When you start X, Debian runs some shell scripts that start your window manager and other X clients. By default, a window manager, an xconsole, and an xterm are started for you.
To customize your X startup, the file /etc/X11/config must contain the line allow-user-xsession. If it does not, log in as root and add the line now. Then log back in as yourself and continue the tutorial.
You can see how Debian's X startup works in the file /etc/X11/ Xsession. Note that you can change the behavior of /etc/X11/Xsession by modifying the file /etc/X11/config, which specifies a few system-wide preferences.
To run the clients of your choice when X starts, you create an executable shell script called .xsession in your home directory.
Once .xsession is created, you need to edit it to do something useful with your favorite text editor. You can do anything you want to in this script. However, when the script's process terminates, X also terminates.
In practical terms, this means that you often end the script with a call to exec. Whatever program you exec will replace the script process with itself, so commands found after the exec line will be ignored. The program you exec will become the new owner of the script process, which means that X will terminate when this new program's process terminates.
Say you end your .xsession with the line exec fvwm. This means that the fvwm window manager will be run when X starts. When you quit the fvwm window manager, your X session will end, and all other clients will be shut down. You do not have to use a window manager here; you could exec xterm, in which case typing exit in that particular xterm would cause the entire X session to end.
If you want to run other clients before you use exec, you will need to run them in the background. Otherwise .xsession will pause until each client exits and then continue to the next line. See the previous section on running jobs in the background (basically you want to put an ampersand at the end, as in xterm &).
You can take advantage of this behavior, though. If you want to run commands at the end of your X session, you can have your .xsession run a window manager or the like and wait for it to finish. That is, leave off the exec and the &; just enter fvwm by itself. Then put the commands of your choice after fvwm.
It would probably help to look at a few sample .xsession files. In all the examples, replace fvwm with the window manager of your choice.
The simplest .xsession just runs a window manager:
A more useful .xsession runs a few clients before starting the window manager. For example, you might want some xterms and an xclock whenever you start X. No problem; just enter xterm & xterm & xclock & exec fvwm. Two xterms and an xclock start up in the background, and then the window manager is launched. When you quit the window manager, you'll also quit X.
You might try it without the backgrounding just to see what happens. Enter this command: xterm xclock exec fvwm. xterm will start, and wait for you to exit it. Then xclock will start; you'll have to exit xclock before fvwm will start. The commands are run in sequence, since the script waits for each one to exit.
You can use sequential execution to your advantage. Perhaps you want to keep track of when you stop working every day:
date >> ~/logout-time
Finally, you can have a program other than the window manager determine when X exits:
The best way to learn how to use .xsession is to try some of these things out. Again, be sure you use chmod to make it executable; failure to do so is a common error.