Chapter 4. Files and Directories in Unix
4.3.6 Locating Files in System Directories
While all your own files should be created in your home directory or in other areas specifically designated for users to share, you need to be aware of the locations of files in other parts of the system. One benefit of a system environment designed for multiple users is that many users can share common resources while controlling access to their own files.
To say there is a standard Unix filesystem is somewhat of an overstatement, but, like Plato's vision of the perfect chair, we will attempt to imagine one out in the ether. Since Linux is being developed by thousands of programmers on different continents and has the benefit of the development of both Berkeley and AT&T's SysV Unix, along with the POSIX standards, we will use the Linux filesystem as a template and point out major discrepancies when necessary. The current standard for the Linux filesystem is described at http://www.pathname.com/fhs/. Here, we present a brief skeleton of the complete filesystem and point out a few salient features. Most directories described in this section are configurable only by the system administrator; however, as a user, you may sometimes need to know where system files and programs can be found. Figure 4-2 illustrates the major subdirectories, which are further described in the following list.
Figure 4-2. Unix subdirectories
/dev
Contains all the device drivers needed to connect peripherals to the system. Drivers for SCSI, audio, IDE drives, PPP, mice, and most other devices are found here. In general there are no user-
configurable options here.
/etc
Houses all the configuration files local to your machine. This includes items such as the system name, Internet address, password file (unless your machine is part of some larger cluster), filesystem
information, and Unix initialization information.
/home
A common, but not standard, part of Unix. /home is usually a fairly large, separate partition that houses all user home directories. Having /home on a separate partition has the advantage of allowing it to be shared in a cluster environment, and it also makes it difficult for users to completely fill an important system partition and cause it to lock up.
/lost+found
A system directory that is a repository for files and directories that have somehow been misplaced by the system. Typically, users can't cd into this directory. Files usually end up in the lost+found because of a system crash or a disk problem. At times it's possible that your system administrator can recover files that appear to be lost simply by moving them from lost+found and renaming them. There's a separate lost+found for each partition on the system.
/mnt
While not found on all systems, this is the typical place to mount any partitions not described by the standard Unix filesystem description. Under Linux, this is where you will find a mounted CD-ROM or floppy drive.
/nfs
Often used as the top-level directory for any mount points for partitions that are mounted from remote machines.
/opt
A relatively new addition to the Unix filesystem. This is where optional, usually commercial, packages are installed. On many systems you will find higher-end, optimizing compilers installed here.
/root
The home directory for root, i.e., for the system administrator when she is logged in as root.
/sbin, /bin, and /lib
Since the machine may need to start the boot process without the /usr partition present, any programs that are using it prior to mounting the /usr partition must reside on the main or root partition. The contents of the /sbin directory, for instance, are a subset of the /usr/sbin directory. Labeling
directories sbin indicates that only system-level commands are present and that normal users probably won't need them, and therefore don't need to include these directories in their path. The /lib directory is a small subset of system libraries that are needed by programs in /bin and /sbin. Current Unix programs use shared libraries, which means that many programs can use functions from the same library, and so the library needs to be loaded into memory only once. What this means for practical purposes is that programs don't take as much memory as they would if each program included all the library routines, and the programs don't actually run if the correct library has been deleted or hasn't been mounted yet.
/tmp and /var/tmp
Typically configured to be readable/writable/executable by all users. Many standard programs, such as vi, write temporary files to one of these directories while they are running. Normally the system cleans out these directories automatically on a regular basis or when the machine is rebooted. This is a good place to write temporary files, but you can't assume that the system will wait for you to erase them.
/usr
The repository for the majority of programs, compilers, libraries, and documentation for the Unix filesystem. The current recommendation for most Unix systems is that the system should be able to mount /usr as a separate, read-only partition. In a workstation-cluster environment, this means that a server can export a /usr partition, and all the workstations in that cluster will share the programs. This makes the system administrator's job easier and provides users with a uniform set of machines.
/usr/local