Introduction to the Command Line


The command line is such a useful tool that it won't be long before you need to have access to the command line on a computer that is not sitting in front of you.  In the old days, before security was a concern, people used telnet to get a command line on a remote computer.  For most purposes, telnet is no longer a good idea, because data is transmitted in a raw, unencrypted format.  The standard secure way to gain access to a command line on a remote computer is via ssh (secure shell).  The simplest invocation of the command is

$ ssh

This command assumes that your username on the remote machine is the same as your username on the local machine at which you type the command.  The remote machine prompts you for your password.  If your username on the remote machine is different than your username on the local machine, use the -l (lower-case "L") option to indicate your username on the remote machine.

$ ssh -l remoteusername

Alternatively, you can use email-style notation to indicate a different username.

$ ssh

So far, all these commands display a command line on the remote machine from which you can then execute whatever commands that machine provides to you.  Sometimes you may want to execute a single command on a remote machine, returning afterward to the command line on your local machine. This can be achieved by placing the command to be executed by the remote machine in single quotes.

$ ssh 'mkdir /home/myname/newdir'

Sometimes what you need is to execute time consuming commands on a remote machine, but you aren't sure to have sufficient time during your current ssh session.  If you close the remote connection before a command execution has been completed, that command will be aborted.  To avoid losing your work, you may start via ssh a remote screen session and then detach it and reconnect to it whenever you want.  To detach a remote screen session, simply close the ssh connection: a detached screen session will remain running on the remote machine.

ssh offers many other options, which are described on the manual page. You can also set up your favorite systems to allow you to log in or run commands without specifying your password each time. The setup is complicated but can save you a lot of typing; try doing some Web searches for "ssh-keygen", "ssh-add", and "authorized_keys".

scp: file copying

The SSH protocol extends beyond the basic ssh command.  A particularly useful command based on the SSH protocol is scp, the secure copy command.  The following example copies a file from the current directory on your local machine to the directory /home/me/stuff on a remote machine.

$ scp

Be warned that the command will overwrite any file that's already present with the name /home/me/stuff/ (Or you'll get an error message if there's a file of that name and you don't have the privilege to overwrite it.) If /home/me is your home directory, the target directory can be abbreviated.

$ scp

You can just as easily copy in the other direction: from the remote machine to your local one.

$ scp yesterday-interview.txt

The file on the remote machine is interview.txt in the docs subdirectory of your home directory. The file will be copied to yesterday-interview.txt in the home directory of your local system

scp can be used to copy a file from one remote machine to another.

$ scp user1@host1:file1 user2@host2:otherdir

To recursively copy all of the files and subdirectories in a directory, use the -r option.

$ scp -r user1@host1:dir1 user2@host2:dir2

See the scp man page for more options.

rsync: automated bulk transfers and backups

rsync is a very useful command that keeps a remote directory in sync with a local directory. We mention it here because it's a useful command-line way to do networking, like ssh, and because the SSH protocol is recommended as the underlying transmission for rsync.

The following is a simple and useful example. It copies files from your local /home/myname/docs directory to a directory named backup/ in your home directory on the system rsync actually minimizes the amount of copying necessary through various sophisticated checks.

$ rsync -e ssh -a /home/myname/docs

The -e option to ssh uses the SSH protocol underneath for transmission, as recommended. The -a option (which stands for "archive") copies everything within the specified directory. If you want to delete the files on the local system as they're copied, include a --delete option. See the rsync manual page for more details about rsync.

Making life easier when you use SSH often

If you use SSH to connect to a lot of different servers, you will often make mistakes by mistyping usernames or even host names (imagine trying to remember 20 different username/host combinations). Thankfully, SSH offers a simple method to manage session information through a configuration file.

The configuration file is hidden in your home directory under the directory .ssh (the full path would be something like /home/jsmith/.ssh/config --if this file does not exist you can create it). Use your favorite editor to open this file and specify hosts like this:

Host dev
User fc

You can set up multiple hosts like this in your configuration file, and after you have saved it, connect to the host you called "dev" by running the following command.

$ ssh dev

Remember, the more often you use these commands the more time you save.