Interacting with a server

Overview

Teaching: 15 min
Exercises: 0 min

Questions

• How do I get started with a Linux server?

Objectives

• Understand how to run commands on a server

• Explain how to copy files between a server and my computer

Connecting to and Disconnecting from a Remote Computer

In this lesson we will show you how to connect to a remote computer and transfer data you would like to work on to this remote machine. To start, you should have a newly opened shell window on your computer. Currently you can use this shell to interact with your computer. Since we want to work on a remote computer, we need to connect to a shell on that computer. To do this, we will use a command called ssh (secure shell). This is a standard protocol used to securely connect to computers over a network.

Let’s say a user named Jane wants to connect to a computer with IP address 192.0.2.5. The IP is a unique address identifying a particular computer on the network.

After following the initial Setup instructions, Jane should have a local terminal window open where she can type Linux shell commands. To connect to her server, she would type the following shell command

local$ ssh jane@192.0.2.5

Are you connected?

It can get confusing when you are switching between your own computer’s shell and remote servers’ shells. All computers have a name that can be shown using the hostname command. This lets you know what computer you are working on:

$ hostname

If you are using Carleton’s RCDC, both your username and your temporary password will be provided. When you execute the ssh command and provide a username and IP address you will be prompted to add your password. In any context, if you are provided a temporary password (espcially if it was communicated to you over email), you should change it as soon as you can. Once you have successfully connected to the remote machine, you will likely be presented with some text describing the computer and operating system. After this you should then be left with a shell prompt, similar to what you had before connecting. To change your password, enter the following command:

server$ passwd

Changing password for jane.
(current) UNIX password: 
Enter new UNIX password: 
Retype new UNIX password: 
passwd: password updated successfully

You will then be asked to enter your current password followed by your new password twice. If you followed the instructions correctly, your password will be changed.

OK, so now we know how to connect, but how do we then disconnect?. To do this, enter the following command:

server$ exit

This will close the ssh connection and return to the shell on your own computer.

Copying Files to and from a Remote Computer

Now that we know how to connect and disconnect from a remote computer, it is very likely that you would want to transfer files to the remote computer to process and then copy the results back to your computer. To do this we will use a standard command called scp (secure copy). This command works a lot like the standard unix command cp (copy), which we expect you have used before. To copy a file from your computer to a remote computer, your scp command would look as follows:

local$ scp <local-path/local-file> <remote-user-name>@<remote-computer-IP>:<remote-path>

On the other hand, if you’d like to copy a file from the remote computer onto your computer, the scp command would resemble the following:

local$ scp <remote-user-name>@<remote-computer-IP>:<remote-path/remote-file> <local-path>

You can also copy entire directories, including all their contents, by adding the -r flag:

local$ scp -r <remote-user-name>@<remote-computer-IP>:<remote-path> <local-path>

Now let’s see how Jane would copy our data to the remote machine so she can start working on it. You can do the same using your computer. Navigate to the folder containing the data.zip prepared for this lesson. Here is how Jane would copy the data to the remote machine and then unpack the ZIP file:

local$ scp data.zip jane@192.0.2.5:~
local$ ssh jane@192.0.2.5
jane@192.0.2.5's password: 
server$ unzip data.zip
server$ exit

To do this yourself, you would need to replace “jane” with your username, and “192.0.2.5” with your IP address. The ~ at the end of the command is short hand for our home directory.

Downloading files from webpages

If you have a direct link to a web file, then you can also just directly download it to the server. The command to do this is called wget. So instead of downloading the data.zip file to your own computer and then copying it to the server:

server$ wget https://researchcomputingservices.github.io/remote-access/data/data.zip

As mentioned, this requires less steps so it is easier and faster - you aren’t copying the file twice. Working with large data files won’t take up space on your own computer, plus the network connection to the server may be faster than home internet or wireless.

Connect to a server

If Jane wants to connect to her favourite RCDC server with address 192.0.2.5, which command should she use?

1. ssh jane 192.0.2.5
2. scp jane@192.0.2.5:~ .
3. ssh jane@192.0.2.5
4. hostname 192.0.2.5
5. ssh jane:192.0.2.5

Solution

The correct answer is 3. The ssh command will connect Jane to a shell on a remote server, and the @ sign separates her name from the computer address.

Finding where you are

You think you have used ssh to connect to a remote server, but you aren’t sure. Which of the following commands will show you what server you are connected to?

1. hostname
2. whoami
3. pwd
4. computername

Solution

1. Yes: hostname will show the name of the computer you are currently interacting with.
2. No: whoami will show you the name of your user account
3. No: pwd will show you current directory path
4. No: computername is not a valid bash command

Copying Data

Assuming you do have access to remoteserver as user charlie, what is the error in this scp command?

$ mkdir mydirectory
$ scp mydirectory charlie@remoteserver:~

Solution

If you run the above command, then you would get an error like this:

mydirectory: not a regular file

To copy a directory, you must use the -r option:

$ scp -r mydirectory localhost:~

Key Points

• ssh connects to a shell session on the server.

• scp copies files to and from the server.