path: root/man/file-hierarchy.xml

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<refentry id="file-hierarchy">

        <refentryinfo>
                <title>file-hierarchy</title>
                <productname>systemd</productname>

                <authorgroup>
                        <author>
                                <contrib>Developer</contrib>
                                <firstname>Lennart</firstname>
                                <surname>Poettering</surname>
                                <email>lennart@poettering.net</email>
                        </author>
                </authorgroup>
        </refentryinfo>

        <refmeta>
                <refentrytitle>file-hierarchy</refentrytitle>
                <manvolnum>7</manvolnum>
        </refmeta>

        <refnamediv>
                <refname>file-hierarchy</refname>
                <refpurpose>File system hierarchy overview</refpurpose>
        </refnamediv>

        <refsect1>
                <title>Description</title>

                <para>Operating systems using the
                <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
                system and service manager are organized based on a
                file system hierarchy inspired by UNIX, more
                specifically the hierarchy described in the <ulink
                url="http://refspecs.linuxfoundation.org/FHS_2.3/fhs-2.3.html">File
                System Hierarchy</ulink> specification and
                <citerefentry><refentrytitle>hier</refentrytitle><manvolnum>7</manvolnum></citerefentry>. This
                manual page describes a more minimal, modernized
                subset of these specifications that defines more
                strictly the suggestions and restrictions systemd
                makes on the file system hierarchy.</para>
        </refsect1>

        <refsect1>
                <title>General Structure</title>

                <variablelist>
                        <varlistentry>
                                <term><filename>/</filename></term>
                                <listitem><para>The file system
                                root. Usually writable, but this is
                                not required. Possibly a temporary
                                file system (<literal>tmpfs</literal>). Not shared with
                                other hosts (unless read-only). The
                                administrator may create additional
                                top-level subdirectories in this tree,
                                if required and the name does not
                                conflict with any of the directories
                                listed below.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><filename>/boot</filename></term>
                                <listitem><para>The boot partition
                                used for bringing up the system. On
                                EFI systems this is possibly the EFI
                                System Partition, also see
                                <citerefentry><refentrytitle>systemd-boot-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>. This
                                directory is usually strictly local
                                the host, and should be considered
                                read-only, except when a new kernel or
                                boot loader is installed. This
                                directory only exists on systems that
                                run on physical or emulated hardware
                                that requires boot
                                loaders.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><filename>/etc</filename></term>
                                <listitem><para>System-specific
                                configuration. This directory may or
                                may not be read-only. Frequently, this
                                directory is pre-populated with
                                vendor-supplied configuration files,
                                but applications should not make
                                assumptions about this directory
                                being fully populated or populated at
                                all, and should fall back to defaults
                                if configuration is missing.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><filename>/home</filename></term>
                                <listitem><para>The location for
                                normal user's home
                                directories. Possibly shared with
                                other systems, and never
                                read-only. This directory should only
                                be used for normal users, never for
                                system users. This directory and
                                possibly the directories contained
                                within it might only become available
                                or writable in late boot or even on
                                user login only. This directory might
                                be placed on limited-functionality
                                network file systems, hence
                                applications should not assume the
                                full set of file API is available on
                                this directory. Applications should
                                generally not reference this directory
                                directly, but via the per-user
                                <varname>$HOME</varname> environment
                                variable, or via the home directory
                                field of the user
                                database.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><filename>/root</filename></term>
                                <listitem><para>The home directory of
                                the root user. The root user's home
                                directory is located outside of
                                <filename>/home</filename> in order to
                                make sure the root user may log in
                                even without <filename>/home</filename>
                                being available and
                                mounted.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><filename>/srv</filename></term>
                                <listitem><para>The place to store
                                general server payload, managed by the
                                administrator. No restrictions are
                                made how this directory is organized
                                internally. Generally writable, and
                                possibly shared among systems. This
                                directory might become available or
                                writable only very late during
                                boot.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><filename>/tmp</filename></term>
                                <listitem><para>The place for small
                                temporary files. This directory is
                                usually mounted as
                                <literal>tmpfs</literal> instance, and
                                should hence not be used for larger
                                files. (Use
                                <filename>/var/tmp</filename> for
                                larger files.) Since the directory is
                                accessible to other users of the
                                system it is essential that this
                                directory is only written to with the
                                <citerefentry><refentrytitle>mkstemp</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
                                <citerefentry><refentrytitle>mkdtemp</refentrytitle><manvolnum>3</manvolnum></citerefentry>
                                and related calls. This directory is
                                usually flushed at boot-up. Also,
                                files that are not accessed within a
                                certain time are usually automatically
                                deleted. If applications find the
                                environment variable
                                <varname>$TMP</varname> set they
                                should prefer using the directory
                                specified in it over directly
                                referencing
                                <filename>/tmp</filename>.</para></listitem>
                        </varlistentry>

                </variablelist>
        </refsect1>

        <refsect1>
                <title>Runtime Data</title>

                <variablelist>
                        <varlistentry>
                                <term><filename>/run</filename></term>
                                <listitem><para>A
                                <literal>tmpfs</literal> file system
                                for system packages to place runtime
                                data in. This directory is flushed on
                                boot, and generally writable for
                                privileged programs
                                only. Always writable.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><filename>/run/log</filename></term>
                                <listitem><para>Runtime system
                                logs. System components may place
                                private logs in this directory. Always
                                writable, even when
                                <filename>/var/log</filename> might
                                not be accessible
                                yet.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><filename>/run/user</filename></term>
                                <listitem><para>Contains per-user
                                runtime directories, each usually
                                individually mounted
                                <literal>tmpfs</literal>
                                instances. Always writable, flushed at
                                each reboot and when the user logs
                                out. User code should not reference
                                this directory directly, but via the
                                <varname>$XDG_RUNTIME_DIR</varname>
                                environment variable, as documented in
                                the <ulink
                                url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG
                                Base Directory
                                Specification</ulink>.</para></listitem>
                        </varlistentry>
                </variablelist>
        </refsect1>

        <refsect1>
                <title>Vendor-supplied Operating System Resources</title>

                <variablelist>

                        <varlistentry>
                                <term><filename>/usr</filename></term>
                                <listitem><para>Vendor-supplied
                                operating system resources. Usually
                                read-only, but this is not
                                required. Possibly shared between
                                multiple hosts. This directory should
                                not be modified by the administrator,
                                except when installing or removing
                                vendor-supplied
                                packages.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><filename>/usr/bin</filename></term>
                                <listitem><para>Binaries for user
                                commands, that shall appear in the
                                <varname>$PATH</varname> search
                                path. It is recommended not to place
                                binaries in this directory that are
                                not useful for invocation from a shell
                                (such as daemon binaries); these
                                should be placed in a subdirectory of
                                <filename>/usr/lib</filename>
                                instead.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><filename>/usr/include</filename></term>
                                <listitem><para>C and C++ API header
                                files of system
                                libraries.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><filename>/usr/lib</filename></term>
                                <listitem><para>Static vendor data
                                that is compatible with all
                                architectures (though not necessarily
                                architecture-independent). Note that
                                this includes internal
                                executables or other binaries that are
                                not regularly invoked from a
                                shell. Such binaries may be for any
                                architecture supported by the
                                system. Do not place public libraries
                                in this directory, use
                                <varname>$libdir</varname> (see
                                below), instead.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>$libdir</varname></term>
                                <listitem><para>Location for placing
                                dynamic libraries in. The precise
                                location depends on the operating
                                system and the architecture, and is
                                sometimes
                                <filename>/usr/lib</filename>,
                                <filename>/use/lib64</filename> or
                                <filename>/usr/lib/</filename>
                                suffixed by an architecture
                                identifier. This directory should not
                                be used for package-specific data,
                                unless this data is
                                architecture-dependent, too. To query
                                <varname>$libdir</varname> for the
                                primary architecture of the system,
                                invoke
                                <programlisting># pkg-config --variable=libdir systemd</programlisting></para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><filename>/usr/share</filename></term>
                                <listitem><para>Resources shared
                                between multiple packages, such as
                                documentation, man pages, time zone
                                information, fonts and other
                                resources. Usually, the precise
                                location and format of files stored
                                below this directory is subject to
                                specifications that ensure
                                interoperability.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><filename>/usr/share/doc</filename></term>
                                <listitem><para>Documentation for the
                                operating system or system
                                packages.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><filename>/usr/share/factory/etc</filename></term>
                                <listitem><para>Repository for
                                vendor-supplied default configuration
                                files. This directory should be
                                populated with pristine vendor versions
                                of all configuration files that may be
                                placed in
                                <filename>/etc</filename>. This is
                                useful to compare the local
                                configuration of a system with vendor
                                defaults and to populate the local
                                configuration with
                                defaults.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><filename>/usr/share/factory/var</filename></term>

                                <listitem><para>Similar to
                                <filename>/usr/share/factory/etc</filename>
                                but for vendor versions of files in
                                the variable, persistent data
                                directory
                                <filename>/var</filename>.</para></listitem>

                        </varlistentry>
                </variablelist>
        </refsect1>

        <refsect1>
                <title>Persistent Variable System Data</title>

                <variablelist>
                        <varlistentry>
                                <term><filename>/var</filename></term>
                                <listitem><para>Persistent, variable
                                system data. Must be writable. This
                                directory might be pre-populated with
                                vendor-supplied data, but applications
                                should be able to reconstruct
                                necessary files and directories in
                                this subhierarchy should they be
                                missing, as the system might start up
                                without this directory being
                                populated. Persistency is recommended,
                                but optional, to support ephemeral
                                systems. This directory might become
                                available or writable only very late
                                during boot. Components that are
                                required to operate during early boot
                                hence shall not unconditionally rely
                                on this directory.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><filename>/var/cache</filename></term>
                                <listitem><para>Persistent system
                                cache data. System components may
                                place non-essential data in this
                                directory. Flushing this directory
                                should have no effect on operation of
                                programs, except for increased
                                runtimes necessary to rebuild these
                                caches.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><filename>/var/lib</filename></term>
                                <listitem><para>Persistent system
                                data. System components may
                                place private data in this
                                directory.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><filename>/var/log</filename></term>
                                <listitem><para>Persistent system
                                logs. System components may place
                                private logs in this directory, though
                                it is recommended to do most logging
                                via the
                                <citerefentry><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
                                and
                                <citerefentry><refentrytitle>sd_journal_print</refentrytitle><manvolnum>3</manvolnum></citerefentry>
                                calls.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><filename>/var/spool</filename></term>
                                <listitem><para>Persistent system
                                spool data, such as printer or mail
                                queues.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><filename>/var/tmp</filename></term>
                                <listitem><para>The place for larger
                                and persistent temporary files. In
                                contrast to <filename>/tmp</filename>
                                this directory is usually mounted from
                                a persistent physical file system and
                                can thus accept larger files. (Use
                                <filename>/tmp</filename> for smaller
                                files.) This directory is generally
                                not flushed at boot-up, but time-based
                                cleanup of files that have not been
                                accessed for a certain time is
                                applied. The same security
                                restrictions as with
                                <filename>/tmp</filename> apply, and
                                hence only
                                <citerefentry><refentrytitle>mkstemp</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
                                <citerefentry><refentrytitle>mkdtemp</refentrytitle><manvolnum>3</manvolnum></citerefentry>
                                or similar calls should be used to
                                make use of this directory. If
                                applications find the environment
                                variable <varname>$TMP</varname> set
                                they should prefer using the directory
                                specified in it over directly
                                referencing
                                <filename>/var/tmp</filename>.
                                </para></listitem>
                        </varlistentry>

                </variablelist>
        </refsect1>

        <refsect1>
                <title>Virtual Kernel and API File Systems</title>

                <variablelist>
                        <varlistentry>
                                <term><filename>/dev</filename></term>
                                <listitem><para>The root directory for
                                device nodes. Usually this directory
                                is mounted as
                                <literal>devtmpfs</literal> instance,
                                but might be of a different type in
                                sandboxed/containerized setups. This
                                directory is managed jointly by the
                                kernel and
                                <citerefentry><refentrytitle>systemd-udevd</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
                                and should not be written to by other
                                components. A number of special
                                purpose virtual file systems might be
                                mounted below this
                                directory.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><filename>/dev/shm</filename></term>
                                <listitem><para>Place for POSIX shared
                                memory segments, as created via
                                <citerefentry><refentrytitle>shm_open</refentrytitle><manvolnum>3</manvolnum></citerefentry>. This
                                directory is flushed on boot, and is a
                                <literal>tmpfs</literal> file
                                system. Since all users have write
                                access to this directory, special care
                                should be taken to avoid name clashes
                                and vulnerabilities. For normal users,
                                shared memory segments in this
                                directory are usually deleted when the
                                user logs out. Usually it is a better
                                idea to use memory mapped files in
                                <filename>/run</filename> (for system
                                programs) or
                                <varname>$XDG_RUNTIME_DIR</varname>
                                (for user programs) instead of POSIX
                                shared memory segments, since they
                                directories are not world-writable and
                                hence not vulnerable to
                                security-sensitive name
                                clashes.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><filename>/proc</filename></term>
                                <listitem><para>A virtual kernel file
                                system exposing the process list and
                                other functionality. This file system
                                is mostly an API to interface with the
                                kernel and not a place where normal
                                files may be stored. For details, see
                                <citerefentry><refentrytitle>proc</refentrytitle><manvolnum>5</manvolnum></citerefentry>. A
                                number of special purpose virtual file
                                systems might be mounted below this
                                directory.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><filename>/proc/sys</filename></term>
                                <listitem><para>A hierarchy below
                                <filename>/proc</filename> that
                                exposes a number of kernel
                                tunables. The primary way to configure
                                the settings in this API file tree is
                                via
                                <citerefentry><refentrytitle>sysctl.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>
                                files. In sandboxed/containerized
                                setups this directory is generally
                                mounted read-only.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><filename>/sys</filename></term>
                                <listitem><para>A virtual kernel file
                                system exposing discovered devices and
                                other functionality. This file system
                                is mostly an API to interface with the
                                kernel and not a place where normal
                                files may be stored. In
                                sandboxed/containerized setups this
                                directory is generally mounted
                                read-only. A number of special purpose
                                virtual file systems might be mounted
                                below this
                                directory.</para></listitem>
                        </varlistentry>


                </variablelist>
        </refsect1>

        <refsect1>
                <title>Compatibility Symlinks</title>

                <variablelist>
                        <varlistentry>
                                <term><filename>/bin</filename></term>
                                <term><filename>/sbin</filename></term>
                                <term><filename>/usr/sbin</filename></term>

                                <listitem><para>These compatibility
                                symlinks point to
                                <filename>/usr/bin</filename>,
                                ensuring that scripts and binaries
                                referencing these legacy paths
                                correctly find their binaries.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><filename>/lib</filename></term>

                                <listitem><para>This compatibility
                                symlink points to
                                <filename>/usr/lib</filename>,
                                ensuring that programs referencing
                                this legacy path correctly find
                                their resources.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><filename>/lib64</filename></term>

                                <listitem><para>On some architecture
                                ABIs this compatibility symlink points
                                to <varname>$libdir</varname>,
                                ensuring that binaries referencing
                                this legacy path correctly find their
                                dynamic loader. This symlink only
                                exists on architectures whose ABI
                                places the dynamic loader in this
                                path.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><filename>/var/run</filename></term>

                                <listitem><para>This compatibility
                                symlink points to
                                <filename>/run</filename>, ensuring
                                that programs referencing this legacy
                                path correctly find their runtime
                                data.</para></listitem>
                        </varlistentry>

                </variablelist>
        </refsect1>

        <refsect1>
                <title>System Packages</title>

                <para>Developers of system packages should follow
                strict rules when placing their own files in the file
                system. The following table lists recommended
                locations for specific types of files.</para>

                <table>
                  <title>System Package Data Location</title>
                  <tgroup cols='2' align='left' colsep='1' rowsep='1'>
                    <colspec colname="directory" />
                    <colspec colname="purpose" />
                    <thead>
                      <row>
                        <entry>Directory</entry>
                        <entry>Purpose</entry>
                      </row>
                    </thead>
                    <tbody>
                      <row>
                        <entry><filename>/usr/bin</filename></entry>
                        <entry>Package executables that shall appear in the <varname>$PATH</varname> executable search path, compiled for the primary architecture of the operating system. It is not recommended to place internal binaries or binaries that are not commonly invoked from the shell in this directory, such as daemon binaries. As this directory is shared with most other packages of the system special care should be taken to pick unique names for files placed here, that are unlikely to clash with other package's files.</entry>
                      </row>
                      <row>
                        <entry><filename>$libdir</filename></entry>
                        <entry>Public shared libraries of the package. As above, be careful with using too generic names, and pick unique names for your libraries to place here to avoid name clashes.</entry>
                      </row>
                      <row>
                        <entry><filename>/usr/lib/<replaceable>package</replaceable></filename></entry>
                        <entry>Private static vendor resources of the package, including private binaries and libraries, or any other kind of read-only vendor data.</entry>
                      </row>
                      <row>
                        <entry><filename>$libdir/<replaceable>package</replaceable></filename></entry>
                        <entry>Private other vendor resources of the package that are architecture-specific and cannot be shared between architectures. Note that this generally does not include private exectuables since binaries of a specific architecture may be freely invoked from any other supported system architecture.</entry>
                      </row>
                      <row>
                        <entry><filename>/usr/include/<replaceable>package</replaceable></filename></entry>
                        <entry>Public C/C++ APIs of public shared libraries of the package.</entry>
                      </row>
                      <row>
                        <entry><filename>/etc/<replaceable>package</replaceable></filename></entry>
                        <entry>System-specific configuration for the package. It is recommended to default to safe fallbacks if this configuration is missing, if this is possible. Alternatively, a <citerefentry><refentrytitle>tmpfiles.d</refentrytitle><manvolnum>5</manvolnum></citerefentry> fragment may be used to copy or symlink the necessary files and directories from <filename>/usr/share/factory</filename> during boot, via the <literal>L</literal> or <literal>C</literal> directives.</entry>
                      </row>
                      <row>
                        <entry><filename>/run/<replaceable>package</replaceable></filename></entry>
                        <entry>Runtime data for the package. Packages must be able to create the necessary subdirectories in this tree on their own, since the directory is flushed automatically on boot. Alternatively, a <citerefentry><refentrytitle>tmpfiles.d</refentrytitle><manvolnum>5</manvolnum></citerefentry> fragment may be used to create the necessary directories during boot.</entry>
                      </row>
                      <row>
                        <entry><filename>/run/log/<replaceable>package</replaceable></filename></entry>
                        <entry>Runtime log data for the package.</entry>
                      </row>
                      <row>
                        <entry><filename>/var/cache/<replaceable>package</replaceable></filename></entry>
                        <entry>Persistent cache data of the package. If this directory is flushed the application should work correctly on next invocation, though possibly slowed done due to the need to rebuild any local cache files.</entry>
                      </row>
                      <row>
                        <entry><filename>/var/lib/<replaceable>package</replaceable></filename></entry>
                        <entry>Persistent private data of the package. This is the primary place to put persistent data that does not fall into the other categories listed. Packages should be able to create the necessary subdirectories in this tree on their own, since the directory might be missing on boot. Alternatively, a <citerefentry><refentrytitle>tmpfiles.d</refentrytitle><manvolnum>5</manvolnum></citerefentry> fragment may be used to create the necessary directories during boot.</entry>
                      </row>
                      <row>
                        <entry><filename>/var/log/<replaceable>package</replaceable></filename></entry>
                        <entry>Persistent log data of the package.</entry>
                      </row>
                      <row>
                        <entry><filename>/var/spool/<replaceable>package</replaceable></filename></entry>
                        <entry>Persistent spool/queue data of the package.</entry>
                      </row>
                    </tbody>
                  </tgroup>
                </table>
        </refsect1>

        <refsect1>
                <title>See Also</title>
                <para>
                        <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>hier</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>systemd-boot-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>sysctl.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
                </para>
        </refsect1>

</refentry>