Monday, 5 July 2010

Using Screen

The bash shell is great, but it can be awkward to use because a task will continue to run until it's completed. It's possible to push a task into the background, but this isn't the best way to do so. What you really need is a way to have more than one shell operating at once.

Now, on a graphical desktop this is no problem, as you can just open a new terminal, or a new tab in the same terminal. However, this is less practical on a command-line only system, or one you've remoted into via SSH. But there is a solution, and it's called Screen.

Screen is a terminal multiplexer - it allows you to access multiple terminal sessions inside a single terminal window or remote terminal session (including SSH). Think of it as being like having a tabbed terminal, but in the command line. You can leave Screen running and detach from a session, then reconnect to it later, so for instance if you had an Ubuntu Server install you could connect to it via SSH, issue a command to update the system, then detach and attach again later once it had finished.

Screen is almost certainly preinstalled on your system, but if not you can install it via apt-get in the usual way (sudo apt-get install screen). Once you have it installed, you can start it by issuing the following command:
You're now running screen! You should be confronted by a standard copyright notice (you can remove this by editing /etc/screenrc and removing the hash at the start of the line that reads #startup_message off).

All of screen's internal commands require you hit Ctrl-a, followed by the appropriate key for what you want to do. Try hitting Ctrl-a, then c, and you'll create a new virtual console. Then hit Ctrl-a, then n to cycle to the next virtual console, or Ctrl-a then p to go to the previous one. Ctrl-a, then " will get you a list of all currently running virtual consoles that you can select from using the cursor keys, while Ctrl-a then d will detach you from your current screen session (it will continue to run in the background, so you could potentially log out and leave the processes to run while you do something else). When you want to reattach, just start screen with screen -r and it will reattach you to the running processes. To stop screen, just exit each of the running consoles with Ctrl-d or the exit command in the usual way.
Screen is of limited use on a graphical desktop, but you can still use it there to get used to the idea. It's most useful when dealing with command-line systems. For more details, check out the man page for Screen, or try searching on Google. It's a great tool that makes a lot of tasks easier, and is indispensable if you have to log into a server via SSH a lot. It's also quite handy if you use a lot of command-line tools because it means you can have everything in one terminal session - for instance, you could have mutt in one screen, irssi in another, lynx in a third, Vim in a fourth, and be compiling code using GCC in a fifth, all at the same time, and in a way that makes it easy to switch between them.

It's also worth checking out byobu, which is an Ubuntu-specific tool that makes Screen a lot easier to use if you're new to it. Byobu makes it easy to set Screen up according to your preferences, without having to edit the configuration files by hand.

Sunday, 23 May 2010

SSH - using the shell remotely

If you came to Ubuntu from a Windows background, or are otherwise familiar with Windows (let's face it, not many of us are in a position where they never use Windows at all!), you'll probably be familiar with Windows Remote Desktop, which is often used as a troubleshooting tool. You can do the same kind of thing in Ubuntu, but because the Linux command line is so much more powerful and flexible than its Windows counterpart, it's often much more practical to use a command-line only tool for the same purpose. This is often a better solution because sending a constantly updated image of your desktop to somewhere else is inevitably going to use a lot more bandwidth than a few shell commands, and it's very likely you can resolve the issue from the shell. It can also be useful for connecting to servers, shell accounts, web hosts etc.

Telnet was the original method of connecting to a remote computer, however it has declined in popularity due to security issues. SSH has largely replaced it because it's more secure, and that's what I'm going to tell you about today.

SSH requires two components to work - a client and a server. Think of it like surfing the web - when you visit a web page, you use a client (the web browser) to connect to a server (the web server). Similarly, with SSH you use an SSH client to connect to an SSH server on the machine you want to connect to. The most common SSH client and server on Linux are maintained by the OpenSSH project. Ubuntu ships with the OpenSSH client by default, and if you have a machine you'd like to be able to connect to remotely via SSH, just install the openssh-server package in the usual way.

How do you use it? It's simple. Open a terminal and enter something using the following format:

ssh user@server
Let's go into this in a little more detail. First of all, user is just the user name you want to log in as on the remote server. For instance, if you've installed openssh-server on your desktop and your user account is called eric on that machine, then you should put eric as the username. If you want to connect to a shell account in the name of ebarnes, then it should be ebarnes.

The server can be either a domain name or an IP address. So if you have a shell account on a machine which has the domain name and your user name is ebarnes, you'd enter ssh If you want to connect to a machine that doesn't have a domain name associated with it, then you'd need to enter the IP address to connect to. There's nothing stopping you connecting to another machine on your home network via this method, so if you have an old machine running Ubuntu Server, for instance, on your home network, and it's been assigned the IP address by your router, you can just connect like this:

ssh ebarnes@
If you'd like to be able to use SSH to connect to your home machine from outside your home network, that's a bit more demanding. You'd need to either obtain a static IP address from your ISP, which can be expensive, or use a service like DynDNS, and would also need to set up port forwarding on your router. However, there's all sorts of possibilites that offers, such as managing BitTorrent downloads remotely by using a command-line client like rTorrent, or administering your machine remotely over SSH.

Once you're connected, SSH works just like any other shell session - you can do exactly the same things with SSH as you would in the terminal. When you're finished, just type in exit or hit Ctrl-D as you would normally to close the shell and that will finish your session.

Note that you aren't limited to connecting to an SSH server from Linux or Unix hosts. Although most Linux or Unix-based operating systems ship with an SSH client, you can use PUTTY to connect from a Windows box (so, for instance, if you have a desktop running Ubuntu with an OpenSSH server installed, and you want to remote into it from your laptop running Windows 7, you can do that). There's even SSH clients for a number of smartphones, including the iPhone and Android, so you can remote into your machine from anywhere.

If you want to try it out, and you have an old desktop you don't use, it's quite interesting to install Ubuntu Server on it, connect it to your router and disconnect the monitor, managing it entirely via SSH. It can be a useful resource to have around, and offers a lot of possibilities, such as:
  • File server
  • uPnP server (allows you to stream content to many electronic devices, including the Sony PlayStation 3)
  • BitTorrent box
By managing it entirely via SSH, you don't need the additional overhead of running a desktop, and can devote more system resources to whatever job you want it to do. You can use SSH to install whatever software you want via apt-get to customise it for the job you want it to do.

Alternatively, if you don't have an old machine spare or don't wish to try this, why not try it in a virtual machine? Virtualbox is good for this - just install Ubuntu Server in a VM, and under the network settings, attach it to a bridged adapter. This will mean the virtual machine gets allocated its own IP address by your router, and you can connect to it via SSH.

If you have a web hosting account, you may be able to connect to this via SSH - the host will provide the details if this is an option. Finally, another option is to get a shell account - essentially it's a user account on a remote server. I highly recommend, who are a free shell account provider running on OpenBSD.

Have fun with SSH! Next time I'll show you a few tricks to make your experience with it better!

Monday, 17 May 2010

Going further with environment variables

First of all, I apologise for how long it's taken to get round to this (merely days away from being a full year) but I've been busy! I was frantically busy at work for much of the last year, and have my own web development studies to deal with too, so I've only just gotten around to doing this post. Hopefully I should be able to maintain a better frequency of posts in future!

Last time I explained a bit about how environment variables work. Now we'll get to use them a little. For instance, say you want to set up a shortcut to a specific folder called mystuff to make it easy to get to. Well, you can easily set up an environment variable to hold the path to that folder. Just enter something like this:

export mystuff=/home/user/my/stuff
This will set mystuff up as a local variable that you can call at any time in the same way you'd refer to it normally, like this:

cd $mystuff
Note the dollar sign is only used when calling it, not when setting it.

If you want to set variables permanently, just add them to the .profile file in your home directory. Then, when you next start up a shell, it will automatically load these settings.

There's a number of different settings you can add to customise your bash shell. For instance, the other day I added this to my .profile so that Vim was set as my text editor when using Subversion:

export SVN_EDITOR=/usr/bin/vim

I also added the following to my shell account at (who, incidentally are hands down the best shell account providers I have ever found, and I recommend) so I could use Vim in colour, even over SSH:
export TERM=xterm-256color
There's a huge range of settings you can change to customise your bash shell, so I won't attempt to cover them all. Instead, I'll refer you to the guide in the Ubuntu documentation. A few I will mention, however, are PAGER (typically /usr/bin/less), EDITOR (the default text editor, you can set it to /usr/bin/vim, /usr/bin/emacs, or /usr/bin/nano, or any other CLI editor you want), VISUAL (the visual editor, normally /usr/bin/gedit, although may be different on Kubuntu or Xubuntu), or BROWSER (pretty much what you'd expect).

Now, a related subject to environment variables is aliases. These are handy shortcuts you can write for commonly used commands, and can be a great timesaver. For instance, if you often connect to a specific host via ssh, you can write an alias for it. To set up an alias, just enter something like the following:

alias example="ssh user@server"

Now, just enter example, on its own, into the shell, and it will run the command. But, of course, this is only temporary. To make it permanent, set up a file in your home directory called .bash_aliases and add the alias you want to set up to it. Once you've done that, every time you start a new shell, all the aliases you've defined in the file will be available to you!

I hope this has given you some idea of how to customise your bash shell to your own liking and improve your workflow in the process!