Note: This was copied from fluxbox's site. Someone using gdm should check this etc.
How do I add fluxbox to my GDM sessions menu.
Create the file: /etc/gdm/Sessions/fluxbox with contents:
# global fluxbox session file -- used by gdm
exec /etc/X11/Xsession /usr/bin/startfluxbox
Of course, change /usr/bin/startfluxbox to wherever your startfluxbox startup script is.
If the /etc/gdm directory doesn't exist, it may be /etc/X11/gdm/Sessions/fluxbox on your computer.
If your on debian distro you need to touch ~/.Xsession use your favorite editor edit ~/.Xsession and put exec /usr/bin/startfluxbox then save the file and restart your GDM session.
On some distros you need to do something a bit different. For Debian, Ubuntu and a few other distros you need to follow this little guide instead.
If you decide you need to compile Fluxbox from git on your Ubuntu system you might have noticed, after it installed, that there was no way to start using Fluxbox, even if you tried to do
$ startx `which fluxbox` -- :1
you get premission denied and are unable to connect to the X server (I only tried this on Hoary). So on to the fix....
$ sudo gedit /usr/share/xsessions/fluxbox.desktop
Add to that file the following information:
Comment=Highly configureable low resource X11 Window Manager
NOTE: The bold lines, /usr/local/bin/startfluxbox are for custom built fluxboxs. If you have installed with the package manager, use /usr/bin/startfluxbox.
NOTE: For FreeBSD 6.0 try to put fluxbox.desktop on /usr/X11R6/share/gdm/xsessions/fluxbox.desktop
Then save the file and restart your GDM session. This can be done by logging out of your current X session then pressing ctrl+alt+backspace note: NOT ctrl+alt+del (that would work but will also reboot your computer)
To customize the startup script see this page You will find information explaining how to start applications at startup, how to execute apps automatically after fluxbox has already started, and other topics.