COA1 – Using SSH
This page does not represent the most current semester of this course; it is present merely as an archive.

SSH (the Secure SHell) is a protocol for allowing people to access a computer over the Internet and run programs on it as if they were physically present.

1 Software

If you are running Then you should use
Linux open a terminal and use the built-in ssh command
MacOS open the Terminal app, then use the built-in ssh command
Windows 10 enable it once; then open the PowerShell app and use the ssh command
Haiku open a terminal and use the built-in ssh command
FreeBSD open a terminal and use the built-in ssh command
OpenBSD open a terminal and use the built-in ssh command
Irix open a terminal and use the built-in ssh command

2 Using key pairs instead of passwords

Typing passwords is both less secure (key-sniffers, typos, typing wrong passwords, etc) and more tedious than using a private key.

2.1 Concept

You’ll place a file on your computer and a file on the remote computer. They are matched, and each provides half of the work needed to do a job. When you log in, the remote computer will do half the work with its file, then send that to your computer to do the other half, then send it back, thus allowing both computers to be confident the other computer is who it says it is1.

2.2 Setup

The following commands should work on any system with SSH installed, with appropriate changes to username@the.server.edu;2

ssh-keygen -f ~/.ssh/id_rsa -t rsa -b 2048

When prompted for a passphrase by ssh-keygen, just press enter without typing anything.

ssh-copy-id -i ~/.ssh/id_rsa.pub username@the.server.edu

When prompted for a passphrase by ssh-copy-id, use your UVA CS account password.

2.2.1 Multiple machines

You’ll need to do the ssh-keygen once per client machine you use (e.g., you laptop)

3 Short summary

The following copies files to the CS SSH portal, then compiles and runs them on the server and displays the output on your laptop.

scp myfile1.c myfile2.c mst3k@portal.cs.virginia.edu:project1/
            ssh mst3k@portal.cs.virginia.edu "cd project1/; clang *.c; ./a.out"
Interactive Terminal

Open with ssh username@the.server.edu

Close with exit or Ctrl+D

ssh mst3k@portal.cs.virginia.edu

Run remote command and see output

ssh username@the.server.edu command arg1 arg2 ...

–or–

ssh username@the.server.edu "commands to execute"

ssh mst3k@portal.cs.virginia.edu ls -l

ssh mst3k@portal.cs.virginia.edu "cd /tmp; ls"

Copy files from local to remote

scp file file2 file3 ... user@the.server.edu:path/to/destination/

scp testfile.c mst3k@portal.cs.virginia.edu:code/demo1/

Note that scp will not create directories, but ssh can:

ssh mst3k@portal.cs.virginia.edu mkdir -p code/demo1/

Copy files from remote to local

scp user@the.server.edu:path/to/source/filename path/to/destination/

(use ./ for “put it where I am”)

scp mst3k@portal.cs.virginia.edu:code/demo1/testfile.c ./

  1. This is a gross over-simplification, but gets the core idea across. We’ll see what’s really going on when we discuss digital certificates in COA2↩︎

  2. If on Windows, you also may need to use \ instead of / (whether you do or not depends on which command line tool you use).

    There is a slight chance that ~/.ssh will not already exist. In that case ssh-keygen will fail; if you see such a failure you can fix it by running

    mkdir ~/.ssh
            chmod 700 ~/.ssh

    and then re-run the above commands↩︎