Remote Computers

Initial Login

You can run the tutorial hands-on sections using either option:

(A) lxplus

or

(B) A Tier3 machine (a site, your own laptop, or the CernVM guest machine). You may first want to ensure that you are able to do password-less logins or preferably Kerberos authentications to these machines. Instructions to do this can be found here.

As of November 2020, we strongly suggest you use lxplus for the purpose of this tutorial

However, below you will find instructions for either lxplus or the tier3 option, choose one. There may be problems with using some of the tier3 sites for analysis, it is difficult to test everything! In theory though, they should be a viable option.

On lxplus

These machines are ATLAS ready and you do not have to install additional software.

To log into lxplus you should type:

ssh -Y -l <username> lxplus.cern.ch

For this tutorial we will use EOS to store our source code (more info on next page).

It can sometimes be helpful to have a long-running session. In case your connection is unstable, or if you need to be able to close your laptop and return to your work, this tutorial provides some useful tips for using tmux and screen, which can help.

On Tier3 Machines

Check that your machine is ATLAS-ready. If you are managing your machine, the minimal requirements are

• CVMFS is installed, mounted and working

but otherwise this will be checked in the next sections.

Login to your Tier3 (if you are not already logged on) with the -Y option so that your X-windows are exported back.

ssh -Y -l <username> <your tier3 hostname>

X11 Forwarding

Whether you are working on lxplus or another machine, you need to use X11 forwarding, which is done with the -Y in the ssh command. This also requires X11 to be installed locally on your computer. There are multiple X11 server options depending on your OS:

• Linux: X11 should already be installed
• MacOS: XQuartz
• Windows: Cygwin/X or Xming

To check X11 forwarding is done correctly when connected to the remote machine, type xterm. If a new terminal-like window opens, X11 forwarding is working and you can proceed.

Alternatively, you can start ROOT on your remote machine using the command:

root -a

If you have X11 forwarding set up correctly, it will display a splash screen.

You need to make sure that ROOT is set up correctly on your remote machine before you can use this method. This is shown in the next section.

Passwordless ssh (optional)

Whether you are logging into lxplus or another machine, it is possible to connect without needing to input your password every time using passwordless ssh.