Demo using readabilityHandler to read data asynchronously from FileHandle

readabilityHandler.swift

import Foundation

let outputPipe = Pipe()

let process = Process()
process.executableURL = URL(fileURLWithPath:"/bin/bash")
process.arguments = ["-c", "ls"]
process.standardOutput = outputPipe

outputPipe.fileHandleForReading.readabilityHandler = { fileHandle in
    print("handler called")
    let data = fileHandle.availableData
    if data.isEmpty {
        outputPipe.fileHandleForReading.readabilityHandler = nil
        return
    }
    if let outputString = String(data: data, encoding: .utf8) {
        print(outputString)
    }
}

try? process.run()
let standardInputFH = process.standardInput as! FileHandle
let standardErrorFH = process.standardError as! FileHandle

print("PID: \(process.processIdentifier)")
print("stdin FD: \(standardInputFH.fileDescriptor)")
print("stdout FD: \(outputPipe.fileHandleForReading.fileDescriptor)")
print("stderr FD: \(standardErrorFH.fileDescriptor)")