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.

/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

/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 us ers. 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

The typical directory in which to install programs and documentation so that they aren't overwritten by the operating system. You will often find programs such as Perl and various others that have been downloaded from the Internet installed in this location.

/var

The directory used by all system programs that write output to the disk. All system logs, spools, and temporary data are written here. This includes logging information such as that written during the boot process, by the mailer, by the login program, and by all other system processes. Incoming and outgoing mail is stored in the /var/spool directory, as are files being sent to printers. Information needed for cron, batch, and at jobs is also found here.

Chapter 5. Working on a Unix System

Unix has a wealth of functions, and you'll want to be aware of a particular subset of them before you start running programs and collecting data. In Chapter 4, we talked about how to organize and manage your files in Unix, as well as how to move around the filesystem. In this chapter we take you on a whirlwind tour through the common Unix commands you'll need to know to work efficiently. We discuss the Unix shell itself, issuing commands in Unix, viewing, editing, and extracting information from your files, shell scripts, and working in a multiuser environment.

Once you've learned to use some of these Unix commands, you'll find that they are astonishingly powerful and flexible, allowing you to modify files in ways that are impossible, or at least not easy, with a conventional word-processing program. For example, with a single command you can find all the instances of a pattern in every file under your home directory. A few simple tricks can create a script that will process every file in your source data directory identically. Another simple script can update a customized local copy of a database every night while you're sleeping.