Cookbook: Running screen
Using IPython via ssh with X-forwarding is a really great way to work, but it can be a pain with the DISPLAY settings. The tricks outlined below have been really helpful for me and are now a part of my standard 'workflow'. For the most part, I assume Ubuntu (or some 'deb' package OS variant) and bash in my descriptions below. YMMV.
When working via ssh (and X forwarding) on IPython, sometimes your DISPLAY environment variable gets mangled. One approach to deal with this is outlined below, but recently I found a new approach.
There is a program called xpra which creates a virtual monitor (x server) to which you can point your DISPLAY variable.
You can start a screen session with the DISPLAY setting pointing to this virtual display. Then when you run IPython (inside screen) anything sent to X-display is sent to the virtual environment. You can attach and detach to this environment and your GUIs / matplotlib windows, etc will remain alive.
To install xpra:
sudo apt-get install xpra
Then a session:
First, start a screen session pointing to a virtual display
xpra start :100 DISPLAY=:100 screen
Now you've started screen and should be "inside" screen with all your X display sent to :100. You can start Ipython in the screen session:
ipython
In another terminal, attach to the xpra display to 'see' the X programs:
xpra attach :100
You can detach from both these sessions, and they'll keep running, get home or wherever... reattach to screen and also reattach to your xpra virtual display. Good luck!
Often you'll want to ssh to a server, start a job and leave. Knowing that you want the job to stay alive after you log out, you can just use:
nohup myjob.sh > nohup_output.nh
But sometimes you'd rather have an actual term or shell session stay alive. This is often the case when working with IPython. The problem is that your $DISPLAY environment variable may change, so that when you log into the machine later, what screen thinks is the present $DISPLAY variable will no longer be current and you'll get errors.
My workaround for this has been the following:
1 On the host machine, that is the one you are logging into, in your .bash_profile (which gets read when you log in via ssh) you should add the line:
echo "export DISPLAY=$DISPLAY" > .display.`whoami`.`hostname`
2 Again, on your host machine,make sure the following is in your .bashrc:
## set the prompt command to read the .disply file export PROMPT_COMMAND=". ~/.display.`whoami`.`hostname`"
What is happening here is that everytime you log in, the .bash_profile file creates a new file in your home directory, specific to your user, and the hostname. Whenever you log out and log back in, that file is updated with the appropriate DISPLAY information. In the .bashrc file, we've created a PROMPT_COMMAND that will read that file everytime you hit return in your shell. This can cause some annoyances if the file does not exist, for example if you use the 'su' command to become another user. However, overall it works quite well, such that when you return to a running screen session, the DISPLAY variable will be updated and you'll be able to send X-display windows back to your client machine.
Often you may have several screen sessions running. It is recommended to start screen with the following command:
screen -S my_description
Where my_description is some string that has meaning to you as a 'session' identifier. If only one screen session is running then simply screen -r will reconnect. However, if you have more, things can get complicated. The following script is helpful in such a case:
#!/bin/bash
# filters the screen -ls output to show the sesssions
sessions=`screen -ls | sed -ne 's/[[:space:]]//' -ne 's/\((Attached)\|(Detached)\|(Multi, detached)\|(Multi, attached)\)// p'`
#echo $sessions
#echo $sessions | wc -w
res=`echo "$sessions" | wc -w`
if [[ "$res" != "0" ]]
then
echo ''
echo " CURRENT SESSIONS"
echo " ------------------------"
#screen -ls | sed -ne 's/[[:space:]]//' -ne 's/\((Attached)\|(Detached)\)// p' | cat -n
echo "$sessions" | cat -n
echo " ------------------------"
echo ''
#prompt for the session to join
echo -n " Reattach to session, or ENTER for None: "
read session
if [[ $session != 0 ]]
then
#attach to specified session
linenum=0
name=`screen -ls | sed -ne 's/[[:space:]]//' -ne 's/\((Attached)\|(Detached)\|(Multi, detached)\|(Multi, attached)\)// p' |
while read line
do
let "linenum += 1"
if [[ "$linenum" -eq "$session" ]]
then
echo $line
break
fi
done`
shrtname=`echo $name | sed -e 's/[[:space:]]//' -e 's/(.*)//' -e 's/\t//g'`
echo "Reattaching to: " $shrtname
if [[ "$name" != "" ]]; then
if
#echo $shrtname
screen -r "$shrtname"
then
echo -n " Bye bye screen.. Exit?[y] "
read nbye
if [[ "$nbye" == "" ]]
then
exit
else
goodbye=$nbye
fi
if [[ "$goodbye" == "y" ]]
then
exit
else
echo "What's next?"
fi
else
if
# try multiuser connect
screen -Ax "$shrtname"
then
echo "Hope that was fun!"
else
echo "Trying to Force connection"
screen -D -r "$shrtname"
fi
fi
else
echo "Could not reattach to $session"
fi
else
echo " Have fun..."
fi
else
echo " No existing SCREEN session to reattach to..."
fi
Save this file in your $HOME/bin directory as 'screenlist' and make it executable: chmod +x screenlist. Then at the command prompt you can type:
[user@host ~]$screenlist
and you should get some output similar to:
CURRENT SESSIONS
------------------------
1 3841.coding (09/26/2011 11:35:01 PM)
2 3791.Ipython (09/26/2011 11:34:42 PM)
3 17716.test3 (09/26/2011 10:12:15 PM)
------------------------
Reattach to session, or ENTER for None: