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The CS2030S Programming Environment

Java version

Java is a language that continues to evolve. A new version is released every six months. For CS2030S, we will only use Java 17, the most recent version with long-term support. Specifically, we use openjdk 17 2021-09-14 on Ubuntu 20.04.5.

PE Hosts

The school has provided a list of computing servers for you to use, with all the required software for CS2030S installed. You can access them remotely via ssh, or Secure SHell. The hosts are named pe111, pe112, ..., pe120. (pe stands for "programming environment"). We will refer to these servers generally as the PE hosts.

For this semester, the two servers pe115 and pe116 are not available.

You can choose which of the eight hosts to use. You share the same home directory across all the hosts (this home directory, however, is different from that of stu1). If you notice that one host is crowded, you can use another host to spread out the load.

While you can complete the programming assignments on your computers, the practical exams are done in a controlled environment using servers similar to the PE hosts. It is therefore advisable for you to familiarize yourself with accessing the PE servers via ssh and edit your program with either vim or emacs (vim is recommended and supported).

Accessing the PE Hosts

Basic Requirements

  1. You should be familiar with the terms Unix, command-line interface, command prompt, terminal, and shell. Read this background article if you don't.

  2. You need to have an SoC Unix account. If you do not have one, you can apply for one online.

  3. Once you have an account, you need to activate your access to the PE hosts, which are part of the SoC computer clusters.

  4. You need a command line ssh client. Windows 10, macOS, and Linux users should already have it installed by default.

  5. You need a good terminal emulator. For Microsoft Windows users, you can use either PowerShell or Windows Terminal; Mac users can use the default Terminal or iTerm2.

The Command to SSH

To connect to a remote host, run the following in your terminal on your local computer: 

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ssh <username>@<hostname>

Replace <username> with your SoC Unix username and <hostname> with the name of the host you want to connect to. For instance, I would do: 

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ssh ooiwt@pe112.comp.nus.edu.sg

if I want to connect to pe112.comp.nus.edu.sg.

After the command above, follow the instructions on the screen. The first time you ever connect to pe112.comp.nus.edu.sg, you will be warned that you are connecting to a previously unknown host. Answer yes. After that, you will be prompted with your SoC Unix password. Note that nothing is shown on the screen when your password is entered.

Accessing The PE Hosts from Outside SoC

The PE hosts can only be accessed from within the School of Computing networks. If you want to access it from outside, you need to connect through SoC VPN.

First, you need to set up a Virtual Private Network (VPN) (See instructions here). The staff at the IT helpdesk in COM1, Level 1, will be able to help you with setting up if needed.

SoC VPN vs NUS VPN

Note that SoC VPN is different from NUS VPN. Connecting to NUS VPN only allows you access to the NUS internal network, but not the SoC internal network.

Troubleshooting

Some common error messages and what they mean:

  1. ssh: Could not resolve hostname pe1xx.comp.nus.edu.sg

    ssh cannot recognize the name pe1xx. Likely, you tried to connect to the PE hosts directly from outside of the SoC network.

  2. Connection closed by 192.168.48.xxx port 22

    You have connected to the PE host, but you are kicked out because you have no permission to use the host.

    Make sure you have activated your access to "SoC computer clusters" here: https://mysoc.nus.edu.sg/~myacct/services.cgi

  3. Permission denied, please try again

    You did not enter the correct password or username. Please use the username and password of your SoC Unix account which you have created here: https://mysoc.nus.edu.sg/~newacct/.

    Check that you have entered your username correctly. It is case-sensitive.

    If you have lost your password, go here: https://mysoc.nus.edu.sg/~myacct/resetpass.cgi

  4. Could not chdir to home directory /home/o/ooiwt: Permission denied

    This error means that you have successfully connected to the PE hosts, but you have no access to your home directory.

    This should not happen. Please file a service request with SoC IT Unit at https://rt.comp.nus.edu.sg/. Include the error message, the PE hosts that you connected to, and your username. The system administrator can reset the permission of your home directory for you.

Copying Files between PE Nodes and Local Computer

Secure copy, or scp, is one way to transfer files between the programming environments and your local computer. scp behaves just like cp (see Unix: Essentials). The command takes in two arguments, the source, and the destination. The difference is that we use the <username>@<hostname>:<filename> notation to specify a file on a remote host.

Let's say you want to transfer a set of C files from the directory lab01 to your local computer. Then, on the local computer, run:

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ooiwt@macbook:~$ scp ooiwt@pe111.comp.nus.edu.sg:~/lab01/*.java .

Warning

If you have files with the same name in the remote directory, the files will be overwritten without warning. I have lost my code a few times due to scp.

The expression *.java is a regular expression that means all files with a filename ending with .java (see Advanced Topics on Unix). You can copy specific files as well. For instance, to copy the file Hello.java from your local computer to your ~/lab01 directory:

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ooiwt@macbook:~$ scp Hello.java ooiwt@pe111.comp.nus.edu.sg:~/lab01

scp supports -r (recursive copy) as well.

Note that we always run scp on your local computer in the examples above, since the SSH server runs on the PE host.

Setting up SSH Keys

The next step is not required but is a time-saver and a huge quality-of-life improvement. You need to be familiar with basic Unix commands, including how to copy files to remote hosts (using scp) and how to check/change file permissions (using ls -l and chmod). If you are still not comfortable with these commands, make sure you play with the basic Unix commands first. You can come back and complete this step later.

Our goal here is to set up a pair of public/private keys for authentication so that you do not need to type your password every time you log into a PE host.

You can use the following command on your local computer to generate a pair of keys: 

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ssh-keygen -t rsa

This command will generate two keys, a private key id_rsa, and a public key id_rsa.pub. Keep the private key id_rsa on your local machine in the hidden ~/.ssh directory and copy the public key id_rsa.pub to your account on PE pe111.

There are two methods to do this:

Method 1: Using ssh-copy-id

If your local machine has ssh-copy-id installed, then, run: 

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ssh-copy-id <username>@pe111.comp.nus.edu.sg

You will be prompted to enter your password for the PE host. After this step is completed, your public key will be copied to and configured for password-less login to the PE hosts.

Method 2: Using scp

First, use scp to copy the public key id_rsa.pub from your local machine to your home directory on PE pe111.

On pe111, run 

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cat id_rsa.pub >> ~/.ssh/authorized_keys

Make sure that the permission for .ssh both on the local machine and on PE is set to 700 and the files id_rsa on the local machine and authorized_keys on the remote machine are set to 600. See the guide on using ls and chmod if you are unsure how to do this.

Once set up, you need not enter your password every time you run ssh or scp.

Stability of Network Connection

Note that a stable network connection is required to use the PE hosts for a long period without interruption. If you encounter frequent disconnections while working at home or on campus while connected wirelessly, please make sure that your Wi-Fi signal is strong and that there is no interference from other sources.

If your connection is disconnected in the middle of editing, vim saves the state of the buffer for you. See the section on recovery files on how to recover your files.

If you find yourself facing frequent disconnection, you can consider running screen. After logging into a PE host, run: 

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screen

You will see some messages, press Enter to go to the command prompt. You can now use the PE host as usual. In case you are disconnected (e.g., in the middle of editing), you can log into the same PE host again, and run:

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screen -r

to resume your previous session.