Unix Tutorial

This set of notes assumes that you have never used Unix before. It does assume, however, that you are comfortable using computers with a graphical user interface, such as the Macintosh operating system or Microsoft Windows. You should sit down at a Mac in the lab and go through this tutorial trying things out as they are explained. Upon completion, you should be able to understand the basics of entering commands, move around in the file system and view existing files, perform basic editing with the Emacs text editor, and know where to find help.

To gain experience with these topics, try things out as you read.

Logging In

To start, you must log in to one of the Macintosh computers in our lab. Begin by entering the user name and password for your computer science account in the login window and click ``Log in''. Your user name for this account may be different than the one used for computer accounts provided by the Office of Information Technology (typically something like 09ewr), and the initial password will also be different. You must obtain this password from your instructor. Once you log in, the screen should display a window containing icons for some of the files you can access on the machine.

Changing Your Password

Before proceeding, you should change the password we assigned to your account to something that only you will know and that will be easier for you to remember. Don't, of course, make your new password too easy to guess. In particular, at least use something that is a mix of letters and numbers. If you forget your password, the system administrator can give you a new password.

To change your password you should:

Now, log out and log in again with your new password. To log out select the last item in the menu that appears when you depress the mouse on the little apple image that appears in the upper left corner of your screen. Then click the "Log out" button that appears in the dialog box.

The files you create for our course laboratory projects will not actually be stored in the computer on which you are working. Instead, they will be stored on our department's Macintosh file server, cortland. By entering the account information for your account, you have instructed the computer you are using to access your files on cortland. You can access your files on cortland from any computer in our lab by simply logging in to that computer.

Using the Mac Interface

Some of you may be new to Macintoshes, and this section highlights the most important elements of the interface. The files on the hard drive can be viewed by clicking on the "Mac HD" icon in the top right-hand corner of the screen. To go to your home directory, click on the house icon under "Places" in the Finder window.

All Mac Windows roughly look like the following window:

Here are some basic window management operations:

You can start to get the feel for the environment by clicking on the Safari icon to start a web browser. (You are welcome to use Firefox if you are more comfortable with it.) The safari icon is in the tool bar and looks like the following:

Go to the class web page at http://www.cs.williams.edu/~morgan/cs136 and find the electronic version of this page. Bookmark this page--you will be referring to it often during lab.

Using Unix Commands

In order to use the Unix interface on the Macintosh, you must first start the Terminal Program, which you can do by clicking on the following icon in the tool bar:

The Terminal Program will open a shell window in which will type Unix commands. For example, type the command "ls" in the shell window and press return. This command will list the files in your directory. A command consists of the name of a program, zero or more options preceded by '-', and zero or more arguments, which are themselves words. The command line is terminated with a carriage return.

If you make mistakes in typing on the command line, use the Delete or Backspace key to erase the previous character.

Here are other useful commands related to files:

 
ls List the files in the current directory
cp file1 file2 Copy the file named file1 to a new file named file2
mv file1 file2 Rename the file named file1 to have the name file2. (mv is actually short for move, but it doesn't necessarily move anything!)
rm file1 Remove the file named file1
more file1 Display the contents of a file
lpr file1 Print the file named file1

One thing you will notice with Unix is that "no news is good news". Often if a Unix command is successful, it will produce no output! If it fails, you will receive an error message. Of the list of commands above, only ls and more produce output when they succeed.

The "more" command deserves some more explanation because it is an interactive command. It displays your file one "screenful" at a time. You can try out "more" by typing: 

more /usr/mac-cs-local/share/cs136/examples/alice.txt

This will show you a file containing the first chapter of Alice in Wonderland. (One useful tidbit: hitting the tab key will autocomplete directory and file names. Try typing "/usr/mac" and then hitting the tab key. It should autocomplete /usr/mac-cs-local for you.)

After each screenful, it displays a prompt either consisting of the name of the file being displayed (for the first screenful), END (for the last screenful), or : (for all other screenfuls). The prompt appears at the bottom of the terminal screen. At this prompt, you have several options, the most useful being:

 
space Display the next screenful
b Display the previous screenful
q Quit more, returning to a shell prompt.

When you use ls to list your files, a directory will appear with / at the end of its name. At all times you have a current directory. Your commands are interpreted with respect to your current directory by default. So, for example, ls lists the files in your current directory if you give it no arguments. Here are some useful commands to manipulate directories:

 
mkdir dir1 Create a new directory named dir1.
rmdir dir1 Remove the directory named dir1. You can only do this if the directory is empty.
cd dir1 Make dir1 be the current directory
pwd Display the name of the current working directory

Filenames should consist only of letters, numbers, and some punctuation marks. A filename is typically divided into a descriptive name and an extension, separated by a period. Extensions are purely by convention but typically indicate the type of file. For example, "myfile.txt" would be a text file, while "myfile.java" would contain a Java program. Extensions are generally optional although some programs, such as the Java compiler, expect them to be there.

Filenames can be given as names that are relative to the current working directory or as absolute names from the special root directory. Thus far, we have assumed relative names that refer simply to files in the current directory. We can also use relative names to identify files in subdirectories. For example, "mydir/myfile" is the file named "myfile" in the directory named "mydir", where "mydir" is located in the current working directory.

Each user also has a home directory. This is the directory that is the current directory when you start a new shell window. (You have already found your home directory through the standard Mac interface). No matter what the absolute pathname is to your home directory, you can always refer to it with the special relative pathname '~'. For example, no matter what your current directory is, if you type "ls ~", you will see a listing of the files in your home directory.

An absolute pathname always begins with / while a relative pathname never does. To find out the pathname that corresponds to your current directory, type pwd. Use pwd to find the absolute path for your home directory.

One last useful feature of the command line: Pressing the up arrow will give the last command you executed. Pressing return will run that command again. Try this. You can access older commands in your history by hitting the up arrow multiple times. To practice these commands, copy the alice.txt file to your home directory with the command 

cp /usr/mac-cs-local/share/cs136/examples/alice.txt ~/alice.txt

Rename the file, move it into a newly created subdirectory, and practice the other basic Unix commands as well.

Additional Shell Windows

You can create additional shell windows to work in more than one directory or on different tasks at the same time. When any terminal window has the focus (by clicking in it), the menus across the top of the screen should be "Terminal", "Shell", and so on. Select "New Window" or "New Tab" from the "Shell" menu. This will create a second terminal window or tab in your current window which you can use just as the one already open. (Or just press Command-T, as you would in a web browser for a new tab!)

To close a terminal window, click on the red circle in the title bar or press Command-W. To close only a single tab, click on the gray X in the right corner of the tab's title bar.

Basic Emacs

Emacs is a widely used text editor for Unix. To start Emacs, perform the following steps:

"nw" means "no window"; it forces Emacs to run inside the Terminal window instead of making its own window. This is the fastest way to launch Emacs and will work even if you are connected into a computer remotely by SSH. If you have time on your hands or can't live without icons and menus, you can type "emacs &" instead and a more friendly GUI-driven version of Emacs will launch...eventually.

Emacs relies heavily upon control-characters as a means of entering editing commands. Many commands are also available via pulldown menus, but experienced users tend to use control commands almost exclusively. Over the semester, your repertoire of commands will grow and will move away from menus and toward keyboard commands. For now, let's create a new file. To do this, type Control-x followed by Control-f. (That is, hold down the control key and press x and then f.) At the bottom of the window, you will see "Find file: ~/" with a small black box following it. The box indicates where characters will go when you type. Type in the name of the file that you want to create and hit carriage return. (Alternatively, you can create a new file by selecting "Visit New File" from the "File" menu.

The last line of the Emacs window is where command prompts and messages appear. The black line immediately preceding it is a status line. It shows you the name of the file you are editing. There's also some other information that is occasionally useful, but which you don't need to worry about too much right now. For what it's worth, before the filename you should see something like --:--. This means that the file is up-to-date. If it says, --:**, it means the buffer has been changed since it was last saved. If it says --:%%, it means that you cannot modify the file. After the filename is the current line number (L1 indicates the first line) and an indication of how far the current line is in the file in terms of percentage. "All" indicates that the entire file is showing. "Top" indicates that you are on the first screenful. "Bot" indicates that you are on the last (or bottom) screenful. The final entry is the "mode" that you are editing in. This should say "(Java/l Abbrev)" when editing a Java file.

The area between the menu titles and the status line is the buffer that displays the contents of the file. Most of your typing goes directly into this buffer. Type some text. Notice the status line changes. You can move the cursor around using the arrow keys. To move more quickly up and down, use Control-v and Meta-v as Page Down and Page Up. There are lots of little tricks like this.

Many Emacs commands are executed by pressing Control and then another key. This is written as C- and then the other key. For example, C-g is "cancel" in Emacs. To execute it, press and hold Control, then press and release the G key, and then release the Control key. Sometimes there are two other keys. For example, to load a file, type C-x C-f. To save a file, type C-x C-s. To quit Emacs, type C-x C-c.

Another common way to enter commands is using the Meta key. This is abbreviated M-. For example, you can launch a terminal shell inside of Emacs by pressing M-x and then typing "shell" (without the quotation marks). Try it now. Oh, wait! There's no Meta key on the keyboard! To fix this oversight on Apple's part, go to the Terminal menu and select "Preferences". Then go to "Settings" and select the Keyboard tab. You should see a checkbox labeled "Option sends Meta". Check it. Now your Alt/Option key is a Meta key. There's a backup you can use when connected into the computer in a way that doesn't let you do this: whenever you see Meta-something, press the escape key and then press the other key, separately. For example M-x represents typing the Escape key followed by the "x" key. Here are the keyboard sequences for the most common Emacs commands:

 
C-v Next screenful
M-v Previous screenful
C-d Delete next character
Backspace Delete previous character
C-x C-s Save buffer to a file
C-x C-f Open a file
C-x C-c Exit Emacs

You can find more Emacs commands in the Emacs Hints Page.

Using keyboard commands is generally faster than moving one's hands back and forth between the mouse and keyboard as one must do to use the menus and scrollbar. While it takes extra effort to learn, the payoff is worth it in the long run in terms of speed of using the editor.

Likewise, Emacs is able to have multiple buffers (e.g., files) open at the same time. Don't open and close Emacs a lot--that just wastes time as you try to find where you were. Open all of the files that you need in a single Emacs session and use C-x b to switch between those buffers. Run a command shell inside Emacs and use that to compile.

Try editing your first file in Emacs. Load ~/.bash_profile and add the following lines to the bottom: 

PS1="\h:\w -> "
export PS1

Save the file and exit Emacs, and then close Terminal (Command-Q) and restart it. The prompt at the command line will now tell you the name of the machine that you're logged into, and what directory you're in. This will help keep you oriented when typing commands.

It would be too overwhelming to learn all of the commands in Emacs right now, but I expect you to pick up lots more Emacs commands throughout the semester. For example, you might like to know how to copy and paste, or how to see two buffers side-by-side. Swap Emacs tips with other students and don't be shy about asking TAs and the professor if there is an easy way to do something in Emacs.

Using Java

To execute a Java program, you perform two steps. First, you compile the Java source file. For example, to compile the file "Hello.java", type

javac Hello.java

Assuming the program is free of compilation errors, a class file "Hello.class" will be created. This files contains the compiled representation of the program. To execute the program, you run the Java virtual machine with the command

java Hello

The command line argument to java is the name of the class whose main method you would like to run. Copy the file 

/usr/mac-cs-local/share/cs136/examples/Hello.java

to your home directory, compile it, and run it.

Getting Help

The Unix command man provides information about all Unix commands and programs. For example, to learn about the ls command, type man ls. You can then page through the manual information as you did with more. To learn more about the manual, you can read the manual about the manual by entering man man at the command prompt.

You can get fancy and run man from inside Emacs as well...with the command "M-x man", of course!

Logging Out

As mentioned earlier, to logout you select "Log Out..." from the Apple menu in the top left-hand corner of the screen. Always close all Emacs and shell windows prior to logging out to prevent you from losing any work.