I installed Xcode a long time ago.
Apparently I didn't check back then the "UNIX Development Support" checkbox.
Now I want to have them but when I click on the installation this is what appears:
The UNIX Development Support check box is disabled
Q: How can I install the UNIX Development Support? Is there a way to run some script that creates all the needed links from /Developer/ to /usr/bin ?
EDIT: Note: This is for old Xcode 3. Screens and tool names differ for Xcode 4 ("Unix Command Line Tools")
With Xcode 4.3 (from Apple App Store) you can enable Unix Command Line Tools via the Xcode Menu: Preferences -> Download -> Components
In xcode > 4.5 select Preferences -> Download -> Components -> Command Line Tools
Finally.
I had to download from Apple's website the latest version of Xcode 3 (3.2) along with the iPhone SDK that I won't use in the near feature.
This time the "UNIX Development Support" was enabled:
So now I can compile from the command line!
In the Xcode .dmg file, there is a Packages folder. You can manually install DeveloperToolsCLI.pkg which creates the link in /usr/bin.
At least, it worked for me.
You can also right click the "Install Xcode X.X" icon and Show Package Contents then in Contents > Resources > Packages, you will find many packages from among which resides DeveloperToolsCLI.pkg. This package installs the files you want in /usr/bin.
You should uninstall and then re-install the developer tools.
To uninstall the tools, run the following command in Terminal:
sudo /Developer/Library/uninstall-devtools --mode=all
来源：https://stackoverflow.com/questions/2693336/how-to-update-xcode-to-install-unix-development-support