Using screen to keep things running
Learning Objectives
Create a new
screen
sessionDisconnect from a
screen
sessionReconnect to an existing
screen
sessionEnd an existing
screen
sessionHandling 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
tmux
tmux
is another program you can use to keep things running remotely.
In practice, it is very similar to screen, but it has some minor
differences. You can find syntax guides easily online, including guides
showing the equivalent commands in tmux and screen
(this one is quite good), but
here’s a quick list of equivalent commands to those used in this lesson:
tmux ls
instead ofscreen -list
Ctrl-b d
to detach instead ofCtrl-a d
(tmux
in general usesCtrl-b
instead ofCtrl-a
)tmux a
ortmux attach
instead ofscreen -rD