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02-screen.md

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First Steps in LHCb
Using screen to keep things running
10

Learning Objectives {.objectives}

  • Create a new screen session
  • Disconnect from a screen session
  • Reconnect to an existing screen session
  • End an existing screen session
  • Handling of Kerberos tokens

Often we want to run a program for a long time on a computer we connected to via ssh, like the lxplus machines. The screen program allows you to keep programs running on the remote computer even if you disconnect from it. For example when putting your laptop to sleep or losing wifi.

Connect to lxplus:

$ ssh lxplus.cern.ch

You can start screen by simply typing its name:

$ screen

Once you do this it will look as if nothing has happened. However you now have a fresh terminal inside what is refered to as a "screen session". Let's type some commands that generate some output:

$ uptime

 11:48:02 up 15 days, 20:36, 105 users,  load average: 2.26, 2.10, 3.16

$ date

Thu Apr 16 11:48:32 CEST 2015

To disconnect from this session press Ctrl-a d. We are now back to the terminal that we first started in. You can check that your screen session is still running by typing screen -list, which will list all active sessions:

$ screen -list

There is a screen on:
      25593.pts-44.lxplus0234   (Detached)
1 Socket in /var/run/screen/S-thead.

To reconnect to the session you use screen -rD. When there is just one session running (like now) then it will reconnect you to that session. Try it and you should see the date and time output by the date command we ran earlier.

When you have more than one session you need to provide the name of the session as an argument to screen -rD. The name of the above session is: 25593.pts-44.lxplus0234. You can reconnect to it with:

$ screen -rD 25593.pts-44.lxplus0234

To end a screen session you are currently connected to, simply press Ctrl-d. Just like you would to disconnect from a ssh session.

A screen session is tied to a specific computer. This means you need to remember which computer you connected to. Even if your screen session keeps running, you can only resume it from the same machine as you started it.

When connecting to lxplus you are assigned to a random computer, but you can find out its name with hostname:

$ hostname

lxplus0081.cern.ch

This tells you that the computer you are connected to is called lxplus0081.cern.ch. Later on you can re-connect to exactly this machine with:

$ ssh lxplus0081.cern.ch

Another complication are your kerberos tokens. These typically expire as soon as you disconnect from a lxplus machine. This means the program you left running inside the screen session will suddenly not be able to write to any files in your home directory anymore. This is particularly annoying if you are running ganga in your screen session.

One way to stop your tokens from expiring is to type kinit when you first start a new screen session. The tokens you get this way will survive you disconnecting. However they will expire after 24 hours, so you will have to type kinit again to renew them if you leave screen running for longer than 24 hours.

$ screen
# now inside the screen session
$ kinit

Finding lost screens {.callout}

Once you start a screen session you need to remember which lxplus node it is running on. If you forget to note that down you can use the following little snippet to find any screen sessions running on lxplus nodes:

for i in $(seq -f "%04g" 1 500); do
 ssh -o ConnectTimeout=10 -o PreferredAuthentications=gssapi-with-mic,gssapi -o GSSAPIAuthentication=yes -o StrictHostKeyChecking=no -o LogLevel=quiet lxplus$i.cern.ch "(screen -list | head -1 | grep -q 'There is a screen on') && hostname && screen -list"
done

This will connect to the first 500 lxplus nodes in turn, checking if a screen session is running and if yes prints the hostname and output of screen -list.