"In het verleden behaalde resultaten bieden geen garanties voor de toekomst"
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These are the ramblings of Matthijs Kooijman, concerning the software he hacks on, hobbies he has and occasionally his personal life.

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Getting Screen and X (and dbus and ssh-agent and ...) to play well

When you use Screen together with Xorg, you'll recognize this: You log in to an X session, start screen and use the terminals within screen to start programs every now and then. Everything works fine so far. Then, you logout and log in again (or X crashes, or whatever). You happily re-attach the still running screen, which allows you to continue whatever you were doing.

But now, whenever you want to start a GUI program, things get wonky. You'll get errors about not being able to find configuration data, connect to gconf or DBUS, or your programs will not start at all, with the ever-informative error message "No protocol specified". You'll also recognize your ssh-agent and gpg-agent to stop working within the screen session...

What is happening here, is that all those programs are using "environment variables" to communicate. In particular, when you log in, various daemons get started (like the DBUS daemon and your ssh-agent). To allow other programs to connect to these daemons, they put their contact info in an environment variable in the login process. Whenever a process starts another process, these environment variables get transfered from the parent process to the child process. Sine these environment variables are set in the X sesssion startup process, which starts everything else, all programs should have access to them.

However, you'll notice that, after logging in a second time, the screen you re-attach to was not started by the current X session. So that means its environment variables still point to the old (no longer runnig) daemons from the previous X session. This includes any shells already running in the screen as well as new shells started within the screen (since the latter inherit the environment variables from the screen process itself).

To fix this, we would like to somehow update the environment of all processes that are already running when we login, to update them with the addresses of the new daemons. Unfortunately, we can't change the environment of other processes (unless we resort to scary stuff like using gdb or poking around in /dev/mem...). So, we'll have to convice those shells to actually update their own environments.

So, this solution has two parts: First, after login, saving the relevant variables from the environment into a file. Then, we'll need to get our shell to load those variables.

The first part is fairly easy: Just run a script after login that writes out the values to a file. I have a script called ~/bin/save-env to do exactly that. It looks like this (full version here):


# Save a bunch of environment variables. This script should be run just
# after login. The saved variables can then be sourced by every bash
# shell, so long running shells (e.g., in screen) or incoming SSH shells
# can also use these services.

# Save the DBUS sessions address on each login
if [ -n "$DBUS_SESSION_BUS_ADDRESS" ]; then

if [ -n "$SSH_AUTH_SOCK" ]; then
echo export SSH_AGENT_PID="$SSH_AGENT_PID" > ~/.env.d/ssh
echo export SSH_AUTH_SOCK="$SSH_AUTH_SOCK" >> ~/.env.d/ssh

# Save other variables here

This script fills the directory ~/.env.d with files containg environment variables, separated by application. I could probably have thrown them all into a single file, but it seemed like a good idea to separate them. Anyway, these files are created in such a way that they can be sourced by a running shell to get the new files.

If you download and install this script, don't forget to make it executable and create the ~/.env.d directory. You'll need to make sure it gets run as late as possible after login. I'm running a (stripped down) Gnome session, so I used gnome-session-properties to add it to my list of startup applications. You might call this script from your .xession, KDE's startup program list, or whatever.

For the second part, we need to set our saved variables in all of our shells. This sounds easy, just run for f in ~/.env.d/*; do source "$f"; done in every shell (Don't be tempted to do source ~/.env.d/*, since that sources just the first file with the other files as arguments!). But, of course we don't want to do this manually, but let every shell do it automatically.

For this, we'll use a tool completely unintended, but suitable enough for this job: $PROMPT_COMMAND. Whenever Bash is about to display a prompt, it evals whatever is in the variable $PROMPT_COMMAND. So it ends up evaluating that command all the time, which makes it a prefect place to load the saved variables. By setting the $PROMPT_COMMAND variable in your ~/.bashrc variable, it will become enabled in every shell you start (except for login shells, so you might want to source ~/.bashrc from your ~/.bash_profile):

# Source some variables at every prompt. This is to make stuff like
# ssh agent, dbus, etc. working in long-running shells (e.g., inside
# screen).
PROMPT_COMMAND='for f in ~/.env.d/*; do source "$f"; done'

You might need to be careful where to place this line, in case PROMPT_COMMAND already has some other value, like is default on Debian for example. Here's my full .bashrc file, note the += and starting ; in the second assignment of $PROMPT_COMMAND.

The astute reader will have noticed that this will only work for existing shells when a prompt is displayed, meaning you might need to just press enter at an existing prompt (to force a new one) after logging in the second time to get the values loaded. But that's a small enough burden, right?

So, with these two components, you'll be able to optimally use your long-running screen sessions, even when your X sessions are not so stable ;-)

Additionally, this stuff also allows you to use your faithful daemons when you SSH into the machine. I use this so I can start GUI programs from another machine (in particular, to open up attachments from my email client which runs on a server somewhere). See my recent blogpost about setting that up. However, since running a command through SSH non-interactively never shows a prompt and thus never evaluates $PROMPT_COMMAND, you'll need to manually source the variables at once in your .bashrc directly. I do this at the top of my ~/.bashrc.

Man, I need to learn how to writer shorter posts...

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