Harvey Mudd College
Computer Science 60
Trying out the CS server and the command line

Password

You'll need a CS account - if you're in CS 60, you should have one. This is different from your CIS accounts It's likely the same login name (though there are a few exceptions) and also likely that your password is different from your other accounts - unless you made it the same in the past.

Forgotten password or login name?    If so, simply stop by Beckman B101 and see Tim Buchheim, our system administrator. It will take only a second for you to reset your password to something else.

LOCAL login - from one of the CS Macs

The CS department has about 50 Macs in Beckman B102 and B105. Using those is an easy way to access your CS account and files and, in particular, Rex! If you login to one of the lab Macs, you will be able to navigate to your files using the familiar window-based interface of folders and files.

In order to get to the command-line and continue on with the assignment, continue with the From a Mac instructions below.

REMOTE login - from your machine

You can also access your CS account from your own machine (as long as you're connected to the internet, that is). The program from which you log in is different, depending on whether your machine is a Mac or a PC. We'll look at each one in turn.

   From a Mac    Macs come with a command-line interface program called Terminal. If you type "Terminal" in the spotlight search box available from the upper right corner, the terminal program will be the "top hit" it finds. It's also available via Go - Applications - Utilities - Terminal.

Within Terminal, you will be at a command prompt. You can navigate around your local machine using pwd, cd, and the other commands described below. At least try typing hostname at the prompt: it should tell you the name of your local machine.

Next, to log in remotely to the main CS server, named knuth.cs.hmc.edu, type

     ssh -Y hadas@knuth.cs.hmc.edu

Note: as you type your password, no text will appear (not even the dots that op up on some web forms). If you make a mistake in typing your password, it will simply give you the password prompt again. Once you successfully enter your password, you will now see a "welcome" message and then yet another command prompt. Try typing hostname again -- this time, it should reply knuth, indicating that you're now accessing that server.

For some tips on navigating around, skip down to the getting around section.

   From a windows PC    To login to the command line of another machine from a windows PC, you'll need a secure-shell terminal program. The most popular is named putty, and can be downloaded from this link. The putty.exe top-left version under "binaries" works great.

Once you have putty, perhaps on your desktop, double-click it and type knuth.cs.hmc.edu into the textbox labeled "Host name or IP address" - then hit enter or click on "Open." If all goes well, you will get a window asking you for your login id (for example, hadas) and then your password, e.g., 42aliensLoveSpam. As you type your password, no text will appear; if there is a mistake, it will simply prompt you for your password again. Once you're successfully connected, you'll see a "welcome" message and then a command prompt. Try typing hostname at the prompt, and it should reply with knuth, indicating that you're indeed now on the main CS server.

This section is meant as a quick-as-possible introduction to the basic commands for getting around at a command prompt:

• pwd prints the current directory you're in (print working directory)
• ls lists the contents of a directory
• pico (or nano) are editors for changing or creating text files
• mkdir makes a new directory/folder within the current one
• cd moves to a new directory
• cp copies files from one directory to another
• rm removes files (deletes them entirely) caution - not undo-able!

pwd    The command-line is a text-based interface to all of the computational and storage facilities of a computer. The program that provides the command line is called a shell. The prompt provided by the shell can be customized, but it typically ends in % and awaits your commands... .

Type pwd at the prompt, and it will print your current directory. All of the files and programs on any machine are organized hierarchically. The root of the hierarchy is called root and is represented by a single forward-slash /. In text form, the nested-folder system of displaying this hierarchy becomes a series of directory names separated by forward-slashes, e.g., /home/hadas if you've logged in as Prof. Ran (please don't do this, unless you are Prof. Ran!)

ls    This command lists all of the contents of the current directory. Try it - if you haven't added any files or directories, you'll see the default contents that new accounts are given.

I find ls rather frustrating because I can't tell which of the things listed are files and which are directories themselves (or programs). Use ls -F to get a forward-slash at the end of subdirectories and an asterisk at the end of runnable programs.

pico (or nano)    So, how to create files? There are many editors available, but the one that is both the most hassle-free (and, perhaps, feature-free) is named pico. To create a new text file named test.rex, type

  pico test.rex

answer = 42;

f(x) = x + 1;

In class we used nano -- it's OK, too. These do the same thing... (if one is scrolling through text in odd ways, use the other one).

If you type ls again, you should now see your text.rex file. Try it out with

 rex test.rex

rex > answer;      // you're asking for the value of this expression
42                    // this should be Rex's response
   
rex > f(f(answer));  // again, asking Rex to evaluate an expression
44                      // its response

mkdir and cd    This command allows you to create a new directory. It's probably a good idea to create a cs60 directory with

  mkdir cs60

  cd cs60

cp and rm    These commands manpulate files: copying, removing, and moving them, respectively. For example, there is a file in the directory named /cs/cs60/hwfiles/a1/part1.rex that has a template for the first part of this week's hw and some example code. To copy that file to your current directory, type (or copy-and-paste)

 cp /cs/cs60/hwfiles/a1/part1.rex .

You will also want a file named part2.rex for the second half of the problems. If you already have part1.rex in your current directory, you can make a copy of it with a different name using

 cp part1.rex part2.rex

If all went as expected, you should be able to use ls to see that you have a part1.rex and a part2.rex file in your current directory. You're ready to edit those files (perhaps with pico or nano) and get started with Rex!

Perhaps you don't want that test.rex file that was back in your home directory? You can "go up" a level in the directory hierarchy with cd .. -- those two periods are shorthand for "the next directory up." Another cd .. should bring you to your home directory, where rm test.rex will delete that file. Warning! rm should be used with caution. It does not warn you about deleting files and it does so completely -- it's not "undo-able." (If your file has been around long enough that the backup system has made a copy, you can ask CS staff to retrieve it for you, but that takes some time.)

These commands are only a small portion of the well-designed set of command-line tools that are available on all systems -- even windows PCs have all these same tools, though many have slightly different names, of course! A much more extensive introduction to the unix operating system and interacting with it via the command line is available at this link .

You can invoke Rex from the command line simply by typing rex. You will see a rex > command-prompt, which is identical in spirit to the Python command prompt. Try out a few definitions and evaluations:

rex > x = 60;      // binds x to 60 
1                     // rex says it's OK with that

rex > x;           // asks for the current value of x
60                    // rex says it's 60

rex > f(60) => 42; // a part of the definition for a function f
f                     // rex says it's OK with that

rex > f(x) => x+1; // another part of the definition for a function f
f                     // rex says it's OK with that

rex > f(60);       // asks rex to evaluate this...
42                    

rex > f(41);       // asks rex to evaluate this...
42                    

rex > f("hi");     // rex won't be so happy about this...
*** warning: can't add non-numerics hi
*** aborting to top-level

rex > (control-d)  // hit the control-d sequence to exit rex

%                     // back at the unix prompt

You can load the definitions from your part1.rex file in two ways. First, you can do so when you start rex via

% rex part1.rex

rex > *i part1.rex

When you load a file in, it will run any tests you have included (an easy way to run tests each time you change code!) - it will also provide you with the rex prompt so that you can test any of the definitions by hand, as well.

Efficiency suggestion - two windows:    It's much more efficient to have two windows open to the same directory when developing programs - in Rex or any other language. In one window, you might have the rex prompt open; in the other window you would have the file you're editing -- that way, you won't need to close down the editor or rex in order to go back and forth between writing and testing! You could even have a third one open with a Rex prompt to test language questions, e.g., is 3/2 == 1 or 1.5? (It's 1.)

Good luck with Rex! You're ready to write the problems back on the main hw1 assignment page.

The instructions for submitting your work are at the bottom of the main hw1 assignment page .

Back to the main hw1 assignment page
Back to the main hw2 assignment page

Being comfortable working from the command line is one thing; but it would be crazy not to use graphical interfaces when available and efficient! Although it takes a bit more setup, it's possible to open windows from knuth on a remotely logged-in machine. This section describes how (or at least one way how). Again, the instructions are different, depending on where you are...

   From the Lab Macs    Here, things are easiest -- you can use the computer's ordinary applications to open and edit your file. If you click on the background, you can then use the Go menu at the top of the screen to navigate to the TextMate application (or any other text editor). Open TextMate, and then use it to open your part1.rex file.

   From a remote Mac    If you have the most recent Mac OS (version 10.5.x), this is pretty easy, too. First, click on the background Desktop and then navigate the menu system through Go - Utilities - X11. Double-clicking X11 will bring up a new terminal window with the title xterm on top. Log in to knuth from that window with the -Y flag, that is, ssh -Y name@knuth.cs.hmc.edu. This line sets up a connection, known as X forwarding, over which graphical data can be sent. Then, navigate to the directory where your part1.rex file is located (or any other file). Type

% gedit part1.rex &

   From a remote PC   

The same instructions as described above for a Mac work also on a PC, except that you need to install the local graphics server, known as an X server, first. (On a Mac it's already there.)

The easiest, free X server for a PC is named Xming. You will need to download and install two packages: first, Xming itself and then, Xming's fonts. They are located at this Xming page under "Public Domain Releases" and at

• this link for Xming itself, and
• this link for the Xming fonts

Once those two packages are installed, you can change the settings within Putty to use Xming. To do this, first start Xming by double-clicking its desktop icon. You'll see a small Xming icon in the task bar at the bottom right of the screen. Then, open Putty and type knuth.cs.hmc.edu into the "Host Name" textbox as usual. Next, you'll need to turn on the "X11 forwarding" -- X11 is simply the name of the system allowing you to share graphics across machines:

1. Click on the little + beside the SSH item in the hierarchical menu on the left.
2. Click on the X11 menu item that appears.
3. Click the "Enable X11 forwarding" checkbox.
4. Type localhost:0.0 into the "X display location" textbox.
5. Click the "Open" button and log in as usual.
6. When at the prompt, navigate to a directory that contains your part1.rex file and then type gedit part1.rex &. The ampersand tells the command line to run the graphical text editor in the background and then provide a new prompt for additional commands. That way, you'll be able to run Rex as in hw1.

Much more is possible, but this is certainly enough to get started interacting with a remote server via its command-line.