Re: Executing command on reattaching to screen session

[Axel Beckert <abe@deuxchevaux.org>]

Hi,

On Thu, Feb 06, 2014 at 07:27:43PM +0100, Thorsten Kampe wrote:
> tough question: can I automatically execute a command on reattaching 
> a detached byobu (screen or tmux) session?

Does that command needs to be run inside the session itself or does it
not need a terminal?

I have the later case and simply use a shell script wrapper around
screen which does some ssh-agent magic to get the new ssh-agent
working with the existing screen session upon reconnect via ssh
respectively autossh.

Here's the according shell function:

asc () {
        print -Pn "\e]0;%n@%m: autossh -t $* 'screen -RdU'\a"
        autossh -x -A -t "$@" 'test -x ~/bin/setup-ssh-screen && exec ~/bin/setup-ssh-screen || exec screen -RdU'
}

And here's setup-ssh-screen:

---snip---
#!/bin/sh

HOST=`hostname -s`
if [ -n "$SSH_AUTH_SOCK" ]; then
    # If we ssh in, redirect ~/.ssh/auth_sock to the
    # (forwarded) ssh agent socket. Keep a different
    # forwarded socket for each host, since different
    # vservers do not share the same /tmp (and moving the
    # socket to my homedir seems to break the socket...)
    rm -vf ~/.ssh/auth_sock-$HOST
    echo "Linking SSH auth sock: $SSH_AUTH_SOCK"
    ln -vs $SSH_AUTH_SOCK ~/.ssh/auth_sock-$HOST
    export SSH_AUTH_SOCK="$HOME/.ssh/auth_sock-$HOST"
fi  

exec screen -RdU
---snap---

> Every time I login, /etc/motd is displayed. Since I hardly ever log 
> out and in, I'd like to have the same functionality when reattaching 
> to my detached session. This could be a simple cat /etc/motd kind of 
> "autostart".

As someone else already proposed, you could use "screen -X". While an
"echo" command in a .screenrc is executed also on reattaching to a
session, "exec" and "screen" don't seem to be executed a second time,
so it seems as if this needs to go into some wrapper script as
described above.

To not interfere with the input of the running session, I would not
use "send-key" or such, but rather use screen's "exec" or "screen"
commands. The following two example commands can also be run from
inside a screen session to demonstrate their functionality:

screen -X exec sh -c 'sleep 2; cat /etc/motd'

This may though garble the screen of full-screen text mode
applications (like, vim, emacs, aptitude, mc, etc.), but works ok-ish
e.g. with just a shell. You usually just need to press enter to get a
prompt again. (Just typing the next command also suffices but may feel
strange as you don't see a prompt on that line where you're typing.)

screen -X screen sh -c 'cat /etc/motd; sleep 2'
screen -X screen less /etc/motd

This works fine anywhere as it starts a new screen window inside the
session which is closed automatically again after 2 seconds or, in the
second case, closes when the user presses "q".

The "sh -c '…'" is necessary if there are shell meta characters like
";" in the command, because screen passes it's parameters to "execve"
or similar and not to "sh -c".

HTH

		Kind regards, Axel
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