UNIX-like操作系统文件系统层次结构标准详解

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"FHS-3.0-文件系统层次结构标准.pdf" 《文件系统层次结构标准》（FilesystemHierarchyStandard，简称FHS）是Linux基础组织（Linux Foundation）的 LSB 工作组发布的一项标准，旨在规定类似UNIX的操作系统中的文件和目录应该如何布局。这个标准的最新版本为3.0，于2015年3月19日发布。FHS 的目标是促进应用程序、系统管理工具、开发工具和脚本之间的互操作性，以及提高这些系统的文档一致性。 FHS 规定了操作系统中各个主要目录及其子目录的功能和用途，确保在不同发行版的Linux系统之间有一致的文件系统结构。以下是FHS中一些关键目录及其作用： 1. `/bin`：包含基本的用户命令，这些命令在单用户模式下也能运行，对所有用户都是必需的。 2. `/boot`：存放引导加载器（如GRUB）和内核文件，用于启动系统。 3. `/dev`：设备文件的存放地，用于访问硬件设备。 4. `/etc`：存储系统配置文件，包括服务配置、用户配置等。 5. `/home`：用户个人目录的根，每个用户的私人数据和配置文件应存放在各自的家目录下。 6. `/lib`：存放系统运行所需的动态链接库和内核模块。 7. `/media`：用于挂载临时设备，如USB驱动器或CD/DVD。 8. `/mnt`：传统上用于临时挂载其他文件系统，但在FHS中不推荐作为默认挂载点。 9. `/opt`：存放可选的应用程序软件包，不随系统一起安装。 10. `/proc`：虚拟文件系统，提供系统进程和内核状态的信息。 11. `/root`：系统管理员（root用户）的家目录。 12. `/sbin`：包含系统管理命令，通常只供管理员使用。 13. `/srv`：存放服务的数据，例如Web服务器的网页内容。 14. `/sys`：另一个虚拟文件系统，用于表示内核对象和设备。 15. `/tmp`：存放临时文件，系统重启时会被清空。 16. `/usr`：包含了非系统运行所必需但对用户和系统管理员有用的文件，如应用程序、库和手册页。 17. `/var`：存放系统运行时产生的可变数据，如日志、缓存和锁文件。遵循FHS标准，开发者可以编写出能在各种Linux发行版上工作的程序，而系统管理员则能更方便地管理和维护系统，提高效率。此外，FHS也有助于保持用户和开发者的习惯一致性，降低学习和使用新系统的难度。

Chapter 2. The Filesystem

This standard assumes that the operating system underlying an FHS-compliant file system supports the

same basic security features found in most UNIX filesystems.

It is possible to define two independent distinctions among files: shareable vs. unshareable and variable

vs. static. In general, files that differ in either of these respects should be located in different directories.

This makes it easy to store files with different usage characteristics on different filesystems.

"Shareable" files are those that can be stored on one host and used on others. "Unshareable" files are those

that are not shareable. For example, the files in user home directories are shareable whereas device lock

files are not.

"Static" files include binaries, libraries, documentation files and other files that do not change without

system administrator intervention. "Variable" files are files that are not static.

Rationale

Shareable files can be stored on one host and used on several others. Typically, however, not all

files in the filesystem hierarchy are shareable and so each system has local storage containing at

least its unshareable files. It is convenient if all the files a system requires that are stored on a

foreign host can be made available by mounting one or a few directories from the foreign host.

Static and variable files should be segregated because static files, unlike variable files, can be

stored on read-only media and do not need to be backed up on the same schedule as variable files.

Historical UNIX-like filesystem hierarchies contained both static and variable files under both

/usr and /etc. In order to realize the advantages mentioned above, the /var hierarchy was

created and all variable files were transferred from /usr to /var. Consequently /usr can now

be mounted read-only (if it is a separate filesystem). Variable files have been transferred from /

etc to /var over a longer period as technology has permitted.

Here is an example of a FHS-compliant system. (Other FHS-compliant layouts are possible.)

shareable unshareable

static /usr /etc

/opt /boot

variable /var/mail /var/run

/var/spool/news /var/lock

Chapter 3. The Root Filesystem

3.1. Purpose

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

• To boot a system, enough software and data must be present on the root partition to mount other

filesystems. This includes utilities, configuration, boot loader information, and other essential start-

up data. /usr, /opt, and /var are designed such that they may be located on other partitions or

filesystems.

• To enable recovery and/or repair of a system, those utilities needed by an experienced maintainer to

diagnose and reconstruct a damaged system must be present on the root filesystem.

• To restore a system, those utilities needed to restore from system backups (on floppy, tape, etc.) must

be present on the root filesystem.

Rationale

The minimum requirements for the root filesystem should be as small as reasonably possible, but

no smaller. While many users may not want the extra complexity of a partitioned system, the

option to keep the root small should be preserved for several reasons:

• It is occasionally mounted from very small media.

• The root filesystem contains many system-specific configuration files. Possible examples

include a kernel that is specific to the system, a specific hostname, etc. This means that the root

filesystem isn't always shareable between networked systems. Keeping it small on servers in

networked systems minimizes the amount of lost space for areas of unshareable files. It also

allows workstations with smaller local hard drives.

• While you may have the root filesystem on a large partition, and may be able to fill it to your

heart's content, there will be people with smaller partitions. If you have more files installed,

you may find incompatibilities with other systems using root filesystems on smaller partitions.

If you are a developer then you may be turning your assumption into a problem for a large

number of users.

• Disk errors that corrupt data on the root filesystem are a greater problem than errors on any

other partition. A small root filesystem is less prone to corruption as the result of a system crash.

These considerations must be balanced against the need for a minimally useful operating

environment, for the sake of the boot process as well as in failure recovery situations.

Applications must never create or require special files or subdirectories in the root directory. Other

locations in the FHS hierarchy provide more than enough flexibility for any package.

Rationale

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

• It demands space on a root partition which the system administrator may want kept small and

simple for either performance or security reasons.

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