Xcode 6.0でswift/swiftcを起動する

Never run this procedure on Mountain Lion or former, or you will have unexpected behavior.
Mountain Lion以下では絶対に実行しないでください。

In short, you can just copy everything.

setup_macosx10_10.sh

cd /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/
sudo mkdir MacOSX10.10.sdk
cd MacOSX10.10.sdk
sudo cp ../MacOSX10.9.sdk/SDKSettings.plist ./
sudo sed -i -e 's/10\.9/10.10/' SDKSettings.plist
sudo ln -s ../MacOSX10.9.sdk/System
sudo ln -s ../MacOSX10.9.sdk/usr

And you are all set. Just execute xcrun -sdk macosx10.10 swift program.swift! Have fun!

(You should remove this before updating to upcoming 6.1)

remove_macosx10_10.sh

cd /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/
sudo rm MacOSX10.10.sdk/*
sudo rmdir MacOSX10.10.sdk

Broken SDK path?

If xcrun -sdk macosx10.9 --show-sdk-path is pointed to MacOSX10.10.sdk for some reasons (like <unknown>:0: warning: no such SDK: '/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.10.sdk'), executing xcrun --no-cache -sdk macosx10.9 --show-sdk-path will fix the issue.

SDKのパスが異常な値になった場合は、xcrun --no-cache -sdk macosx10.9 --show-sdk-pathによって復旧することができます。