How to Disable the “Are you sure you want to open this file?” Warning Dialog in Mac OS X
“[Name] is an application downloaded from the internet. Are you sure you want to open it?”
Starting in new-ish versions of Mac OS X, you may have noticed that when you download a file from the web and go to open it, you will get a prompt saying something along these lines, warning that an application has been downloaded from the internet, and asking to confirm if you actually want to open it or not.
This is Mac OS X just being safe, which is generally a good thing for most users, but if you always know for sure what you’re downloading it could even be overly safe. For users who wish to stop those messages, you can turn off that warning dialog by turning to the command line and a defaults write string. We’ll show you how to turn this off (and back on) if you don’t want that message to appear.
How to Turn Off “Application Downloaded from Internet” Warning in OS X with defaults write
You can disable this quarantine message by launching the Mac Terminal and typing the following command:
defaults write com.apple.LaunchServices LSQuarantine -bool NO
You will then need to reboot (although killing the Finder should work as well) for the changes to take effect.
This has changed from time to time after being introduced with OS X Leopard, through Snow Leopard, Mountain Lion, and Mavericks. Even OS X Yosemite carries the warning, though it’s easier in modern versions of OS X to simply toggle the warnings by going through GateKeeper in the Security preference panel on your Mac.
Using Gatekeeper can also bypass these alerts on a one-off basis for applications.
How to Re-Enable File Download Quarantine Warning in OS X with defaults write
To reverse this and get the file quarantine message back, type:
defaults write com.apple.LaunchServices LSQuarantine -bool YES
Again you will need to reboot (or kill the Finder) to have the changes back.