Unix Fundamentals: Directories

You might ask: how do I create and manage folders? UNIX, like other operating systems has folders; we call them directories. Directories in UNIX work just like folders on any graphical operating system. You have been in a directory all along without knowing it. You are in your home directory. We will see how to create directories, store files in them and manage our directories.

The mkdir command makes a new directory. It takes one argument, the name of the directory you intend to create. Let's make a directory.

mkdir Projects

makes a directory called Projects; this directory is now empty. We can always get rid of an empty directory by typing

rmdir <garbageDirectory>

and that directory is removed. The rmdir command will not work if the directory is nonempty. There is a way to snip off directories with their contents, but I will avoid it for now because it is very dangerous. For now, you can delete the contents of a directory, then remove the directory.

To get into our new directory Projects, enter this command:

cd Projects

and type ls. You will see no files. This is because Projects is empty. Open files named moo and baa with vi and place a little text in them. Then enter ls -al at the command line. You will see just the files you created. The command ls displays only files in the directory you are currently occupying.

Sometimes you forget where you are. If you type cd any time you are in UNIX, you will automatically go back to your home directory; you will feel like Dorothy going back to Kansas. You can also type

pwd

This is "print working directory" and this will show your current location. If I type this right now, it says I am in

/home/faculty/morrison/public_html/programming/unixBasics

Let's see what is going on here. I know my home directory: it is

/home/faculty/morrison

This means I am inside a directory called unixBasics, which lives inside of programming, which lives inside of public_html, which lives in my home directory. Each slash represents a layer in the directory structure. Time for a field trip!

To get to our first destination, type cd /. The directory / is the "root" directory. If you think of the directory structure as an upside-down tree (root at top), the directory / is at the top. Type pwd and see where you are. Type ls; you should see the directory home in there. Now type cd home; if you type ls you will see the directory faculty. See if you can follow this all the way down to my unixBasics directory. From this, you see that you can step down through the directory structure. How do you step up?

Try typing cd ..; the special symbol .. represents the directory above where we are. Now you can climb up and down the directory structure!

Practice this; go back to your home directory. Make a new directory called mudpies. Put files in it. Make new directories in mudpies, got down inside these and make more directories and files. Practice using cd to navigate the tree you create. When you are done, get rid of the whole mess; remember you have to go to the bottom, empty out the files using rm and then use rmdir to get rid of the empty directories.

If you type ls in a directory, notice how any directories inside it are in blue type and how files are in white type. You can use the -F option in ls to print directory names with a slash (/) after them. Try this. If you use the -l option in ls, you will see that in the permissions column, the colum begins with a d.

-rw-rw-r--    1 morrison morrison        0 Jun  9 14:54 bar
-rw-rw-r--    1 morrison morrison        0 Jun  9 14:54 foo
drwxrwxr-x    2 morrison morrison     4096 Jun  9 14:54 junk

You can see there that bar and foo are empty files. Notice the d at the beginning of the line in junk; this tells you junk is a directory.

Finally, I shall do a brief discussion of permissions. You are the owner of your home directory and all directories and files it contains. This is your "subtree" of the system's directories belonging to you. There are three layers of permission: you, your group, and others. You is letter u, your group is letter g and others is letter o. There are three types of permission for each of these: read, write and execute. Read means that level can read the file, write means that level can execute the file, ant execute means that level can execute the file. Example:

-rw-rw-r--

on my file bar means the following

• I can read or write to the file. I cannot execute.
• My group can read or write but not execute.
• The world can read this file but neither write nor execute.

If I want to execute this file myself I can use the chmod command as follows

chmod u +x bar

I changed the u(ser's, that's me) permission to allow the user (me) to execute the file.

If I do not want the world to see this file I could enter

chmod o -x bar

and revoke permission for the world to see my file. Since I am the owner of the file, I have this right. In general you can do

 chmod (u or g or o) (+ or -) (r or w or x) fileName

to manage permissions. There are other ways to do this; you can look these up.

While we are on a computer we might just want to program. What a thought!