Working Remotely

Overview

Teaching: 10 min
Exercises: 0 min

Questions

• How do I use ‘ssh’ and ‘scp’ ?

Objectives

• Learn what SSH is

• Learn how to work remotely using ssh and scp

What if we want to run some commands on another machine, such as the server in the basement that manages our database of experimental results? To do this, we have to first log in to that machine. We call this a remote login.

Once our local client is connected to the remote server, everything we type into the client is passed on, by the server, to the shell running on the remote computer. That remote shell runs those commands on our behalf, just as a local shell would, then sends back output, via the server, to our client, for our computer to display.

The SSH protocol uses encryption to ensure that outsiders can’t see what’s in the messages going back and forth between different computers.

The remote login server which accepts connections from client programs is known as the SSH daemon, sshd.

The client program we use to login remotely is the secure shell or ssh.

The ssh login client has a companion program called scp, which allows us to copy files to or from a remote computer using the same kind of encrypted connection.

A remote login using ssh

Depending on security settings on the server and network, we may have to be connected to the local network; or if working remotely, we may have to use the institution’s VPN.

Then, we issue the command ssh username@computer, which tries to make a connection to the SSH daemon running on the remote computer we have specified.

Typing exit, or Control-D on an empty line, terminates the remote shell.

In the example below, the remote machine’s command prompt is moon> instead of just $. To make it clearer which machine is doing what, we’ll indent the commands sent to the remote machine and their output.

$ pwd

/c/Users/nelle/Desktop

$ ssh nelle@moon.euphoric.edu
Password: ********

Assuming this connection works (this specific example will not!), any commands we issue, for example hostname, pwd, ls or some scientific analysis software, will run on the remote server. This will continue until we exit the remote shell, for example:

    moon> exit

pwd confirms we are now running commands on the local computer again (not the remote server):

$ pwd

/c/Users/nelle/Desktop

Copying files to, and from a remote machine using scp

To copy a file, we specify the source and destination paths, either of which may include computer names.

For example, this command might be used to copy our latest results to Nelle’s backups directory of server backupserver.euphoric.edu, printing out its progress as it does so:

$ scp results.dat nelle@backupserver.euphoric.edu:backups/results-2023-16-05.dat
Password: ********

results.dat              100%  9  1.0 MB/s 00:00

Note the colon :, seperating the hostname of the server and the pathname of the file we are copying to.

Copying a whole directory betwen remote machines uses the same syntax as the cp command: we just use the -r option to signal that we want copying to be recursively. For example, this command copies all of our results from the backup directory on the backupserver.euphoric.edu server to our laptop:

$ scp -r nelle@backupserver.euphoric.edu:backups ./backups
Password: ********

Key Points

• SSH is a secure means to access a remote Linux computer

• The ‘ssh’ and ‘scp’ utilities are secure alternatives to walking over to a machine, logging into it, and copying files off it

• ‘ssh’ and ‘scp’ are essential for using remote Linux servers