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Filesystem Hierarchy Standard
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<h2 id="directory-structure">Directory structure</h2> <p>In the FHS, all files and <a href="/facts/Directory_(file_systems)/VLtLcEXY">directories</a> appear under the <a href="/facts/Root_directory/oJN5waeT">root directory</a> /, even if they are stored on different physical or virtual devices. Some of these directories only exist in a particular system if certain subsystems, such as the <a href="/facts/X_Window_System/ac2vRIQL">X Window System</a>, are installed. </p><p>Most of these directories exist in all <a href="/facts/Unix-like/YAKrAxzd">Unix-like</a> operating systems and are generally used in much the same way; however, the descriptions here are those used specifically for the FHS and are not considered authoritative for platforms other than Linux. </p> <table><tbody><tr><th>Directory</th><th>Description</th></tr><tr><td>/</td><td><i>Primary hierarchy</i> root and <a href="/facts/Root_directory/oJN5waeT">root directory</a> of the entire file system hierarchy.</td></tr><tr><td>/bin</td><td>Essential command <a href="/facts/Executable/VSst9eLA">binaries</a> that need to be available in <a href="/facts/Single-user_mode/GFYQSDke">single-user mode</a>, including to bring up the system or repair it,<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a> for all users (e.g., <a href="/facts/Cat_(Unix)/KQPATH6w">cat</a>, <a href="/facts/Ls/5GsH3v2a">ls</a>, <a href="/facts/Cp_(Unix)/T7n2B1Fg">cp</a>).</td></tr><tr><td><a href="/facts/Boot_folder/lDy2aJWz">/boot</a></td><td><a href="/facts/Boot_loader/6sNpcLMK">Boot loader</a> files (e.g., <a href="/facts/Kernel_(operating_system)/uEj4U3Rz">kernels</a>, <a href="/facts/Initrd/St5yDjAz">initrd</a>).</td></tr><tr><td><a href="/facts/%2fdev/t8ISnXft">/dev</a></td><td><a href="/facts/Device_file/t8ISnXft">Device files</a> (e.g., <a href="/facts/Null_device/MdLRSA8E">/dev/null</a>, /dev/disk0, /dev/sda1, /dev/tty, <a href="/facts/%2fdev%2frandom/o7Xrsrcw">/dev/random</a>).</td></tr><tr><td>/etc</td><td>Host-specific system-wide <a href="/facts/Configuration_file/DDzU0FLJ">configuration files</a>.<p>There has been controversy over the meaning of the name itself. In early versions of the UNIX Implementation Document from Bell Labs, /etc is referred to as the <i><a href="/facts/Et_cetera/ybKaYhN6">etcetera</a> directory</i>,<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> as this directory historically held everything that did not belong elsewhere (however, the FHS restricts /etc to static configuration files and may not contain binaries).<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a> Since the publication of early documentation, the directory name has been re-explained in various ways. Recent interpretations include <a href="/facts/Backronym/LhorLRMH">backronyms</a> such as "Editable Text Configuration" or "Extended Tool Chest".<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a></p></td></tr><tr><td>/etc/opt</td><td>Configuration files for add-on packages stored in /opt.</td></tr><tr><td>/etc/sgml</td><td>Configuration files, such as catalogs, for software that processes <a href="/facts/SGML/euIKXwHV">SGML</a>.</td></tr><tr><td>/etc/X11</td><td>Configuration files for the <a href="/facts/X_Window_System/ac2vRIQL">X Window System</a>, version 11.</td></tr><tr><td>/etc/xml</td><td>Configuration files, such as catalogs, for software that processes <a href="/facts/XML/clNgUfWk">XML</a>.</td></tr><tr><td>/home</td><td>Users' <a href="/facts/Home_directory/cQpQ4nvp">home directories</a>, containing saved files, personal settings, etc.</td></tr><tr><td>/lib</td><td><a href="/facts/Library_(computer_science)/xRzL3P7q">Libraries</a> essential for the <a href="/facts/Binaries/ysttM8hh">binaries</a> in /bin and /sbin.</td></tr><tr><td>/lib<qual></td><td>Alternate format essential libraries. These are typically used on systems that support more than one executable code format, such as systems supporting <a href="/facts/32-bit/TDMVce8s">32-bit</a> and <a href="/facts/64-bit/AgvkQHkM">64-bit</a> versions of an <a href="/facts/Instruction_set/8p8vwUeE">instruction set</a>. Such directories are optional, but if they exist, they have some requirements.</td></tr><tr><td>/media</td><td>Mount points for <a href="/facts/Removable_media/CGJoaUF3">removable media</a> such as <a href="/facts/CD-ROM/JwuaYLJv">CD-ROMs</a> (appeared in FHS-2.3 in 2004).</td></tr><tr><td>/mnt</td><td>Temporarily <a href="/facts/Mount_(computing)/INDgdDX9">mounted</a> filesystems.</td></tr><tr><td>/opt</td><td>Add-on <a href="/facts/Application_software/YQuj7Kog">application software</a> <a href="/facts/Software_package_(installation)/kEX4kx7g">packages</a>.<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a></td></tr><tr><td><a href="/facts/%2fproc/qGd3Ncgz">/proc</a></td><td>Virtual <a href="/facts/File_system/FdPm90uc">filesystem</a> providing <a href="/facts/Process_(computing)/Vcq7EnUV">process</a> and <a href="/facts/Kernel_(operating_system)/uEj4U3Rz">kernel</a> information as files. In Linux, corresponds to a <a href="/facts/Procfs/qGd3Ncgz">procfs</a> mount. Generally, automatically generated and populated by the system, on the fly.</td></tr><tr><td>/root</td><td><a href="/facts/Home_directory/cQpQ4nvp">Home directory</a> for the <a href="/facts/Superuser/0Brr6ol7">root</a> user.</td></tr><tr><td>/run</td><td>Run-time variable data: Information about the running system since last boot, e.g., currently logged-in users and running <a href="/facts/Daemon_(computer_software)/bzuEVGGX">daemons</a>. Files under this directory must be either removed or truncated at the beginning of the boot process, but this is not necessary on systems that provide this directory as a <a href="/facts/Temporary_filesystem/7WM9xAbs">temporary filesystem</a> (<a href="/facts/Tmpfs/7WM9xAbs">tmpfs</a>) (appeared in FHS-3.0 in 2015).</td></tr><tr><td>/sbin</td><td>Essential system binaries (e.g., <a href="/facts/Fsck/v9IpnBLS">fsck</a>, <a href="/facts/Init/0NuCuc1g">init</a>, <a href="/facts/Route_(command)/s7u8T2nk">route</a>).</td></tr><tr><td>/srv</td><td>Site-specific data served by this system, such as data and scripts for web servers, data offered by <a href="/facts/FTP/THuWgzfw">FTP</a> servers, and repositories for <a href="/facts/Version_control_systems/pxCmFXqK">version control systems</a> (appeared in FHS-2.3 in 2004).</td></tr><tr><td>/sys</td><td>Contains information about devices, drivers, and some kernel features.<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a></td></tr><tr><td>/tmp</td><td><a href="/facts/Temporary_folder/8USmCS4N">Directory for temporary files</a> (see also /var/tmp). Often not preserved between system reboots and may be severely size-restricted.</td></tr><tr><td>/usr</td><td><i>Secondary hierarchy</i> for read-only user data; contains the majority of (<a href="/facts/Multi-user/XeKcRNjA">multi-</a>)user utilities and applications. Should be shareable and read-only.<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a><a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a></td></tr><tr><td>/usr/bin</td><td>Non-essential command <a href="/facts/Executable/VSst9eLA">binaries</a> (not needed in <a href="/facts/Single-user_mode/GFYQSDke">single-user mode</a>); for all users.</td></tr><tr><td>/usr/include</td><td>Standard <a href="/facts/Header_file/fg3KEG4V">include files</a>.</td></tr><tr><td>/usr/lib</td><td><a href="/facts/Library_(computer_science)/xRzL3P7q">Libraries</a> for the <a href="/facts/Binaries/ysttM8hh">binaries</a> in /usr/bin and /usr/sbin.</td></tr><tr><td>/usr/libexec</td><td>Binaries run by other programs that are not intended to be executed directly by users or shell scripts (optional).</td></tr><tr><td>/usr/lib<qual></td><td>Alternative-format libraries (e.g., /usr/lib32 for 32-bit libraries on a 64-bit machine (optional)).</td></tr><tr><td>/usr/local</td><td><i>Tertiary hierarchy</i> for local data, specific to this host. Typically has further subdirectories (e.g., bin, lib, share).<a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a></td></tr><tr><td>/usr/sbin</td><td>Non-essential system binaries (e.g., <a href="/facts/Daemon_(computer_software)/bzuEVGGX">daemons</a> for various <a href="/facts/Network_service/GdLvd2ml">network services</a>).</td></tr><tr><td>/usr/share</td><td>Architecture-independent (shared) data.</td></tr><tr><td>/usr/src</td><td><a href="/facts/Source_code/o9J6Jv23">Source code</a> (e.g., the kernel source code with its header files).</td></tr><tr><td>/usr/X11R6</td><td><a href="/facts/X_Window_System/ac2vRIQL">X Window System</a>, Version 11, Release 6 (up to FHS-2.3, optional).</td></tr><tr><td>/var</td><td>Variable files: files whose content is expected to continually change during normal operation of the system, such as logs, spool files, and temporary e-mail files.</td></tr><tr><td>/var/cache</td><td>Application cache data. Such data are locally generated as a result of time-consuming I/O or calculation. The application must be able to regenerate or restore the data. The cached files can be deleted without loss of data.</td></tr><tr><td>/var/lib</td><td>State information. Persistent data modified by programs as they run (e.g., databases, packaging system metadata, etc.).</td></tr><tr><td>/var/lock</td><td>Lock files. Files keeping track of resources currently in use.</td></tr><tr><td>/var/log</td><td>Log files. Various logs.</td></tr><tr><td>/var/mail</td><td>Mailbox files. In some distributions, these files may be located in the deprecated /var/spool/mail.</td></tr><tr><td>/var/opt</td><td>Variable data from add-on packages that are stored in /opt.</td></tr><tr><td>/var/run</td><td>Run-time variable data. This directory contains system information data describing the system since it was booted.<a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a><p>In FHS 3.0, /var/run is replaced by /run; a system should either continue to provide a /var/run directory or provide a symbolic link from /var/run to /run for backwards compatibility.<a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a></p></td></tr><tr><td>/var/spool</td><td><a href="/facts/Spooling/fPbanPPu">Spool</a> for tasks waiting to be processed (e.g., print queues and outgoing mail queue).</td></tr><tr><td>/var/spool/mail</td><td><a href="/facts/Deprecated/1aQL7rnP">Deprecated</a> location for users' mailboxes.<a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a></td></tr><tr><td>/var/tmp</td><td>Temporary files to be preserved between reboots.</td></tr></tbody></table> <h2 id="fhs-compliance">FHS compliance</h2> <p>Most <a href="/facts/Linux_distribution/0vVl4N4f">Linux distributions</a> follow the Filesystem Hierarchy Standard and declare it their own policy to maintain FHS compliance.<a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a><a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a><a class="footnote-ref" id="fnref:17" href="#fn:17"><sup>17</sup></a><a class="footnote-ref" id="fnref:18" href="#fn:18"><sup>18</sup></a> <a href="/facts/GoboLinux/a4kUEGDE">GoboLinux</a><a class="footnote-ref" id="fnref:19" href="#fn:19"><sup>19</sup></a> and <a href="/facts/NixOS/lDoDpNXz">NixOS</a><a class="footnote-ref" id="fnref:20" href="#fn:20"><sup>20</sup></a> provide examples of intentionally non-compliant filesystem implementations. </p><p>Some distributions generally follow the standard but deviate from it in some areas. The FHS is a "trailing standard", and so documents common practices at a point in time. Of course, times change, and distribution goals and needs call for experimentation. Some common deviations include: </p> <ul><li>Modern Linux distributions include a /sys directory as a <a href="/facts/Virtual_filesystem/lRI31T4T">virtual filesystem</a> (<a href="/facts/Sysfs/YHLFf08v">sysfs</a>, comparable to /proc, which is a <a href="/facts/Procfs/qGd3Ncgz">procfs</a>), which stores and allows modification of the devices connected to the system,<a class="footnote-ref" id="fnref:21" href="#fn:21"><sup>21</sup></a> whereas many traditional <a href="/facts/Unix-like/YAKrAxzd">Unix-like</a> operating systems use /sys as a <a href="/facts/Symbolic_link/laB0S68t">symbolic link</a> to the <a href="/facts/Kernel_(operating_system)/uEj4U3Rz">kernel</a> source tree.<a class="footnote-ref" id="fnref:22" href="#fn:22"><sup>22</sup></a></li> <li>Many modern Unix-like systems (such as <a href="/facts/FreeBSD/RKsFTm7F">FreeBSD</a> and <a href="/facts/OpenBSD/xmPS9tek">OpenBSD</a>) via their <a href="/facts/Ports_collection/X7NjT4F5">ports systems</a> install third-party packages into /usr/local, while keeping code considered part of the operating system in /usr.</li> <li>Some Linux distributions no longer differentiate between /lib and /usr/lib and have /lib symlinked to /usr/lib.<a class="footnote-ref" id="fnref:23" href="#fn:23"><sup>23</sup></a></li> <li>Some Linux distributions no longer differentiate between /bin and /usr/bin and between /sbin and /usr/sbin. They may symlink /bin to /usr/bin and /sbin to /usr/sbin. Other distributions choose to consolidate all four, symlinking them to /usr/bin.<a class="footnote-ref" id="fnref:24" href="#fn:24"><sup>24</sup></a></li></ul> <p>Modern Linux distributions include a /run directory as a <a href="/facts/Temporary_filesystem/7WM9xAbs">temporary filesystem</a> (<a href="/facts/Tmpfs/7WM9xAbs">tmpfs</a>), which stores volatile runtime data, following the FHS version 3.0. According to the FHS version 2.3, such data were stored in /var/run, but this was a problem in some cases because this directory is not always available at early boot. As a result, these programs have had to resort to trickery, such as using /dev/.udev, /dev/.mdadm, /dev/.systemd or /dev/.mount directories, even though the device directory is not intended for such data.<a class="footnote-ref" id="fnref:25" href="#fn:25"><sup>25</sup></a> Among other advantages, this makes the system easier to use normally with the root filesystem mounted read-only. For example, below are the changes <a href="/facts/Debian/cBd93qfJ">Debian</a> made in its 2013 Wheezy release:<a class="footnote-ref" id="fnref:26" href="#fn:26"><sup>26</sup></a> </p> <ul><li>/dev/.* → /run/*</li> <li>/dev/shm → /run/shm</li> <li>/dev/shm/* → /run/*</li> <li>/etc/* (writeable files) → /run/* </li> <li>/lib/init/rw → /run</li> <li>/var/lock → /run/lock</li> <li>/var/run → /run</li> <li>/tmp → /run/tmp</li></ul> <h2 id="history">History</h2> <h3>The name of usr</h3> <p>/usr originally stood for "user".<a class="footnote-ref" id="fnref:27" href="#fn:27"><sup>27</sup></a> This was an artifact of early Unix programming. Specifically, when <a href="/facts/Ken_Thompson/KQtrw9YM">Ken Thompson</a> and <a href="/facts/Dennis_Ritchie/I1yu1UWU">Dennis Ritchie</a> were migrating Unix to a PDP-11, the contents of the /bin and /lib directories, which were to be the first directories mounted on startup and to contain all essentials for the OS to function, became too large to fit on an <a href="/facts/RK05/U22Vk2ul">RK05</a> disk drive. So they put some of those files on a second RK05, making sure that the first drive contained everything required for loading the second one. The rest of the files were put into the /usr directory.<a class="footnote-ref" id="fnref:28" href="#fn:28"><sup>28</sup></a> When they got a third drive, users' files were moved to a new directory named /home.<a class="footnote-ref" id="fnref:29" href="#fn:29"><sup>29</sup></a> </p><p>FHS was created as the FSSTND (short for "Filesystem Standard"<a class="footnote-ref" id="fnref:30" href="#fn:30"><sup>30</sup></a>), largely based on similar standards for other <a href="/facts/Unix-like/YAKrAxzd">Unix-like</a> operating systems. Notable examples are these: the hier(7) description of file system layout,<a class="footnote-ref" id="fnref:31" href="#fn:31"><sup>31</sup></a> which has existed since the release of <a href="/facts/Version_7_Unix/ju5nRs7k">Version 7 Unix</a> (in 1979);<a class="footnote-ref" id="fnref:32" href="#fn:32"><sup>32</sup></a> the <a href="/facts/SunOS/oWMcGLqc">SunOS</a> filesystem(7)<a class="footnote-ref" id="fnref:33" href="#fn:33"><sup>33</sup></a> and its successor, the <a href="/facts/Solaris_(operating_system)/KPfT98EJ">Solaris</a> filesystem(7).<a class="footnote-ref" id="fnref:34" href="#fn:34"><sup>34</sup></a><a class="footnote-ref" id="fnref:35" href="#fn:35"><sup>35</sup></a> </p> <h3>Release history</h3> <table><tbody><tr><th>Version</th><th>Release date</th><th>Notes</th></tr><tr><td>Old version, not maintained: 1.0</td><td>1994-02-14</td><td>FSSTND<a class="footnote-ref" id="fnref:36" href="#fn:36"><sup>36</sup></a></td></tr><tr><td>Old version, not maintained: 1.1</td><td>1994-10-09</td><td>FSSTND<a class="footnote-ref" id="fnref:37" href="#fn:37"><sup>37</sup></a></td></tr><tr><td>Old version, not maintained: 1.2</td><td>1995-03-28</td><td>FSSTND<a class="footnote-ref" id="fnref:38" href="#fn:38"><sup>38</sup></a></td></tr><tr><td>Old version, not maintained: 2.0</td><td>1997-10-26</td><td>FHS 2.0 is the direct successor for FSSTND 1.2. Name of the standard was changed to Filesystem Hierarchy Standard.<a class="footnote-ref" id="fnref:39" href="#fn:39"><sup>39</sup></a><a class="footnote-ref" id="fnref:40" href="#fn:40"><sup>40</sup></a><a class="footnote-ref" id="fnref:41" href="#fn:41"><sup>41</sup></a></td></tr><tr><td>Old version, not maintained: 2.1</td><td>2000-04-12</td><td>FHS<a class="footnote-ref" id="fnref:42" href="#fn:42"><sup>42</sup></a><a class="footnote-ref" id="fnref:43" href="#fn:43"><sup>43</sup></a><a class="footnote-ref" id="fnref:44" href="#fn:44"><sup>44</sup></a></td></tr><tr><td>Old version, not maintained: 2.2</td><td>2001-05-23</td><td>FHS<a class="footnote-ref" id="fnref:45" href="#fn:45"><sup>45</sup></a></td></tr><tr><td>Old version, not maintained: 2.3</td><td>2004-01-29</td><td>FHS<a class="footnote-ref" id="fnref:46" href="#fn:46"><sup>46</sup></a></td></tr><tr><td>Latest version: 3.0</td><td>2015-05-18</td><td>FHS<a class="footnote-ref" id="fnref:47" href="#fn:47"><sup>47</sup></a></td></tr><tr><td colspan="3">Legend:Old versionOld version, still maintainedLatest versionLatest preview versionFuture version</td></tr></tbody></table> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Hierarchical_file_system/FVv6r1pe">Hierarchical file system</a></li> <li><a href="/facts/Unix_directory_structure/qWU9Tlnh">Unix directory structure</a></li> <li><a href="/facts/Freedesktop.org/qGlctGYA">XDG Base Directory Specification</a></li></ul> <h2 id="notes">Notes</h2> <h2 id="external-links">External links</h2> <ul><li><a href="https://wiki.linuxfoundation.org/lsb/fhs">Official Home of the Filesystem Hierarchy Standard (FHS) at The Linux Foundation</a> <ul><li><a href="https://refspecs.linuxfoundation.org/fhs.shtml">Full specification texts</a></li></ul></li> <li><a href="https://web.archive.org/web/20211016063611/http://objectroot.org/">"Objectroot"</a>. Archived from the original on 16 October 2021. A proposal for a new filesystem hierarchy, based on object-oriented design principles.</li> <li><a href="https://web.archive.org/web/20050508015157/http://ftp.gwdg.de/pub/cLIeNUX/descriptive/DSFH.html">"Dotted Standard Filename Hierarchy"</a>. Archived from <a href="http://ftp.gwdg.de/pub/cLIeNUX/descriptive/DSFH.html">the original</a> on 8 May 2005. Yet another very different hierarchy (used in cLIeNUX).</li> <li><a href="https://manned.org/hier.7">hier(7)</a> – <a href="/facts/Linux/9FvCKSTq">Linux</a> Programmer's <a href="/facts/Man_page/S2yYjy0J">Manual</a> – Overview, Conventions and Miscellanea</li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>"FHS". The Linux Foundation Wiki. Retrieved 4 January 2022. <a href="https://wiki.linuxfoundation.org/lsb/fhs" target="_blank">https://wiki.linuxfoundation.org/lsb/fhs</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>"lsb:fhs-30 [Wiki]". wiki.linuxfoundation.org. Retrieved 22 February 2023. <a href="https://wiki.linuxfoundation.org/lsb/fhs-30" target="_blank">https://wiki.linuxfoundation.org/lsb/fhs-30</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>"hier(7) - Linux manual page". man7.org. Retrieved 6 January 2021. <a href="https://man7.org/linux/man-pages/man7/hier.7.html" target="_blank">https://man7.org/linux/man-pages/man7/hier.7.html</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>J. DeFelicc (17 March 1972). "E.0". Preliminary Release of UNIX Implementation Document (PDF). p. 8. IMO.1-1. Retrieved 6 June 2024. <a href="http://www.bitsavers.org/pdf/att/unix/Early_UNIX/PreliminaryUnixImplementationDocument_Jun72.pdf" target="_blank">http://www.bitsavers.org/pdf/att/unix/Early_UNIX/PreliminaryUnixImplementationDocument_Jun72.pdf</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>"/etc : Host-specific system configuration". Filesystem Hierarchy Standard 2.3. Retrieved 18 February 2016. <a href="http://www.pathname.com/fhs/pub/fhs-2.3.html#ETCHOSTSPECIFICSYSTEMCONFIGURATION" target="_blank">http://www.pathname.com/fhs/pub/fhs-2.3.html#ETCHOSTSPECIFICSYSTEMCONFIGURATION</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>Define - /etc?, Posted by Cliff, 3 March 2007 - Slashdot. <a href="http://ask.slashdot.org/article.pl?sid=07/03/03/028258" target="_blank">http://ask.slashdot.org/article.pl?sid=07/03/03/028258</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>"/opt : Add-on application software packages". Filesystem Hierarchy Standard 2.3. Retrieved 18 February 2016. <a href="http://www.pathname.com/fhs/pub/fhs-2.3.html#OPTADDONAPPLICATIONSOFTWAREPACKAGES" target="_blank">http://www.pathname.com/fhs/pub/fhs-2.3.html#OPTADDONAPPLICATIONSOFTWAREPACKAGES</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>"/sys : Kernel and system information virtual filesystem". Filesystem Hierarchy Standard 3.0. Retrieved 4 June 2017. <a href="https://refspecs.linuxfoundation.org/FHS_3.0/fhs/ch06.html#sysKernelAndSystemInformation" target="_blank">https://refspecs.linuxfoundation.org/FHS_3.0/fhs/ch06.html#sysKernelAndSystemInformation</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>"Chapter 4. The /usr Hierarchy". 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