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.

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 / 11	install OpenSSH Client (following the “from the Settings UI” directions) once; then open the `PowerShell` app and use the `ssh` command (alternatively install Windows Subsystem for Linux and use the built-in `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

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.

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 is^[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 CSO2].

Setup

The following commands should work on any system with SSH installed, with appropriate changes to username@the.server.edu;¹

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.

Multiple machines

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

Short summary

Example: 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 ./

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 ↩