Send command to an already running screen session

DISPLAY and AUTHORITY

An X program needs two pieces of information in order to connect to an X display. (Note that wmctrl is an X program, even if it accesses other processes' windows rather than creating its own.)

• It needs the address of the display, which is typically :0 when you're logged in locally or :10, :11, etc. when you're logged in remotely (but the number can change depending on how many X connections are active). The address of the display is normally indicated in the DISPLAY environment variable.

• It needs the password for the display. X display passwords are called magic cookies. Magic cookies are not specified directly: they are always stored in X authority files, which are a collection of records of the form “display :42 has cookie 123456”. The X authority file is normally indicated in the XAUTHORITY environment variable. If $XAUTHORITY is not set, programs use ~/.Xauthority.

Inside a screen session, the environment variables are determined when the session starts, unless you explicitly change them at some point. So if you start a screen session locally on your desktop machine, then attach to that session remotely, $DISPLAY and $XAUTHORITY are still pointing to your desktop machine. But if you start the screen session from an ssh connection from some machine C to your desktop machine, then the variables are not set. (They would be set to point to C if you had an X server on C and had enabled X forwarding over the ssh session.)

Getting the values of the variables

As far as I understand, you're trying to act on the windows that are displayed on your desktop. If you're the only person using your desktop machine, it's very likely that the display name is :0. Finding the location of the X authority file is harder (under the default setup in Ubuntu, it's in a file with a randomly generated name).

Here are a few ways to obtain the values of DISPLAY and XAUTHORITY:

• The easy solution is to always start a screen session from your desktop, perhaps automatically in your login scripts (from ~/.profile; but do it only if logging in under X: test if DISPLAY is set to a value beginning with : (that should cover all the cases you're likely to encounter)). In ~/.profile:

  case $DISPLAY in
    :*) screen -S local -d -m;;
  esac
  

  In the ssh session:

  screen -d -r local
  

• You could also save the values of DISPLAY and XAUTHORITY in a file and recall the values. In ~/.profile:

  case $DISPLAY in
    :*) export | grep -E ' (DISPLAY|XAUTHORITY)=' >~/.local-display-coordinates.sh;;
  esac
  

  In the ssh session:

  . ~/.local-display-coordinates.sh
  screen
  

• You could detect the values of DISPLAY and XAUTHORITY from a running process. This is harder to automate. You have to figure out the PID of a process that's connected to the display you want to work on, then get the environment variables from /proc/$pid/environ (eval export $(</proc/$pid/environ tr \\0 \\n | grep -E '^(DISPLAY|XAUTHORITY)=')¹).

Copying the cookies

Another approach (following a suggestion by Arrowmaster is to not try to obtain the value of $XAUTHORITY in the ssh session, but instead to make the X session copy its cookies into ~/.Xauthority. Since the cookies are generated each time you log in, it's not a problem if you keep stale values in ~/.Xauthority.

There can be a security issue if your home directory is accessible over NFS or other network file system that allows remote administrators to view its contents. They'd still need to connect to your machine somehow, unless you've enabled X TCP connections (Debian has them off by default). So for most people, this either does not apply (no NFS) or is not a problem (no X TCP connections).

To copy cookies when you log into your desktop X session, add the following lines to ~/.xprofile or ~/.profile (or some other script that is read when you log in):

case :$DISPLAY:$XAUTHORITY in
  :*:?*) XAUTHORITY=~/.Xauthority xauth merge "$XAUTHORITY";;
esac

Then inside screen you'll only have to setenv DISPLAY :0 (or whatever the display number is, but it's likely to be :0 as explained above).

¹ In principle this lacks proper quoting, but in this specific instance $DISPLAY and $XAUTHORITY won't contain any shell metacharacter.

You might want to grep for SCREEN instead to verify that your screen really isn't running.

Some systems have tmp cleaners that delete files in /tmp, /var/tmp, /var/run, or similar. This can result in screen not being able to find its socket files. If you can identify the PID of your session, you can do kill -CHLD <PID> to tell screen to rewrite its socket file. screen -r should then work again.

If this is what's happening, you should probably configure screen to use another directory for its sockets.