From ArchWiki

Contents 1 If you want to startup your computer to a DM(Desktop Manger. Like XDM, GDM, Entrance) or to a shell 2 After INIT X on startup 3 When /bin/bash is being used (through xterm or as login shell etc..) 4 if you run startx or xinit 5 XDM 6 DE startup files 6.1 XFCE, GNOME, KDE 6.2 KDE 6.3 Fluxbox 6.4 Openbox 7 Arch Specific 8 Notes

[edit] If you want to startup your computer to a DM(Desktop Manger. Like XDM, GDM, Entrance) or to a shell

open /etc/inittab

change the last line in the file to the DM of your choice.

examples:

x:5:respawn:/usr/sbin/entranced -nodaemon
x:5:respawn:/usr/sbin/gdm -nodaemon
x:5:respawn:/usr/X11R6/bin/xdm -nodaemon

Then you can set the first line in inittab to 5 to startup a DE as default or 3 to startup a shell(usually /bin/bash, you can set it by using chsh) as default.

example:

id:5:initdefault:

You can also specify this one on GRUB cmd-line when you bootup by adding 'init single' (to not load any modules) 'init 3' or 'init 5'.

[edit] After INIT X on startup

explanation: after you see INIT 3 (or5) when you startup your computer but before you have logged in: this file /etc/rc.local executes.

[edit] When /bin/bash is being used (through xterm or as login shell etc..)

~/.bashrc executes and set's all the ENVIRONMENT VARIABLES for itself. You can put whatever you want here if you want it to startup each time you open an xterm etc.

[edit] if you run startx or xinit

~/.xinitrc executes

[edit] XDM

From the xdm man page:

First, the xdm configuration file should be set up.  Make  a  directory
       (usually  /usr/X11R6/lib/X11/xdm) to contain all of the relevant files.

       Here is a reasonable configuration file, which could be named  xdm-con-
       fig:

            DisplayManager.servers:            /usr/X11R6/lib/X11/xdm/Xservers
            DisplayManager.errorLogFile:       /var/log/xdm.log
            DisplayManager*resources:          /usr/X11R6/lib/X11/xdm/Xresources
            DisplayManager*startup:            /usr/X11R6/lib/X11/xdm/Xstartup
            DisplayManager*session:            /usr/X11R6/lib/X11/xdm/Xsession
            DisplayManager.pidFile:            /usr/X11R6/lib/X11/xdm/xdm-pid
            DisplayManager._0.authorize:       true
            DisplayManager*authorize:          false

       Note  that  this  file mostly contains references to other files.  Note
       also that some of the resources are specified with ``*'' separating the
       components.  These resources can be made unique for each different dis-
       play, by replacing the ``*'' with the display-name, but  normally  this
       is  not  very useful.  See the Resources section for a complete discus-
       sion.

Note that these can also be in ~/.Xstartup and ~/.Xresources

[edit] DE startup files

explanation: when you launch a DE, these files startup things for you.

[edit] XFCE, GNOME, KDE

~/Desktop/Autostart/*

[edit] KDE

~/.kde/Autostart

[edit] Fluxbox

~/.fluxbox/startup

[edit] Openbox

User-specific:

~/.config/openbox/autostart.sh

System-wide (only executes if above file doesn't exist):

/etc/xdg/openbox/autostart.sh

[edit] Arch Specific

/etc/rc.conf: Among other things executes modules and the daemons(scripts) inside /etc/rc.d/ at bootup before you have logged in
/etc/rc.d/* - configured daemons for the system.

[edit] Notes

make sure the various startup files have execute permissions for your user. here's an example.

chmod a+rwx name-of-startup-file