2017-02-11 Persistent SSH sessions with proper scrollback

TL;DR: Put the code snippet below in ~/.profile or whatever startup script for your shell, and then use autoscreen <ssh-arguments> to get persistent SSH sessions.

As many of us, I often find myself working on remote machines through SSH while on the go. Doing this is problematic if you’re on unreliable connections, or simply want to close your laptop and continue working later.

The most common solution for this problem is to use a terminal multiplexer like screen or tmux to make sure that you won’t lose whatever you’re doing if the connection goes down. However, I really don’t like terminal multiplexers, chiefly because I cannot comfortably scroll and copy paste. I know that both screen and tmux and dvtm and probably many others incorporate various facilities to be able to achieve scrolling and copy-pasting, but I’ve always found them much less convenient than just using the cursor, the scrollwheel, and page up/down. Also note that mosh does not offer scrollback either.

However, after much tribulation I think we have found the ideal setup after a nice session with Niklas Hambüchen. The idea is: connect to the remote server and immediately spawn a screen session, but instruct screen to display the entire scrollback rather than just a screenful. Moreover, use autossh to automatically reconnect and reattach to the session.

It all comes together in this bash function to put in your .profile or equivalent:

function autoscreen() {
  AUTOSSH_GATETIME=5 autossh -M 0 -- -o "ServerAliveInterval 5" -o "ServerAliveCountMax 1" -t $@ $'bash -c \'tmpScreenConfig=$(mktemp); echo "termcapinfo xterm* ti@:te" >> $tmpScreenConfig; echo "altscreen on" >> $tmpScreenConfig; echo "maptimeout 0" >> $tmpScreenConfig; echo "startup_message off" >> $tmpScreenConfig; echo "msgwait 0" >> $tmpScreenConfig; exec screen -c $tmpScreenConfig -S "autosession-'$RANDOM$RANDOM$RANDOM$RANDOM$'" -RD\''
}

Now that I’m using NixOS, I just have the following in my systemPackages, which is even handier since it’ll be available everywhere, not only in bash:

autoscreen = pkgs.writeScriptBin "autoscreen" ''
  AUTOSSH_GATETIME=5 ${pkgs.autossh}/bin/autossh -M 0 -- -o "ServerAliveInterval 5" -o "ServerAliveCountMax 1" -t $@ $'bash -c \'tmpScreenConfig=$(mktemp); echo "termcapinfo xterm* ti@:te" >> $tmpScreenConfig; echo "altscreen on" >> $tmpScreenConfig; echo "maptimeout 0" >> $tmpScreenConfig; echo "startup_message off" >> $tmpScreenConfig; echo "msgwait 0" >> $tmpScreenConfig; exec screen -c $tmpScreenConfig -S "autosession-'$RANDOM$RANDOM$RANDOM$RANDOM$'" -RD\' '
'';

You can then use autoscreen like you’d use ssh, e.g.

$ autoscreen your-server.com

and if the connection goes down for whatever reason autossh will soon reconnect and reattach the session. All of this while still having proper scrollback, copy paste, and reflow!

Note that even if your client dies you can still recover the session by manually listing them and trying out the ones with an autosession- prefix.

You can check out a video of the above in action at http://mazzo.li/dump/autoscreen.mov.

For the curious, explanation of the parameters:

• AUTOSSH_GATETIME makes autossh “validate” the connection in just 5 seconds rather than the default 30;
• -M 0 disables autossh built-in keepalive, since we leverage ServerAliveInterval and ServerAliveCountMax to notice when the connection dies;
• On connection we immediately invoke bash to prepare an adequate screen config, save it to a temp file, and then start screen;
• The screen config contains:
  • termcapinfo xterm* ti@:te, which makes screen not display just a screenful of test but rather to have it to just output everything to the terminal’s scrollback (see http://aperiodic.net/screen/faq);
  • A side effect of the above is that you’ll get the output of less or vim in the scrollback, which is inconvenient. altscreen on removes this side effect;
  • maptimeout 0 minimize the delay when pressing ESC, which is very annoying when using vim or in my case kakoune. Note that to make kakoune or vim register ESC keys inside screen you also need to turn off mouse integration.
  • startup_message off and msgwait 0 which removes screen startup messages.
• The session name is composed of autosession- postfixed with 4 random numbers;
• The -RD option that we pass to screen makes it so that if the session does not exists it gets created, if it does screen reattaches it, which means that on the first connection we’ll create a new session, and then we’ll reconnect.

Check the man pages for autossh, ssh, and screen for more info.