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4. Filename and Pathname Syntax

4.1 The Directory Hierarchy

Like VMS, Unix uses a tree-structured directory made up of a root directory and a hierarchy of subdirectories.

The Root Directory

The root of the directory hierarchy is called the root directory, and is designated by / . So, for example, the command: 

        ls /

would list the contents of this directory.

Absolute Paths

Any point in the directory hierarchy may be specified by giving a complete path from the root to the point in question. Such a path will begin with a /, indicating that it starts at the root, and will contain all the directories lying on the path, separated by slashes.

For example, home directories for users are typically put in the directory users, which is a subdirectory of usr, which is a subdirectory of the root. Suppose a user named foo has created a file named bar. Then a complete path specification for this file would be: 

        /usr/users/foo/bar

Your Default Directory

Like VMS, Unix has the notion of a current default directory for a process. For example, when you log in, your default directory is set to a home directory created for you by the system manager at the time your account was created. You may change your default directory at any time by using the cd (change directory) command; and you may show your current default directory at any time by using the pwd (print working directory command). The operation of these commands is analogous to the VMS SET DEFAULT and SHOW DEFAULT commands.

You can change back to your previous default directory with the command 

        cd -

Relative Paths

When specifying a file outside your current default directory, you may either specify an absolute path to the file from the root of the file system (/), or you may specify a relative path, beginning at your current working directory. The distinction is made as follows: absolute paths always begin with /, and relative paths never do. Instead, they begin with the name of a subdirectory of your current working directory.

For example, suppose a user foo has a home directory /usr/users/foo. In his home directory, he has created a subdirectory bar, which contains a file baz. If his home directory is his current directory, then he may reference this file by either one of the following paths: 

        /usr/users/foo/bar/baz

or 

        bar/baz

(Note that the former specification will always work correctly regardless of his current directory, while the latter relies on the fact that /usr/users/foo is his current directory.)

Use of . and .. in Paths

In addition to beginning with / or a directory name, a path specification may also begin with "." or "..". A single dot (.) refers to the current directory, and two dots (..) refers to the parent of the current directory (analogous to [-] on VMS). For example, consider the following partial directory hierarchy: 

                        /
                        |
-------------------------------------------------
|               |               |               |
bin             dev             etc             usr
                                                |
        -------------------------------------------------
        |               |               |               |
        bin             lib             ucb             users
                                                        |
                                --------------------------------------
                                |               |               |
                            aardvark            foo             zebra
                                |               |
                               xxx      -----------------
                                        |               |
                                        bar             bin
                                        |               |
                                -----------------       ---------------
                                |               |       |             |
                                baz         frobbish    ...

Now, suppose that your current working directory is /usr/users/foo/bar.

4.2 Syntax of Filenames

Basic Rules

Under VMS, the full name of a file consists of a directory specification followed by a file name, file type, and version number. We have seen that Unix likewise includes a directory specification as the first part of a full file name. However, unlike VMS, Unix has no concept of file type and version - only of a file name. A filename, then, simply consists of a string of some number of characters, normally drawn from the sets of lower-case alphabetic characters and decimal digits. However, uppercase letters may be used to call special attention to a file, and certain other characters (such as '_' and '.') may also appear.

Special Uses of "Dot" in Filenames

As was noted above, the character '.' may appear in a filename, and is regarded by Unix as any other character. It is not used, as in VMS, to divide the filename into distinct name and type fields. In fact, under Unix a dot may actually appear more than once in a filename - for example, a filename like ch.1.3.4 is perfectly legal. The dot does, however, have two special uses.

Filenames Beginning with Dot

Because Unix treats the dot as just another character in a filename, you can use it anywhere in a name. However, if you create a file whose name begins with a ., the ls command will not ordinarily list it for you when you do a directory (though there is a command option that will cause it to do so.) This feature creates files that are in some sense hidden from every day viewing, though they can be accessed by name like any other files.

It is a Unix convention to use filenames beginning with dot for various initialization tasks when a new session or program is started. The following are some important files used by bash (the Bourne-Again Shell), which you may want to create/edit for your own purposes. (Some of these files may be automatically placed in your home directory when your account is created.)

.bash_profile

contains commands that are executed by bash when you first log in (like LOGIN.COM on VMS)

.bashrc

contains commands that are executed by bash when you create a new shell process in the middle of a session. (It is conventional to have .bash_profile explicitly invoke this file so that it is also executed by your login process.)

.bash_history

used by bash to keep track of commands you have entered during the current session, to enable command-line recall.

(Note: other shells have similar files, but with different names.)

The Use of Dot in Program Source and Object Files

The language compilers on Unix make use of a dot in filenames in much the same way that VMS uses the dot to separate a file's name from it's type. For example:

All three compilers produce assembly-language translations of the higher-level-language source with names ending in .s and from these produce object files with names ending .o, but these files are normally deleted for you automatically as part of the linking process, so you will seldom see them.

Finally, the heading files used by the C-preprocessor have names ending in .h.

Remember, though, that .c, .cc, .p etc. are not file types in the VMS sense, but simply the last two or three characters in the filename!

The Directories . and ..

As was noted above, a dot can be used at the beginning of a path name to stand for the current directory, and ".." to stand for the parent of the current directory. Both of these actually exist as entries in every directory - i.e. when created, a directory contains an entry named "." that points to itself, and one named .. that points to its parent. These entries will show up if you use an ls option that shows other filenames beginning with ".".

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