Invalid active developer path on MAC OS X after installing Ruby

I started getting the same error after updating to OS X El Capitan.
In my case, all I did was to install it:

sudo xcode-select --install

And it works fine for me.

[Update] The steps below are for OS X 10.10 (Yosemite). Those experiencing this because of an upgrade to 10.11 (El Capitan) seem need to only run sudo xcode-select --install as described in the accepted answer.

I found a suggestion here which indicated

sudo xcode-select -switch /

as the fix. However, this threw an error that the developer path was invalid.

Instead, for me, I modified this to

sudo xcode-select -switch /Applications/Xcode.app/Contents/Developer/

(which is a mixture of the two referenced link solutions) and this worked.

[Edit]

Try using

xcode-select --print-path

to find your developer path if /Applications/Xcode.app/Contents/Developer/ is still invalid for you.

I know this is quite old but I got the same issue and then I fixed it by doing couple more extra steps. Problem came when Terminal could not find the actual path of the Command Line Tools.

1. If you had Xcode and then removed it, then Terminal is referring tothe Command Line Tools inside the Xcode folder. Try using thiscommand and check what your path is, xcode-select --print-path. This would most likely print /Applications/Xcode.app/Contents/Developer.
2. Now to fix this, reinstall the command line tools for xcode. Initially remove any previous installations using sudo rm -rf /Library/Developer/CommandLineTools.
3. Then install the command line tools sudo xcode-select --installNow the final step, change the path for the CLT to the actual path using sudo xcode-select -switch /Library/Developer/CommandLineTools/ this will fix it.