Swift Playgrounds are stored as .playground bundles, i.e. directories with a particular hierarchy containing various files. Here's a typical Playground bundle created by XCode:
MyPlayground.playground MyPlayground.playground/Contents.swift MyPlayground.playground/contents.xcplayground MyPlayground.playground/playground.xcworkspace MyPlayground.playground/playground.xcworkspace/contents.xcworkspacedata MyPlayground.playground/playground.xcworkspace/xcuserdata MyPlayground.playground/playground.xcworkspace/xcuserdata/sveinbjorn.xcuserdatad MyPlayground.playground/playground.xcworkspace/xcuserdata/sveinbjorn.xcuserdatad/UserInterfaceState.xcuserstate MyPlayground.playground/timeline.xctimeline
To run a Swift Playground in the command line:
/usr/bin/swift MyPlayground.playground/Contents.swift