FHS

ファイルシステム階層標準（Filesystem Hierarchy Standard）。一般的にLinuxシステムは以下のようなディレクトリ構造を「最低限の骨組みの指針」としている。下記はDebian11の「$man hier」の内容でDebian11にあるものだけ。実際の構造はめっちゃ色々ある。

ディレクトリ	説明
/	This is the root directory. This is where the whole tree starts.
/root	This directory is usually the home directory for the root user (optional).
/home	On machines with home directories for users, these are usually beneath this directory, directly or not. The structure of this directory depends on local administration decisions (optional).
/bin -> usr/bin	This directory contains executable programs which are needed in single user mode and to bring the system up or repair it. This is the primary directory for executable programs. Most programs executed by normal users which are not needed for booting or for repairing the system and which are not installed locally should be placed in this directory.
/sbin -> usr/sbin	Like /bin, this directory holds commands needed to boot the system, but which are usually not executed by normal users. This directory contains program binaries for system administration which are not essential for the boot process, for mounting /usr, or for system repair.
/boot	Contains static files for the boot loader. This directory holds only the files which are needed during the boot process. The map installer and configuration files should go to /sbin and /etc. The operating system kernel (initrd for example) must be located in either / or /boot.
/dev	Special or device files, which refer to physical devices. See mknod(1).
/proc	This is a mount point for the proc filesystem, which provides information about running processes and the kernel. This pseudo-filesystem is described in more detail in proc(5).
/sys	This is a mount point for the sysfs filesystem, which provides information about the kernel like /proc, but better structured, following the formalism of kobject infrastructure.
/run	This directory contains information which describes the system since it was booted. Once this purpose was served by /var/run and programs may continue to use it.
/etc	Contains configuration files which are local to the machine. Some larger software packages, like X11, can have their own subdirectories below /etc.Site-wide configuration files may be placed here or in /usr/etc. Nevertheless, programs should always look for these files in /etc and you may have links for these files to /usr/etc.
/etc/opt	Host-specific configuration files for add-on applications installed in /opt.
/etc/skel	When a new user account is created, files from this directory are usually copied into the user's home directory.
/lib -> /usr/lib	This directory should hold those shared libraries that are necessary to boot the system and to run the commands in the root filesystem.
/lib<qual> -> /usr/lib<qual>	These directories are variants of /lib on system which support more than one binary format requiring separate libraries (optional).
/usr/lib	Object libraries, including dynamic libraries, plus some executables which usually are not invoked directly. More complicated programs may have whole subdirectories there.
/usr/libexec	Directory contains binaries for internal use only and they are not meant to be executed directly by users shell or scripts.
/usr/lib<qual>	These directories are variants of /usr/lib on system which support more than one binary format requiring separate libraries, except that the symbolic link /usr/lib<qual>/X11 is not required (optional).
/lib/modules	Loadable kernel modules (optional).
/tmp	This directory contains temporary files which may be deleted with no notice, such as by a regular job or at system boot up.
/lost+found	（空）This directory contains items lost in the filesystem. These items are usually chunks of files mangled as a consequence of a faulty disk or a system crash.
/mnt	（空）This directory is a mount point for a temporarily mounted filesystem. In some distributions, /mnt contains subdirectories intended to be used as mount points for several temporary filesystems.
/media	（空）This directory contains mount points for removable media such as CD and DVD disks or USB sticks. 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.
/opt	（空）This directory should contain add-on packages that contain static files.
/srv	（空）This directory contains site-specific data that is served by this system.

/usr

ディレクトリ	説明
/usr	This directory is usually mounted from a separate partition. It should hold only shareable, read-only data, so that it can be mounted by various machines running Linux.
/usr/etc	Site-wide configuration files to be shared between several machines may be stored in this directory. However, commands should always reference those files using the /etc directory. Links from files in /etc should point to the appropriate files in /usr/etc.
/usr/include	Include files for the C compiler.
/usr/src	（空）Source files for different parts of the system, included with some packages for reference purposes. Don't work here with your own projects, as files below /usr should be read-only except when installing software (optional).

/usr/lib

ディレクトリ	説明
/usr/lib/groff	Files for the GNU groff document formatting system.

/usr/local

ディレクトリ	説明
/usr/local	This is where programs which are local to the site typically go.
/usr/local/lib	Files associated with locally installed programs.
/usr/local/man	Man pages associated with locally installed programs.
/usr/local/bin	（空）Binaries for programs local to the site.
/usr/local/sbin	（空）Locally installed programs for system administration.
/usr/local/etc	（空）Configuration files associated with locally installed programs.
/usr/local/games	（空）Binaries for locally installed games.
/usr/local/include	（空）Header files for the local C compiler.
/usr/local/share	（空）Local application data that can be shared among different architectures of the same OS.
/usr/local/src	（空）Source code for locally installed software.

/usr/share

ディレクトリ	説明
/usr/share	This directory contains subdirectories with specific application data, that can be shared among different architectures of the same OS. Often one finds stuff here that used to live in /usr/doc or /usr/lib or /usr/man.
/usr/share/dict	Contains the word lists used by spell checkers (optional).
/user/share/dict/words	List of English words (optional).
/usr/share/doc	Documentation about installed programs (optional).
/user/share/info	Info pages go here (optional).
/usr/share/local	Locale information goes here (optional).
/usr/share/man	Manual pages go here in subdirectories according to the man page sections.
/user/share/misc	Miscellaneous data that can be shared among different architectures of the same OS.
/usr/share/terminfo	The database for terminfo (optional).
/usr/sahre/xml	Files for XML (optional).
/usr/sahre/zoneinfo	Files for timezone information (optional).

/var

ディレクトリ	説明
/var	This directory contains files which may change in size, such as spool and log files.
/var/backups	Reserved for historical reasons.
/var/cache	Data cached for programs.
/var/cache/man	Locally formatted man pages (optional).
/var/cache/<package>	Package specific cache data (optional).
/var/cache/man/cat[1-9]	These directories contain preformatted manual pages according to their man page section.
/var/lib	Variable state information for programs.
/var/lib/misc	Miscellaneous state data.
/var/lib/xdm	X display manager variable data (optional).
/var/lib/<editor>	Editor backup files and state (optional).
/var/lib/<name>	These directories must be used for all distribution packaging support.
/var/lib/<package>	State data for packages and subsystems (optional).
/var/lib/<pkgtool>	Packaging support files (optional).
/var/local	Variable data for /usr/local.
/var/lock	Lock files are placed in this directory. The naming convention for device lock files is LCK..<device> where <device> is the device's name in the filesystem. The format used is that of HDU UUCP lock files, that is, lock files contain a PID as a 10-byte ASCII decimal number, followed by a newline character.
/var/log	Miscellaneous log files.
/var/mail	Users' mailboxes. Replaces /var/spool/mail.
/var/opt	Variable data for /opt.
/var/run	Run-time variable files, like files holding process identifiers (PIDs) and logged user information (utmp). Files in this directory are usually cleared when the system boots.
/var/spool	Spooled (or queued) files for various programs.
/var/spool/cron	Spooled jobs for cron(8).
/var/spool/mail	Replaced by /var/mail.
/var/tmp	Like /tmp, this directory holds temporary files stored for an unspecified duration.