Reads JSON from standard input and writes pretty-printed JSON to standard output

prettyjson.swift

#!/usr/bin/swift

import Cocoa

let stdin = FileHandle.standardInput
let stdout = FileHandle.standardOutput
let stderr = FileHandle.standardError

do {
    let inputData = stdin.readDataToEndOfFile()

    let jsonObject = try JSONSerialization.jsonObject(with: inputData,
                                                      options: [])

    let outputData = try JSONSerialization.data(withJSONObject: jsonObject,
                                                options: [.prettyPrinted])

    stdout.write(outputData)
    print()
}
catch let error as NSError {
    let message = "error: \(error.localizedDescription)\n"
    stderr.write(message.data(using: .utf8)!)
}

Save this as a file, do chmod +x prettyjson.swift to make it executable, then you can use it like this:

prettyjson.swift <myuglyjson.json >myprettyjson.json