UNIX II: The File System

In the old days, computers had flat file systems: all files were equally accessible and were "in one place." This system had many problems once there were a large number of files or more than one user:
- How can I find my files when there are 50,000 files on the system?
- What happens if two people want to have a file called readme?

Hierarchical file systems get around this by storing files in a tree like structure of directories.

                      / (root directory)
                                |
    +-------+--------+------+---+---+------+-------+--------+-------+
    |       |        |      |       |      |       |        |       |
  /bin    /home    /dev   /etc    /tmp   /usr    /mnt     /lib    /var
            |                              |
            |                              |
  +---------+--+---------+        +-----+--+--+----------+------+-----+
  |            |         |        |     |     |          |      |     |
/thomasc    /sethp     /sli     /adm  /bin  /include  /local  /lib   /src
    |                                                    |
    |                                                    |
+---+----+                                             /bin
|        |
/c    /18.02


      Partial Tree of the File System on my Home Computer

In UNIX, directories are delimitted by the forward slash character "/" and the fullname of a directory is obtained by recursively prepending the names of its parents: for example, /home/thomasc/c and /usr/local are both valid directory names on my system.
At any given time, you are "in" a single directory and only the files in that directory are directly accesible.
- To print the name of this directory, type pwd (print working directory).
- To list the files in this directory, type ls.
- To change the directory you are in, type cd (directory name) . (directory name) can either be a fullname like /home/sethp or a partial name if it is a child directory. For example, if I'm in /usr then I can type cd bin to get into /usr/bin or cd local/bin to get into /usr/local/bin.
- Typing cd .. puts you in the parent directory of the current directory.
You are also (mostly) free to change the directory structure:
- Typing mkdir (directory name) makes a directory called (directory name).
- Typing rmdir (directory name) removes (directory name), if it exists.
- Typing rm (file name) removes (file name).
- Typing mv (file name 1) (file name 2) changes the name of (file name 1) to (file name 2).
- Typing cp (file name 1) (file name 2) copies (file name 1) to (file name 2).
Finally, each user has a home directory. This is the directory that the user is first put when he or she logs on. On my home system, my home directory is /home/thomasc and on athena it is /afs/athena/users/t/h/thomasc. Typing just cd puts you back into your home directory.

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