How can I “Open an app from an unidentified developer” without using the GUI? Basically, I used the native Safari.app web browser to download a third party application and I want to run / open it from the comfort of my bash shell / Terminal.app CLI; despite the following error message:

“&lt;unidentified_thirdparty.app&gt;” can’t be opened because it is from an unidentified 
 developer. Your security preferences allow installation of only apps from the Mac 
 App Store and identified developers. Safari downloaded this file on &lt;today's_date&gt; 
 from &lt;the.inter.net&gt;.
                                          |[OK]|
You'll either have to change the settings under System Preferences > Security &amp; Privacy > General > Allow applications downloaded from: to Anywhere or in a Terminal, strip the xattr com.apple.quarantine attribute from the downloaded file(s).

xattr -dr com.apple.quarantine "unidentified_thirdparty.app"