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Interactive GPVM sessions with terminal multiplexers

This is an introduction to the concepts and a few hints on how to use a terminal multiplexer.
Feel free to expand with your own content and experience.

A terminal multiplexer is a program that acts as a container of multiple shell sessions. By using one of them, you can open a single SSH connection to a remote server (say, a General Purpose Virtual Machine of your experiment), and have many shells open at the same time.
For people working on GPVMs, terminal multiplexers are a life changer.
The most popular ones are:

  • screen: text-based; ancient and ubiquitous
  • tmux: also text-based; inspired by screen, shares many aspects with it; if your server does not have it, ask for its installation!

tmux is the most featured and better maintained of them, and it is now expected to be installed on all Fermilab GPVM's. In the following text I will talk mostly about it.
Advantages of using a text multiplexer include:

  • a single entry point into a server: you don't need to open a SSH connection each time
  • session persistency: a multiplexer session can be detached, and recovered at a later time

Some common operations

Commands and command lists

The interaction with the shells is performed mainly with key strokes from your keyboard (this is a description fitting any terminal session after all). There is some mouse interaction support in tmux, but that has ben in my experience unreliable and discontinuous.
So, tmux will pass your keystrokes to the currently active shell. To give instructions to tmux instead of its current shell, a special keystroke is used, the "prefix key" or "escape key".

The default key combination to send commands to tmux is <Ctrl>+<B>. This can be customised if needed.

So for example, to tell tmux to create a new shell, you will have to type <Ctrl>+<B> <C>, and, to switch to the last window you used, you hit <Ctrl>+<B> <L>.
The manual (man tmux) lists at the very beginning the default key bindings. I will mention here only two:

<Ctrl>+<B> list of current key bindings
<Ctrl>+<B> <:> type a tmux command

Starting a new session

From a terminal, you can start a new tmux session with:

tmux -S SessionName

Specifying the -S SessionName is optional, and it allows a better control in case you are running multiple sessions (for example, one for Purity, another for Xsec...).

Detaching and attaching to an existing session

You can detach the current session (default binding is <Ctrl>+<B> <D>), and you will be sent back to the shell you started tmux from. At this point, tmux (server) is still running, but no session is visualised (client).
The processes that were running are not affected: your LArSoft job started in a window is still running, and your local graphic emacs session is still ongoing. You can exit the shell, switch your laptop off and go home.

When at home, you can turn your desktop on and open a SSH session to the GPVM. The command tmux ls will list all the active sessions and their name (the one you used with the -S argument when starting the session).
You can choose which session to attach: tmux attach -t SessionName. But if your style is to have just one session, just tmux attach will do.
You'll get back exactly the same window structure that you had left, and if you were lucky with the GPVM or unlucky with the traffic, LArSoft might even have finished building by them.

Also, see the notes below about graphic programs.

Opening a new shell

The default key binding for a new shell is <Ctrl>+<B> <C>.

This will create a new tmux window and make it current, in the same directory where the previous window was. That means that if you are using your working area for both build and run, you are better served to create a build window, cd into your working area, and then creating another window that will be the "run" shell.
When the session starts being crowded, it makes a lot of sense to give a name to each window. In the example, one may be called "LArSoft build 6.11", another "LArSoft run 6.11", and so on. When browsing the list of windows (hint: @<Ctrl>+<B>

Closing a shell

The recommended way to close a shell is by typing the exit command in that shell. When the shell is over, tmux will automatically close its window, and when no window is left tmux will automatically close the session.
The kill-window command (<Ctrl>+<B> <&>) will ask you whether to kill the current window (and its process).

Scripting tmux behaviour

It is possible, although sometimes not completely trivial, to write configurations that open a set of windows (and panes) and set them up in a certain layout.
When you find out that you are creating the same layout over and over again, it may be time to hit a search engine and copy and customise some existing configuration.

Issues and pitfalls

Scrolling the terminal output and copy and paste

tmux manages the output buffer directly. This typically confuses your terminal application (xterm, iTerm, konsole...) which tries to do the same. A lot of adaptation is needed here.

While copy and paste is definitely possible within tmux, the habit of tmux of intercepting mouse events will make it more... adventurous.
tmux has a copy mode that allows scrolling back in the previous console output, to search it, and to select from it. By default, once the copy mode is entered, hitting <Enter> will exit it and copy in a copy buffer the current selection (then it can be pasted by <Ctrl>+<B> <]>).
Your mileage with using the mouse and its scroll wheel may vary according to your @tmux
settings and on its version. In my experience with konsole, selecting while keeping <Shift> pressed will tell the terminal do directly manage the mouse for that action, which turns into konsole copying whatever characters have been selected, ignoring tmux pane structure, and copying it into my clipboard. This will not be effective when using a window split in vertical panes. In the end, I can paste by using the central mouse button (Linux style), again while pressing <Shift>.

Graphic applications

tmux does not interfere with graphic systems, which is a good start.
GPVM are running GNU/Linux operating system and an XWindow server (we usually call it "X" or "X11"). That is the base of a client/server system where the X server is on the GPVM, the clients are anywhere (e.g., your laptop), and they communicate through a connection.
The X connection is tunnelled through the SSH connection. On each new SSH connection, ssh and the X server agree on a connection name, which is stored in the DISPLAY variable on the shell, server side (try echo $DISPLAY and you'll see something like localhost:10.0).
When you close the SSH connection, the X connection is also broken, and when you open a new SSH connection, the X connection might be different (e.g., localhost:11.0).
In that case, the existing tmux shells will still remember the old DISPLAY setting and will try to use that broken connection. So you start root and you get an X error and a batch session. The solution is to fix that DISPLAY variable first:

  1. open a new tmux window
  2. print the DISPLAY value (the new window will have the updated one!):
    $ echo $DISPLAY
    localhost:11.0
  3. copy that text and close the tmux window you just created
  4. in the old shell, update the DISPLAY variable:
    export DISPLAY="localhost:11.0"

At this point, root should show the splash screen and be able to render graphics.

I am not aware of any way to preserve running graphic applications after the SSH connection (and therefore the X connection) is broken. That is the area for remote desktop applications (e.g. Virtual Network Computing).

Lifetime of a session

Your tmux sessions will last until you close them, or until the server machine is rebooted.
GPVM's can be rebooted on a planned schedule (like the nefarious maintenance Thursdays), or an unplanned one because of any emergency. When that happens, your session is lost. There is no way I am aware of to tell tmux to save any part of the current session (window layout, current directories, or scrollback buffer) on disk.
So: don't count too much on your session being there when you are back: it usually will be, that is much better than without a multiplexer, but that is just as far as it goes.


Contact: Gianluca Petrillo ().