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can be manipulated. A slice of pie is akin to a partition on a drive, and the whole pie could represent a single partition that takes up a whole disk. There are more advanced meanings as well, but for this chapter, only a hard drive or the systematic division of a hard drive is considered to be a partition. A partition usually contains only one file system, such as one file system housing the / file system or another containing the /home file system. One file system per partition allows for the logical maintenance and management of differing file systems. These partitions are invisible to users, who can move effortlessly among any number of file systems on their Unix systems without even knowing they ve gone from one to another. Everything in Unix is considered to be a file, including physical devices such as DVD-ROMs, USB devices, floppy drives, and so forth. This use of files allows Unix to be consistent in its treatment of resources and gives the user a consistent mechanism of interaction with the system. It s easy to understand, then, why file systems are an integral part of a Unix operating system. Unix uses a hierarchical structure to organize files, providing a from-the-top approach to finding information by drilling down through successive layers in an organized fashion to locate what s needed. It s similar to the way that a filing cabinet works. The file cabinet itself is the holder of all information in other words, it s the base of the filing system. To find hiring information about a certain employee, for example, you need to locate the correct file cabinet, the correct drawer in the cabinet, the correct folder in the drawer, and the correct page of information inside the folder. In Unix, everything starts with the root directory, often designated only by /. (This directory is not to be confused with the user account named root, which was discussed in Chapter 3.) All other files and directories originate there. The root directory generally includes a set of commonplace directories (see the Root s Basic Directories section of this chapter), then subdirectories within those directories, and so on. To find specific information in Unix, you need to locate the correct directory, the correct subdirectories, and the correct file. Directory Structure Unix uses a hierarchical file system structure, much like an upside-down tree, with root (/) at the base of the file system and all other directories spreading from there. The vast majority of Unix systems use the directories shown in Figure 4-1 and described in the next table. (For information about the directory structure on Mac OS X, see Chapter 20.) Not every version of Unix will have all the directories listed, nor is this an all-inclusive list, because Unix vendors may incorporate their own directories. Every vendor s Unix systems implement their own directory structures as needed by the vendor and its customers. No system has the exact same directory structure as another, but they generally have the directories described in this chapter and usually follow the conventions outlined as well. Essentially, you always start with the root directory to find any other directory or file. If the hiring information you wanted to find earlier is stored in your Unix computer, you might find the specific information (let s call the employee John Doe) in /home/hr/A_J/John_Doe, where / is the root directory, home is a subdirectory of root, hr is a subdirectory of home, A_J is a subdirectory of hr, and John_Doe, the file you want, is in the A_J directory. 54 Chapter 4
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