The SSH Command

SSH

If you want to run a program on your laptop, you launch the program (maybe double-clicking on some icon) and so foth. Or maybe, for a program like Python, Javac or Java, you open a terminal and run the program by typing the command to the terminal.

But if you need to run the program on tempest, that's not going to work. You need to open up a terminal (also called a shell) that is connected to tempest rather than to your laptop. To do that, we use a program that is usually called SSH (for secure shell.

Running SSH on a Mac

These directions are for running SSH on a Mac. If your laptop is a Windows computer, I suggest using OpenSSH via PowerShell. See Windows 10 ssh-copy-id Hopefully something like that will work on your personal laptop.

SSH is a command, so you'll have to run it via a terminal. As with any program on the Mac, there are many ways to launch the program. You can use the Finder to locate it in the Utilities folder inside the Applications Folder. You'll see an icon that looks like this:

Terminal Icon

Alternatively, what I do is to type Command-space, which brings up the "Spotlight Search" and then I type "terminal" and press return. That works quickly and easily. You can usually abbreviate "terminal" to "term".

Once you have a terminal open, you can use the ssh program. The program takes a command-line argument which is of the form user@host. In the example below, I'm connecting to my account named anderson on the host cs.wellesley.edu (same as tempest):

Last login: Thu Jul 23 16:55:53 on ttys012
sanderso-imac-0719:~ sanderso$ hostname
sanderso-imac-0719
sanderso-imac-0719:~ sanderso$ ssh anderson@cs.wellesley.edu
The authenticity of host 'cs.wellesley.edu (149.130.15.5)' can't be established.
ECDSA key fingerprint is SHA256:3e8AVhpb4iJyRQcgoyyOaeZOEE0BOtOKCw5FfcbUmaI.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added 'cs.wellesley.edu,149.130.15.5' (ECDSA) to the list of known hosts.
anderson@cs.wellesley.edu's password: 
Last failed login: Mon Jul 20 22:06:58 EDT 2020 from 212.64.69.175 on ssh:notty
There were 5 failed login attempts since the last successful login.
Last login: Wed Jul 15 11:13:02 2020 from pool-108-20-176-82.bstnma.fios.verizon.net
This is the new virtual server running CentOS 7
[anderson@tempest ~]$ hostname
tempest
[anderson@tempest ~]$ whoami
anderson
[anderson@tempest ~]$ logout
Connection to cs.wellesley.edu closed.
sanderso-imac-0719:~ sanderso$ hostname
sanderso-imac-0719
sanderso-imac-0719:~ sanderso$ whoami
sanderso
sanderso-imac-0719:~ sanderso$ ssh anderson@cs.wellesley.edu
anderson@cs.wellesley.edu's password: 
Last login: Thu Jul 23 16:57:26 2020 from pool-108-20-176-82.bstnma.fios.verizon.net
This is the new virtual server running CentOS 7
[anderson@tempest ~]$ hostname
tempest
[anderson@tempest ~]$ logout
Connection to cs.wellesley.edu closed.
sanderso-imac-0719:~ sanderso$ hostname
sanderso-imac-0719
sanderso-imac-0719:~ sanderso$

In the transcript above, I used the following commands:

  • hostname, which tells me which machine I'm "talking to." Notice the two different values:
    • sanderso-imac-0719; that's my laptop. (Well, the desktop computer that the college gave me.)
    • tempest; that's our server
  • whoami which tells me who I'm logged in as. Notice two different values:
    • sanderso on my laptop, and
    • anderson on the server
  • logout which ends my remote session and returns me to my laptop.

The important command is, of course, this one:

ssh anderson@cs.wellesley.edu

The first time I used it, I got a complex and scary response:

The authenticity of host 'cs.wellesley.edu (149.130.15.5)' can't be established.
ECDSA key fingerprint is SHA256:3e8AVhpb4iJyRQcgoyyOaeZOEE0BOtOKCw5FfcbUmaI.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added 'cs.wellesley.edu,149.130.15.5' (ECDSA) to the list of known hosts.

SSH will print this message the first time we connect to a server that we've never connect to before. Basically, there are some security issues that can occur (for example, some malicious person manages to re-directs our SSH connection to some other machine, and this warning would alert us to that fact). Here, it's harmless, and so I just said "yes".

Notice that I didn't get it the second time I ssh'd to Tempest. Yes, "ssh" is a verb.

Accounts

Above, I connected to my personal account on the server, anderson. I can also connect to other accounts that I have the password to, such as the cs304 account. That works exactly the same way:

sanderso-imac-0719:~ sanderso$ ssh cs304@cs.wellesley.edu
cs304@cs.wellesley.edu's password: 
Last login: Thu Jul 23 17:28:17 2020 from pool-108-20-176-82.bstnma.fios.verizon.net
This is the new virtual server running CentOS 7
[cs304@tempest ~]$ logout
Connection to cs.wellesley.edu closed.
sanderso-imac-0719:~ sanderso$

Notice that the "prompt" after I logged in reminds me who I am logged in as and where I'm logged in: cs304@tempest

In this course, you'll use your personal account for assignments and you'll create a team account for your semester project. You'll share the team account with your teammates. So you will have occasion to use two different accounts on the server.