file-hierarchy - File system hierarchy overview
Operating systems using the systemd(1) system and service
manager are organized based on a file system hierarchy inspired by UNIX,
more specifically the hierarchy described in the File System
Hierarchy[1] specification and hier(7), with various extensions,
partially documented in the XDG Base Directory Specification[2] and
XDG User Directories[3]. This manual page describes a more
generalized, though minimal and modernized subset of these specifications
that defines more strictly the suggestions and restrictions systemd makes on
the file system hierarchy. Note that this document makes no attempt to
define the directory structure comprehensively, it only documents a skeleton
of a directory tree, that downstreams can extend. Because of that
traditional directories such as /usr/include/ or /var/spool/ are not
covered, even though it might (or might not) make a lot of sense to include
them in the structure of an actually deployed OS.
Many of the paths described here can be queried with the
systemd-path(1) tool.
/
The file system root. Usually writable, but this is not
required. Possibly a temporary file system ("tmpfs"). Not shared
with other hosts (unless read-only).
/boot/
The boot partition used for bringing up the system. On
EFI systems, this is possibly the EFI System Partition (ESP), also see
systemd-gpt-auto-generator(8). This directory is usually strictly local
to 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.
/efi/
If the boot partition /boot/ is maintained separately
from the EFI System Partition (ESP), the latter is mounted here. Tools that
need to operate on the EFI system partition should look for it at this mount
point first, and fall back to /boot/ — if the former doesn't qualify
(for example if it is not a mount point or does not have the correct file
system type MSDOS_SUPER_MAGIC).
/etc/
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.
/home/
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 only after user authentication. 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 $HOME environment variable, or via the home directory
field of the user database.
/root/
The home directory of the root user. The root user's home
directory is located outside of /home/ in order to make sure the root user may
log in even without /home/ being available and mounted.
/srv/
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.
/tmp/
The place for small temporary files. This directory is
usually mounted as a "tmpfs" instance, and should hence not be used
for larger files. (Use /var/tmp/ for larger files.) This directory is usually
flushed at boot-up. Also, files that are not accessed within a certain time
may be automatically deleted.
If applications find the environment variable $TMPDIR set,
they should use the directory specified in it instead of /tmp/ (see
environ(7) and IEEE Std 1003.1[4] for details).
Since /tmp/ is accessible to other users of the system, it is
essential that files and subdirectories under this directory are only
created with mkstemp(3), mkdtemp(3), and similar calls. For
more details, see Using /tmp/ and /var/tmp/ Safely[5].
/run/
A "tmpfs" file system for system packages to
place runtime data, socket files, and similar. This directory is flushed on
boot, and generally writable for privileged programs only. Always
writable.
/run/log/
Runtime system logs. System components may place private
logs in this directory. Always writable, even when /var/log/ might not be
accessible yet.
/run/user/
Contains per-user runtime directories, each usually
individually mounted "tmpfs" instances. Always writable, flushed at
each reboot and when the user logs out. User code should not reference this
directory directly, but via the $XDG_RUNTIME_DIR environment variable,
as documented in the XDG Base Directory Specification[2].
/usr/
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.
/usr/bin/
Binaries and executables for user commands that shall
appear in the $PATH 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
/usr/lib/ instead.
/usr/lib/
Static, private 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
$libdir (see below), instead.
/usr/lib/arch-id/
Location for placing dynamic libraries into, also called
$libdir. The architecture identifier to use is defined on
Multiarch
Architecture Specifiers (Tuples)[6] list. Legacy locations of
$libdir are /usr/lib/, /usr/lib64/. This directory should not be used
for package-specific data, unless this data is architecture-dependent, too.
To query $libdir for the primary architecture of the
system, invoke:
systemd-path system-library-arch
/usr/share/
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.
Note that resources placed in this directory typically are under
shared ownership, i.e. multiple different packages have provided and
consumed these resources, on equal footing, without any obvious primary
owner. This makes things systematically different from /usr/lib/, where
ownership is generally not shared.
/usr/share/doc/
Documentation for the operating system or system
packages.
/usr/share/factory/etc/
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 /etc/. This is useful to compare the
local configuration of a system with vendor defaults and to populate the local
configuration with defaults.
/usr/share/factory/var/
Similar to /usr/share/factory/etc/, but for vendor
versions of files in the variable, persistent data directory /var/.
/var/
Persistent, variable system data. Writable during normal
system operation. 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.
/var/cache/
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.
/var/lib/
Persistent system data. System components may place
private data in this directory.
/var/log/
Persistent system logs. System components may place
private logs in this directory, though it is recommended to do most logging
via the syslog(3) and sd_journal_print(3) calls.
/var/tmp/
The place for larger and persistent temporary files. In
contrast to /tmp/, this directory is usually mounted from a persistent
physical file system and can thus accept larger files. (Use /tmp/ for small
ephemeral 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.
If applications find the environment variable $TMPDIR set,
they should use the directory specified in it instead of /var/tmp/ (see
environ(7) for details).
The same security restrictions as with /tmp/ apply:
mkstemp(3), mkdtemp(3), and similar calls should be used. For
further details about this directory, see Using /tmp/ and /var/tmp/
Safely[5].
/dev/
The root directory for device nodes. Usually, this
directory is mounted as a "devtmpfs" instance, but might be of a
different type in sandboxed/containerized setups. This directory is managed
jointly by the kernel and systemd-udevd(8), and should not be written
to by other components. A number of special purpose virtual file systems might
be mounted below this directory.
/dev/shm/
Place for POSIX shared memory segments, as created via
shm_open(3). This directory is flushed on boot, and is a
"tmpfs" 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 /run/ (for system programs) or
$XDG_RUNTIME_DIR (for user programs) instead of POSIX shared memory
segments, since these directories are not world-writable and hence not
vulnerable to security-sensitive name clashes.
/proc/
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
proc(5). A number of special purpose virtual file systems might be
mounted below this directory.
/proc/sys/
A hierarchy below /proc/ that exposes a number of kernel
tunables. The primary way to configure the settings in this API file tree is
via sysctl.d(5) files. In sandboxed/containerized setups, this
directory is generally mounted read-only.
/sys/
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.
/sys/fs/cgroup/
A virtual kernel file system exposing process control
groups (cgroups). This file system is an API to interface with the kernel and
not a place where normal files may be stored. On current systems running in
the default "unified" mode, this directory serves as the mount point
for the "cgroup2" filesystem, which provides a unified cgroup
hierarchy for all resource controllers. On systems with non-default
configurations, this directory may instead be a tmpfs filesystem containing
mount points for various "cgroup" (v1) resource controllers; in such
configurations, if "cgroup2" is mounted it will be mounted on
/sys/fs/cgroup/unified/, but cgroup2 will not have resource controllers
attached. In sandboxed/containerized setups, this directory may either not
exist or may include a subset of functionality.
/bin/, /sbin/, /usr/sbin/
These compatibility symlinks point to /usr/bin/, ensuring
that scripts and binaries referencing these legacy paths correctly find their
binaries.
/lib/
This compatibility symlink points to /usr/lib/, ensuring
that programs referencing this legacy path correctly find their
resources.
/lib64/
On some architecture ABIs, this compatibility symlink
points to $libdir, 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.
/var/run/
This compatibility symlink points to /run/, ensuring that
programs referencing this legacy path correctly find their runtime data.
User applications may want to place files and directories in the
user's home directory. They should follow the following basic structure.
Note that some of these directories are also standardized (though more
weakly) by the XDG Base Directory Specification[2]. Additional
locations for high-level user resources are defined by
xdg-user-dirs[3].
~/.cache/
Persistent user cache data. User programs 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. If an application finds $XDG_CACHE_HOME set, it
should use the directory specified in it instead of this directory.
~/.config/
Application configuration. When a new user is created,
this directory will be empty or not exist at all. Applications should fall
back to defaults should their configuration in this directory be missing. If
an application finds $XDG_CONFIG_HOME set, it should use the directory
specified in it instead of this directory.
~/.local/bin/
Executables that shall appear in the user's $PATH
search path. It is recommended not to place executables in this directory that
are not useful for invocation from a shell; these should be placed in a
subdirectory of ~/.local/lib/ instead. Care should be taken when placing
architecture-dependent binaries in this place, which might be problematic if
the home directory is shared between multiple hosts with different
architectures.
~/.local/lib/
Static, private vendor data that is compatible with all
architectures.
~/.local/lib/arch-id/
Location for placing public dynamic libraries. The
architecture identifier to use is defined on Multiarch Architecture
Specifiers (Tuples)[6] list.
~/.local/share/
Resources shared between multiple packages, such as fonts
or artwork. Usually, the precise location and format of files stored below
this directory is subject to specifications that ensure interoperability. If
an application finds $XDG_DATA_HOME set, it should use the directory
specified in it instead of this directory.
~/.local/state/
Application state. When a new user is created, this
directory will be empty or not exist at all. Applications should fall back to
defaults should their state in this directory be missing. If an application
finds $XDG_STATE_HOME set, it should use the directory specified in it
instead of this directory.
Unprivileged processes generally lack write access to most of the
hierarchy.
The exceptions for normal users are /tmp/, /var/tmp/, /dev/shm/,
as well as the home directory $HOME (usually found below /home/) and
the runtime directory $XDG_RUNTIME_DIR (found below /run/user/) of
the user, which are all writable.
For unprivileged system processes, only /tmp/, /var/tmp/ and
/dev/shm/ are writable. If an unprivileged system process needs a private
writable directory in /var/ or /run/, it is recommended to either create it
before dropping privileges in the daemon code, to create it via
tmpfiles.d(5) fragments during boot, or via the
StateDirectory= and RuntimeDirectory= directives of service
units (see systemd.unit(5) for details).
/tmp/, /var/tmp/ and /dev/shm/ should be mounted nosuid and
nodev, which means that set-user-id mode and character or block
special devices are not interpreted on those file systems. In general it is
not possible to mount them noexec, because various programs use those
directories for dynamically generated or optimized code, and with that flag
those use cases would break. Using this flag is OK on special-purpose
installations or systems where all software that may be installed is known
and doesn't require such functionality. See the discussion of
nosuid/nodev/noexec in mount(8) and
PROT_EXEC in mmap(2).
As noted above, some systems operate with the /usr and /etc
hierarchies mounted read-only, possibly only allowing write access during
package upgrades. Other part of the hierarchy are generally mounted
read-write (in particular /var and /var/tmp), but may be read-only when the
kernel remounts the file system read-only in response to errors, or when the
system is booted read-only for recovery purposes. To the extent reasonable,
applications should be prepared to execute without write access, so that for
example, failure to save non-essential data to /var/cache/ or failure to
create a custom log file under /var/log does not prevent the application
from running.
The /run/ directory is available since the earliest boot and is
always writable. It should be used for any runtime data and sockets, so that
write access to e.g. /etc or /var is not needed.
Unix file systems support different types of file nodes, including
regular files, directories, symlinks, character and block device nodes,
sockets and FIFOs.
It is strongly recommended that /dev/ is the only location below
which device nodes shall be placed. Similarly, /run/ shall be the only
location to place sockets and FIFOs. Regular files, directories and symlinks
may be used in all directories.
Applications should expect that a security policy might be
enforced on a system that enforces these rules.
Developers of system packages should follow strict rules when
placing their files in the file system. The following table lists
recommended locations for specific types of files supplied by the
vendor.
Table 1. System package vendor files
locations
| Directory |
Purpose |
| /usr/bin/ |
Package executables that shall appear in the $PATH executable
search path, compiled for any of the supported architectures compatible
with 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. |
| /usr/lib/arch-id/ |
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. |
| /usr/lib/package/ |
Private static vendor resources of the package, including private
binaries and libraries, or any other kind of read-only vendor data. |
| /usr/lib/arch-id/package/ |
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 executables since binaries of
a specific architecture may be freely invoked from any other supported
system architecture. |
Additional static vendor files with shared ownership may be
installed in the
/usr/share/ hierarchy to the locations defined by the various relevant
specifications.
The following directories shall be used by the package for local
configuration and files created during runtime:
Table 2. System package variable files
locations
| Directory |
Purpose |
| /etc/package/ |
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 tmpfiles.d(5) fragment may be used to
copy or symlink the necessary files and directories from
/usr/share/factory/ during boot, via the "L" or "C"
directives. |
| /run/package/ |
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 tmpfiles.d(5)
fragment may be used to create the necessary directories during boot, or
the RuntimeDirectory= directive of service units may be used to
create them at service startup (see systemd.unit(5) for
details). |
| /run/log/package/ |
Runtime log data for the package. As above, the package needs to make
sure to create this directory if necessary, as it will be flushed on every
boot. |
| /var/cache/package/ |
Persistent cache data of the package. If this directory is flushed, the
application should work correctly on next invocation, though possibly
slowed down due to the need to rebuild any local cache files. The
application must be capable of recreating this directory should it be
missing and necessary. To create an empty directory, a
tmpfiles.d(5) fragment or the CacheDirectory= directive of
service units (see systemd.unit(5)) may be used. |
| /var/lib/package/ |
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. To create
an empty directory, a tmpfiles.d(5) fragment or the
StateDirectory= directive of service units (see
systemd.unit(5)) may be used. |
| /var/log/package/ |
Persistent log data of the package. As above, the package should make
sure to create this directory if necessary, possibly using
tmpfiles.d(5) or LogsDirectory= (see
systemd.exec(5)), as it might be missing. |
Programs running in user context should follow strict rules when
placing their own files in the user's home directory. The following table
lists recommended locations in the home directory for specific types of
files supplied by the vendor if the application is installed in the home
directory. (User applications installed system-wide are covered by the rules
outlined above for vendor files.)
Table 3. Vendor package file locations under the
home directory of the user
| Directory |
Purpose |
| ~/.local/bin/ |
Package executables that shall appear in the $PATH executable
search path. It is not recommended to place internal executables or
executables that are not commonly invoked from the shell in this
directory, such as daemon executables. As this directory is shared with
most other packages of the user, special care should be taken to pick
unique names for files placed here, that are unlikely to clash with other
package's files. |
| ~/.local/lib/arch-id/ |
Public shared libraries of the package. As above, be careful with using
overly generic names, and pick unique names for your libraries to place
here to avoid name clashes. |
| ~/.local/lib/package/ |
Private, static vendor resources of the package, compatible with any
architecture, or any other kind of read-only vendor data. |
| ~/.local/lib/arch-id/package/ |
Private other vendor resources of the package that are
architecture-specific and cannot be shared between architectures. |
Additional static vendor files with shared ownership may be
installed in the
~/.local/share/ hierarchy, mirroring the subdirectories specified in the
section "Vendor-supplied operating system resources" above.
The following directories shall be used by the package for
per-user local configuration and files created during runtime:
Table 4. User package variable file
locations
| Directory |
Purpose |
| ~/.config/package/ |
User-specific configuration for the package. It is required to default
to safe fallbacks if this configuration is missing. |
| $XDG_RUNTIME_DIR/package/ |
User runtime data for the package. |
| ~/.cache/package/ |
Persistent cache data of the package. If this directory is flushed, the
application should work correctly on next invocation, though possibly
slowed down due to the need to rebuild any local cache files. The
application must be capable of recreating this directory should it be
missing and necessary. |
| ~/.local/state/package/ |
Persistent state data of the package. |
systemd(1), hier(7), systemd-path(1),
systemd-gpt-auto-generator(8), sysctl.d(5),
tmpfiles.d(5), pkg-config(1), systemd.unit(5)
- 1.
- File System Hierarchy
http://refspecs.linuxfoundation.org/FHS_3.0/fhs-3.0.html
- 2.
- XDG Base Directory Specification
https://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html
- 3.
- XDG User Directories
https://www.freedesktop.org/wiki/Software/xdg-user-dirs
- 4.
- IEEE Std 1003.1
http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03
- 5.
- Using /tmp/ and /var/tmp/ Safely
https://systemd.io/TEMPORARY_DIRECTORIES
- 6.
- Multiarch Architecture Specifiers (Tuples)
https://wiki.debian.org/Multiarch/Tuples