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File System

The UNIX system knows about three kinds of files, namely, directories, ordinary files, and special files. The UNIX file system is a hierarchical directory structure (tree-structure) starting at the `root', called /. The levels on the tree are also separated by a /---the leaves are ordinary files and nodes which are not leaves are files known as `directories.'

Each directory always contains the two special characters . and .., which are pointers to itself and its parent, respectively. Thus, /usr/man/man1/./tcsh is equivalent to /usr/man/man1/tcsh and /usr/man/man1/../cat/tcsh is the same as /usr/man/cat/tcsh. This brings up the idea of absolute and relative pathnames: a directory or file specified without a preceding ~ or / will be sought for relative to the current directory; any other specification will be sought for under /, the system root.

File and directory names can be made up of any characters. To avoid conflict with shell, however, you should stick with the characters A-Z, a-z, 0-9, . , _ , and -. Upper and lower case letters are distinct. The following is a listing of some of the most useful commands used for working with files:

• cat - prints the contents of a file on the screen (by default). Useful concurrent commands include head and tail (prints default first and last 10 lines, respectively).
• more - presents the contents of a file a page at a time on the screen. Pressing SPC will allow you to scroll through the display.
• rm - remove a file or many files.
• mv - move a file from one place to another.
• cp - copy a file or a number of files.

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Directories