Using Linux in 351

Logging onto attu or cancun

attu and cancun are powerful clusters of computers sitting in the CSE building that are ready to run your work, except that they have no screens! They run Linux and can only be accessed over the network. But they have all of the commands that you will need for the 351 labs and are built to be fast, so more often than not it's most convenient to do your work directly there.

From Windows (PuTTY)

1. Open putty.exe from wherever you downloaded it.
2. For the host name, type either <your-csenetid>@attu.cs.washington.edu or <your-uwnetid>@cancun.cs.washington.edu.
3. Click Open at the bottom.
4. If you're prompted about an RSA key, click Yes.
5. Put in your CSENetID (for attu) or UWNetID (for cancun) password when asked for a password. It will look like you're not typing anything – that's done on purpose so people looking over your shoulder won't know your password.

From Windows (bash), OS X, or Linux

1. Open the terminal on your computer (see "Terminal and Shell Access" above).
2. Type either ssh <your-csenetid>@attu.cs.washington.edu or ssh <your-uwnetid>@cancun.cs.washington.edu at the command prompt and press [Enter].
3. If prompted about a server key, type yes and press [Enter].
4. When prompted, enter your CSENetID (for attu) or UWNetID (for cancun) password and press [Enter]. It will look like you're not typing anything – that's done on purpose so people looking over your shoulder won't know your password.

Troubleshooting your connection

Computers are prone to fail every now and then, especially servers since they are constantly available and often handling multiple connected users at a time. Throughout the quarter, you will likely encounter some instances where you are rejected by attu or cancun or your files are missing when you do connect. This is likely because at least one of the servers is having a problem/issue and you were unfortunately routed to it. The quickest solution is to try to connect to a different individual cluster machine.

• For attu, append a number from 1-8 after "attu" (e.g., ssh <netid>@attu1.cs.washington.edu.
• For cancun, contact the course staff or email support@cs.washington.edu directly.

Changing your password

If you would like to change your password, type passwd into the command prompt once you're logged in and press [Enter], then follow the instructions given.

Linux Files and Directories

All of the files we will create, edit, and use in this class are stored in a directory (of your choosing) in the Linux file system. The most important part for 351 is understanding the directory structure layout so that you can properly locate your files. File paths will be more important in future courses for specifying file locations within your code or in shell commands. Do note that paths in Linux are case-sensitive (e.g., /home/test.txt is not the same as /home/Test.txt).

The root directory (/)

The Linux file system consists of a single, hierarchical directory structure that starts from the root directory, represented by '/', and then expands into subdirectories. Where DOS/Windows had various drives (e.g., C:, D:) with directories under those partitions, Linux places all the drives under the root directory by 'mounting' them under specific directories. The closest parallel to the root directory in Windows would probably be C:.

Note that Linux uses the frontslash '/' as the directory separator in paths instead of the backslash '\' as in Windows/DOS. So C:\windows\system in Windows would be /c/windows/system in Linux.

Your home directory (~)

To keep people (users) from stepping on each other's toes, everyone has one personal directory that they can freely write and maker changes to. This is called your "home directory." On any Unix-like system, you can refer to the home directory with a tilde, so a directory named frogs inside of your home directory could be referenced via the path: ~/frogs/.

When you run a command, your shell will replace ~ with the full path to your home directory. If you want to know what that is or how to manipulate the shell, read the next section on basic commands.

Other important directories (. and ..)

The shell also allows you to use/create paths relative to your current working directory.

• '.' represents the current/present working directory.
• '..' refers to the direct parent of the current/present working directory (i.e., the directory that contains the current working directory).
• Every additional '.' refers to the directory one level higher.

These are very convenient in working your way around your directory structure. For example, if you are in the folder lab1, a sibling directory lab2 would be referred to as: ../lab2.

Basic Unix Commands

To move around through the directory structure in your terminal, you'll need to know a few basic Unix commands. Note that you always start in your home directory when you first open a terminal or SSH into a server.

Below is a sample execution of some Unix commands. The [attu]$ is just the prompt telling you that you're logged into attu and will differ on your terminal. Lines that don't start with this prompt are output returned by the shell in response to the previous command.

[attu]$ mkdir mydir
[attu]$ ls
mail  mydir
[attu]$ cd mydir
[attu]$ pwd
/homes/iws/jhsia/mydir
[attu]$ exit

Transferring Files Remotely

You will frequently want/need to transfer files from one computer to another (e.g., between your local machine and attu). Some use cases include downloading files from attu to your local machine to submit to Gradescope or getting a copy of a starter file from the course website.

File transfer

The following are suggested methods for transferring files between your local machine and attu (i.e., either from your machine to attu or from attu to your machine):

• scp: A shell command of the form scp <source> <destination> used in your terminal. A quick overview can be found on this blog post.
  • Your local shell and the attu shell can likely both run this command, so it's important to remember which one you're currently using/viewing in your terminal. It is recommended that you always use scp with your local shell (i.e., disconnect from attu first) to avoid differences between file systems.
• FileZilla: GUI version of scp that allows for drag-and-drop and limited remote file system manipulation (e.g., file renaming, file deletion, file permissions, directory creation). Works on all platforms (Windows, MacOS, Linux).
• WinSCP: GUI version of scp that allows for drag-and-drop and limited remote file system manipulation (e.g., file renaming, file deletion, file permissions, directory creation). Windows only!

File download

If the source file is accessible via a URL (e.g., files hosted on the 351 course website), then you can use the shell command wget <URL> to download the file to your current/present working directory. More information can be found in the wget manual.

Changing your Shell

There are different shell programs, which are all similar but have different rules and features. For sake of uniformity, we will use a shell called "bash," and your account should use this by default. You can check using the following command: echo $0. In case the output isn't bash or -bash (or you're updating your personal Linux machine), you can change the shell temporarily or permanently but we strongly recommend the latter.

Temporarily

Issue the command bash.

Now you may have a different-looking prompt (such as [bash-3.00]$), but otherwise at this point you will not notice any differences. Note that instead of changing your shell, you've actually opened up a new shell (we'll discuss processes in this course), so when you issue the exit command, you'll actually return to the original shell (i.e., the one you typed bash into).

Permanently

Issue the command chsh -s /bin/bash.

This runs the "change shell" program and tells the operating system that the "first shell" for your account for every terminal should be the one found at /bin/bash.

• To be sure you have the right to do this, you'll be prompted for your password.
• Before that, you'll be warned that typing passwords can be dangerous. Say "y".
• As the message says, the change may take a few hours. In the meantime, see the "Temporarily" instructions above.

If your shell already is bash, chsh will just say "Shell not changed."