row>
       <entry>Directory</entry>
       <entry>Description</entry>
     </row>
  </thead>

  <tbody>

     <row>
       <entry><filename>floppy</filename></entry>
       <entry>Floppy drive (optional)</entry>
     </row>

     <row>
       <entry><filename>cdrom</filename></entry>
       <entry>CD-ROM drive (optional)</entry>
     </row>

     <row>
       <entry><filename>cdrecorder</filename></entry>
       <entry>CD writer (optional)</entry>
     </row>

     <row>
       <entry><filename>zip</filename></entry>
       <entry>Zip drive (optional)</entry>
     </row>

   </tbody>
  </tgroup>
</informaltable>

<para>On systems where more than one device exists for mounting a
certain type of media, mount directories can be created by appending a
digit to the name of those available above starting with '0', but the
unqualified name must also exist.

<footnote>
<para>
A compliant distribution with two CDROM drives might have
<filename>/media/cdrom0</filename> and
<filename>/media/cdrom1</filename> with
<filename>/media/cdrom</filename> a symlink to either of these.
</para>

</footnote>

</para>

</section>
</section>

<section id='mntMountPointForATemporarilyMount'>
<title>/mnt : Mount point for a temporarily mounted filesystem</title>

<section id='purpose13'><title>Purpose</title>

<para>This directory is provided so that the system administrator may
temporarily mount a filesystem as needed.  The content of this
directory is a local issue and should not affect the manner in which
any program is run.</para>

<para>This directory must not be used by installation programs: a
suitable temporary directory not in use by the system must be used
instead.</para>

</section>
</section>

<section id='optAddonApplicationSoftwarePackages'>
<title>/opt : Add-on application software packages</title>

<section id='purpose14'><title>Purpose</title>

<para><filename>/opt</filename> is reserved for the installation of
add-on application software packages.</para>

<para>A package to be installed in <filename>/opt</filename> must
locate its static files in a separate
<filename>/opt/&lt;package&gt;</filename> or
<filename>/opt/&lt;provider&gt;</filename> directory
tree, where <filename>&lt;package&gt;</filename> is a name that
describes the software package and
<filename>&lt;provider&gt;</filename> is the provider's LANANA
registered name.</para>

</section>

<section id='requirements7'><title>Requirements</title>

<informaltable frame='none'>
  <tgroup cols='2' align='left'>
  <thead>
     <row>
       <entry>Directory</entry>
       <entry>Description</entry>
     </row>
  </thead>

  <tbody>

     <row>
       <entry>&lt;package&gt;</entry>
       <entry>Static package objects</entry>
     </row>

     <row>
       <entry>&lt;provider&gt;</entry>
       <entry>LANANA registered provider name</entry>
     </row>

   </tbody>
  </tgroup>
</informaltable>


<para>The directories <filename>/opt/bin</filename>,
<filename>/opt/doc</filename>, <filename>/opt/include</filename>,
<filename>/opt/info</filename>, <filename>/opt/lib</filename>, and
<filename>/opt/man</filename> are reserved for local system
administrator use.  Packages may provide "front-end" files intended to
be placed in (by linking or copying) these reserved directories by the
local system administrator, but must function normally in the absence
of these reserved directories.</para>

<para>Programs to be invoked by users must be located in the directory
<filename>/opt/&lt;package&gt;/bin</filename> or under the
<filename>/opt/&lt;provider&gt;</filename> hierarchy. If the package
includes UNIX manual pages, they must be located in
<filename>/opt/&lt;package&gt;/share/man</filename> or under the
<filename>/opt/&lt;provider&gt;</filename> hierarchy, and the same
substructure as <filename>/usr/share/man</filename> must be
used.</para>

<para>Package files that are variable (change in normal operation)
must be installed in <filename>/var/opt</filename>.  See the section
on <filename>/var/opt</filename> for more information.</para>

<para>Host-specific configuration files must be installed in
<filename>/etc/opt</filename>.  See the section on
<filename>/etc</filename> for more information.</para>

<para>No other package files may exist outside the
<filename>/opt</filename>, <filename>/var/opt</filename>, and
<filename>/etc/opt</filename> hierarchies except for those package
files that must reside in specific locations within the filesystem
tree in order to function properly.  For example, device lock files
must be placed in <filename>/var/lock</filename> and devices must be
located in <filename>/dev</filename>.</para>

<para>Distributions may install and otherwise manage software in
<filename>/opt</filename> under an appropriately registered
subdirectory.</para>

<tip><title>Rationale</title>

<para>The use of <filename>/opt</filename> for add-on software is a
well-established practice in the UNIX community.  The System V
Application Binary Interface [AT&amp;T 1990], based on the System V
Interface Definition (Third Edition), provides for an
<filename>/opt</filename> structure very similar to the one defined
here.</para>

<para>The Intel Binary Compatibility Standard v. 2 (iBCS2) also
provides a similar structure for <filename>/opt</filename>.</para>

<para>Generally, all data required to support a package on a system
must be present within <filename>/opt/&lt;package&gt;</filename>,
including files intended to be copied into
<filename>/etc/opt/&lt;package&gt;</filename> and
<filename>/var/opt/&lt;package&gt;</filename> as well as reserved
directories in <filename>/opt</filename>.</para>

<para>The minor restrictions on distributions using
<filename>/opt</filename> are necessary because conflicts are possible
between distribution-installed and locally-installed software,
especially in the case of fixed pathnames found in some binary
software.</para>

<para>The structure of the directories below
<filename>/opt/&lt;provider&gt;</filename> is left up to the packager
of the software, though it is recommended that packages are installed
in <filename>/opt/&lt;provider&gt;/&lt;package&gt;</filename> and
follow a similar structure to the guidelines for
<filename>/opt/&lt;package&gt;</filename>. A valid reason for diverging from
this structure is for support packages which may have files installed
in <filename>/opt/&lt;provider&gt;/lib</filename> or
<filename>/opt/&lt;provider&gt;/bin</filename>.</para>

</tip>
</section>
</section>

<section id='rootHomeDirectoryForTheRootUser'>
<title>/root : Home directory for the root user (optional)</title>

<section id='purpose15'><title>Purpose</title>

<para>The root account's home directory may be determined by developer
or local preference, but this is the recommended default
location.

<footnote><para>If the home directory of the root account is not
stored on the root partition it will be necessary to make certain it
will default to <filename>/</filename> if it cannot be
located.</para>

<para>We recommend against using the root account for tasks that can be
performed as an unprivileged user, and that it be used solely for system
administration.  For this reason, we recommend that subdirectories for
mail and other applications not appear in the root account's home
directory, and that mail for administration roles such as root,
postmaster, and webmaster be forwarded to an appropriate user.
</para>
</footnote>
</para>

</section>
</section>

<section id='runRuntimeVariableData'>
<title>/run : Run-time variable data</title>

<section id='runPurpose'><title>Purpose</title>

<para>This directory contains system information data describing the
system since it was booted.  Files under this directory must be
cleared (removed or truncated as appropriate) at the beginning of the
boot process.</para>

<para>The purposes of this directory were once served by
<filename>/var/run</filename>.  In general, programs may continue to
use <filename>/var/run</filename> to fulfill the requirements set out
for <filename>/run</filename> for the purposes of backwards
compatibility.  Programs which have migrated to use
<filename>/run</filename> should cease their usage of
<filename>/var/run</filename>, except as noted in the section on
<filename>/var/run</filename>.</para>

<para>Programs may have a subdirectory of
<filename>/run</filename>; this is encouraged for programs that
use more than one run-time file.  Users may also have a subdirectory
of <filename>/run</filename>, although care must be taken to
appropriately limit access rights to prevent unauthorized use of
<filename>/run</filename> itself and other subdirectories.

<footnote>
<para>
<filename>/run</filename> should not be writable for unprivileged
users; it is a major security problem if any user can write in this
directory.  User-specific subdirectories should be writable only by
each directory's owner.
</para>
</footnote>
</para>

</section>

<section id='runRequirements'><title>Requirements</title>

<para>
Process identifier (PID) files, which were originally placed in
<filename>/etc</filename>, must be placed in
<filename>/run</filename>.  The naming convention for PID files is
<filename>&lt;program-name&gt;.pid</filename>.  For example, the
<command>crond</command> PID file is named
<filename>/run/crond.pid</filename>.</para>

<para>The internal format of PID files remains unchanged.  The file
must consist of the process identifier in ASCII-encoded decimal,
followed by a newline character.  For example, if
<command>crond</command> was process number 25,
<filename>/run/crond.pid</filename> would contain three characters:
two, five, and newline.</para>

<para>Programs that read PID files should be somewhat flexible in what
they accept; i.e., they should ignore extra whitespace, leading
zeroes, absence of the trailing newline, or additional lines in the
PID file.  Programs that create PID files should use the simple
specification located in the above paragraph.</para>

<para>System programs that maintain transient UNIX-domain sockets must
place them in this directory or an appropriate subdirectory as
outlined above.</para>

</section>
</section>

<section id='sbinSystemBinaries'><title>/sbin : System binaries</title>

<section id='purpose16'><title>Purpose</title>

<para>Utilities used for system administration (and other root-only
commands) are stored in <filename>/sbin</filename>,
<filename>/usr/sbin</filename>, and
<filename>/usr/local/sbin</filename>.  <filename>/sbin</filename>
contains binaries essential for booting, restoring, recovering, and/or
repairing the system in addition to the binaries in
<filename>/bin</filename>.

<footnote>
<para>
Originally, <filename>/sbin</filename> binaries were kept in
<filename>/etc</filename>.  
</para></footnote> 

Programs executed after
<filename>/usr</filename> is known to be mounted (when there are no
problems) are generally placed into <filename>/usr/sbin</filename>.
Locally-installed system administration programs should be placed into
<filename>/usr/local/sbin</filename>.

<footnote> 
<para>Deciding what things go into
<emphasis>"sbin"</emphasis> directories is simple: if a normal (not a
system administrator) user will ever run it directly, then it must be
placed in one of the <emphasis>"bin"</emphasis> directories.  Ordinary
users should not have to place any of the <filename>sbin</filename>
directories in their path.</para>

<para>For example, files such as <command>chfn</command> which users
only occasionally use must still be placed in
<filename>/usr/bin</filename>.  <command>ping</command>, although it
is absolutely necessary for root (network recovery and diagnosis) is
often used by users and must live in <filename>/bin</filename> for
that reason.</para>

<para>We recommend that users have read and execute permission for
everything in <filename>/sbin</filename> except, perhaps, certain
setuid and setgid programs.  The division between
<filename>/bin</filename> and <filename>/sbin</filename> was not
created for security reasons or to prevent users from seeing the
operating system, but to provide a good partition between binaries
that everyone uses and ones that are primarily used for administration
tasks.  There is no inherent security advantage in making
<filename>/sbin</filename> off-limits for users.
</para>
</footnote></para>

</section>

<section id='requirements8'><title>Requirements</title>

<para>There must be no subdirectories in <filename>/sbin</filename>.</para>

<para>The following commands, or symbolic links to commands, are
required in <filename>/sbin</filename>:</para>

<informaltable pgwide='0' frame='none'>
  <tgroup cols='2' align='left'>
    <thead>
      <row>
        <entry>Command</entry>
        <entry>Description</entry>
      </row>
    </thead>
    <tbody>
      <row>
        <entry><command>shutdown</command></entry>
        <entry>Command to bring the system down.</entry>
      </row>
   </tbody>
  </tgroup>
</informaltable>


</section>

<section id='specificOptions8'><title>Specific Options</title>

<para>The following files, or symbolic links to files, must be in
<filename>/sbin</filename> if the corresponding subsystem is
installed:</para>

<informaltable pgwide='0' frame='none'>
  <tgroup cols='2' align='left'>
    <colspec colname='c1'/>
    <colspec colname='c2'/>
    <thead>
      <row><entry>Command</entry><entry>Description</entry></row>
    </thead>
    <tbody>
      <row>
        <entry><filename>fastboot</filename></entry>
        <entry>Reboot the system without checking the disks (optional)</entry>
      </row>
      <row>
        <entry><filename>fasthalt</filename></entry>
        <entry>Stop the system without checking the disks (optional)</entry>
      </row>
      <row>
        <entry><filename>fdisk</filename></entry>
        <entry>Partition table manipulator (optional)</entry>
      </row>
      <row>
        <entry><filename>fsck</filename></entry>
        <entry>File system check and repair utility (optional)</entry>
      </row>
      <row>
        <entry><filename>fsck.*</filename></entry>
        <entry>File system check and repair utility for a specific filesystem (optional)</entry>
      </row>
      <row>
        <entry><filename>getty</filename></entry>
        <entry>The getty program (optional)</entry>
      </row>
      <row>
        <entry><filename>halt</filename></entry>
        <entry>Command to stop the system (optional)</entry>
      </row>
      <row>
        <entry><filename>ifconfig</filename></entry>
        <entry>Configure a network interface (optional)</entry>
      </row>
      <row>
        <entry><filename>init</filename></entry>
        <entry>Initial process (optional)</entry>
      </row>
      <row>
        <entry><filename>mkfs</filename></entry>
        <entry>Command to build a filesystem (optional)</entry>
      </row>
      <row>
        <entry><filename>mkfs.*</filename></entry>
        <entry>Command to build a specific filesystem (optional)</entry>
      </row>
      <row>
        <entry><filename>mkswap</filename></entry>
        <entry>Command to set up a swap area (optional)</entry>
      </row>
      <row>
        <entry><filename>reboot</filename></entry>
        <entry>Command to reboot the system (optional)</entry>
      </row>
      <row>
        <entry><filename>route</filename></entry>
        <entry>IP routing table utility (optional)</entry>
      </row>
      <row>
        <entry><filename>swapon</filename></entry>
        <entry>Enable paging and swapping (optional)</entry>
      </row>
      <row>
        <entry><filename>swapoff</filename></entry>
        <entry>Disable paging and swapping (optional)</entry>
      </row>
      <row>
        <entry><filename>update</filename></entry>
        <entry>Daemon to periodically flush filesystem buffers (optional)</entry>
      </row>
    </tbody>
  </tgroup>
</informaltable>



</section>
</section>

<section id='srvDataForServicesProvidedBySystem'>
<title>/srv : Data for services provided by this system</title>

<section id='purpose16a'>
<title>Purpose</title>

<para><filename>/srv</filename> contains site-specific data which is
served by this system.

<tip><title>Rationale</title>
<para>
This main purpose of specifying this is so that users may find the
location of the data files for a particular service, and so that
services which require a single tree for readonly data, writable data
and scripts (such as cgi scripts) can be reasonably placed. Data that
is only of interest to a specific user should go in that users' home
directory.  If the directory and file structure of the data is not 
exposed to consumers, it should go in <filename>/var/lib</filename>.
</para>

<para>
The methodology used to name subdirectories of
<filename>/srv</filename> is unspecified as there is currently no
consensus on how this should be done.  One method for structuring data
under <filename>/srv</filename> is by protocol,
eg. <filename>ftp</filename>, <filename>rsync</filename>,
<filename>www</filename>, and <filename>cvs</filename>. On large
systems it can be useful to structure <filename>/srv</filename> by
administrative context, such as <filename>/srv/physics/www</filename>,
<filename>/srv/compsci/cvs</filename>, etc. This setup will differ
from host to host. Therefore, no program should rely on a specific
subdirectory structure of <filename>/srv</filename> existing or data
necessarily being stored in <filename>/srv</filename>.  However
<filename>/srv</filename> should always exist on FHS compliant systems
and should be used as the default location for such data.
</para>

<para>
Distributions must take care not to remove locally placed files in
these directories without administrator permission.
<footnote>
<para>
This is particularly important as these areas will often contain both
files initially installed by the distributor, and those added by the
administrator.
</para>
</footnote>
</para>

</tip>
</para>

</section>
</section>

<section id='tmpTemporaryFiles'><title>/tmp : Temporary files</title>


<section id='purpose17'><title>Purpose</title>

<para>The <filename>/tmp</filename> directory must be made available
for programs that require temporary files.</para>

<para>Programs must not assume that any files or directories in
<filename>/tmp</filename> are preserved between invocations of the
program.</para>

<tip><title>Rationale</title>

<para>IEEE standard POSIX.1-2008 lists requirements 
similar to the above section.</para>

<para>Although data stored in <filename>/tmp</filename> may be deleted
in a site-specific manner, it is recommended that files and
directories located in <filename>/tmp</filename> be deleted whenever
the system is booted.</para>

<para>FHS added this recommendation on the basis of historical
precedent and common practice, but did not make it a requirement
because system administration is not within the scope of this
standard.</para>

</tip>
</section>

</section>
</chapter>