THE FILE SYSTEM ON LINUX
Part-01 of Part-03
The most important role of the system is to provide a file system. From the point of view of the user, there are three kinds of files: ordinary disk files, directories, and special files.
* Ordinary files
A file contains whatever information the user places on it, for example, symbolic or binary (object) programs. No particular structuring is expected by the system. A file of text consists simply of a string of characters, with lines demarcated by the newline character. Binary programs are sequences of words as they will appear in core memory when the program starts executing. A few user programs manipulate files with more structure; for example, the assembler generates, and the loader expects, an object file in a particular format. However, the structure of files is controlled by the programs that use them, not by the system.
* Directories
Directories provide the mapping between the names of files and the files themselves, and thus induce a structure on the file system as a whole. Each user has a directory of his own files, he may also create sub-directories to contain groups of files conveniently treated together. A directory behaves exactly like an ordinary file except that it cannot be written on by unprivileged programs, so that the system controls the contents of directories. However, anyone with appropriate permission may read a directory just like any other file. The system maintains several directories for its own use. One of these is the root directory. All files in the system can be found by tracing a path through a chain of directories until the desired file is reached. The starting point for such searches is often the root. Other system directories contain all the programs provided for general use; that is, all the commands. As will be seen, however, it is by no means necessary that a program reside in one of these directories for it to be executed.
Files are named by sequences of 14 or fewer characters. When the name of a file is specified to the system, it may be in the form of a path name, which is a sequence of directory names separated by slashes, 5/??, and ending in a file name. If the sequence begins with a slash, the search begins in the root directory. The name /alpha/beta/gamma causes the system to search the root for directory alpha, then to search alpha for beta, finally to find gamma in beta gamma may be an ordinary file, a directory, or a special file. As a limiting case, the name "/** refers to the root itself.
A path name not starting with */** causes the system to begin the search in the user's current directory. Thus, the name alpha/beta specifies the file named beta in subdirectory alpha of the current directory. The simplest kind of name, for example, alpha, refers to a file that itself is found in the current directory. As another limiting case, the null file name refers to the current directory.
The same non-directory file may appear in several directories under possibly different names. This feature is called linking; a directory entry for a file is sometimes called a link. The Unix system differs from other systems in which linking is permitted in that all links to a file have equal status. That is, a file does not exist within a particular directory: the directory entry for a file consists merely of its name and a pointer to the information actually describing the file. Thus a file exists independently of any directory entry, although in practice a file is made to disappear along with the last link to it.
Each directory always has at least two entries. The name ". ** in each directory refers to the directory itself. Thus a program may read the current directory under the name "." without knowing its complete path name. The name “.." by convention refers to the parent of the directory in which it appears, that is, to the directory in which it was created..
The directory structure is constrained to have the from of a rooted tree. Except for the special entries "." and "..", each directory must appear as an entry in exactly one other directory, which is its parent. The reason for this to simplify the writing of programs that visit subtrees of the directory structure, and more important, to avoid the separation of portions of the hierarchy. If arbitrary links to directories were permitted, it would be quite difficult to detect when the last connection from the root to a directory was severed.
The directory structure is constrained to have the from of a rooted tree. Except for the special entries "." and "..", each directory must appear as an entry in exactly one other directory, which is its parent. The reason for this to simplify the writing of programs that visit subtrees of the directory structure, and more important, to avoid the separation of portions of the hierarchy. If arbitrary links to directories were permitted, it would be quite difficult to detect when the last connection from the root to a directory was severed.
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