How to run a script at login/logout in OS X?

There are several ways to run scripts at login/logout in OS X, some are more recent and only apply to 10.5 and above, some are rather deprecated, but the fastest one would be to add a Login Hook.

First, create the script you want to run. Open up a Terminal and enter:

touch ~/script.sh
open -e !$

This will open a text editor. Enter the script, e.g. with the following contents:

#!/bin/sh
# insert your script here

Save the file. In your terminal, run:

chmod +x ~/script.sh

This will make the file executable. Now, let's add it as a hook:

sudo defaults write com.apple.loginwindow LoginHook /usr/local/bin/script.sh 

There's also the Logout Hook counterpart:

sudo defaults write com.apple.loginwindow LogoutHook /usr/local/bin/script2.sh

I've tested this on OS X 10.6, and it should work even up to 10.8. Keep in mind that the script runs as root and there is only one hook for login and logout respectively.

To undo all that, enter

sudo defaults delete com.apple.loginwindow LoginHook
sudo defaults delete com.apple.loginwindow LogoutHook

Note that this method is not recommended for deployment or anything, but if you're only using it like your question stated, that should be no problem.

Login hooks were deprecated in 10.4 in favor of launchd. To run a script at login, save a plist like this as ~/Library/LaunchAgents/test.plist. It's loaded on the next login even if you don't run launchctl load ~/Library/LaunchAgents/test.plist.

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC -//Apple Computer//DTD PLIST 1.0//EN
http://www.apple.com/DTDs/PropertyList-1.0.dtd>
<plist version="1.0">
<dict>
    <key>Label</key>
    <string>test</string>
    <key>ProgramArguments</key>
    <array>
        <string>say</string>
        <string>test</string>
    </array>
    <key>RunAtLoad</key>
    <true/>
</dict>
</plist>

For more information, see man launchd.plist and this blog post.