Swift function to redirect stdout. Helpful for using print in environments where stdout is not easily accessible

redirect_stdout.swift

import Foundation

func redirectStdout(to path: String) throws {
    let (openResult, posixErrorCode) = path.withCString {
        // idea thanks to https://github.com/leptos-null/ClassDumpRuntime/blob/31e3601/classdumpctl/main.m#L456-L459
        // error checking thanks to https://github.com/leptos-null/Forest/blob/ba4b89a/Shared/Tar.swift#L172-L174
        let result = open($0, O_WRONLY | O_CREAT | O_TRUNC, 0o644)
        let globalError = errno // copy errno since it may change before we access it again later
        return (result, globalError)
    }
    guard openResult >= 0 else {
        throw NSError(domain: NSPOSIXErrorDomain, code: Int(posixErrorCode))
    }
    
    let dupStatus = dup2(openResult, STDOUT_FILENO)
    let globalError = errno // copy errno since it may change before we access it again later
    guard dupStatus >= 0 else {
        throw NSError(domain: NSPOSIXErrorDomain, code: Int(globalError))
    }
}

Testing:

try! redirectStdout(to: "/tmp/app_logs.txt")

let runLoop: RunLoop = .current
let timerCancellable = Timer.publish(every: 0.5, on: runLoop, in: .default)
    .autoconnect()
    .sink { pubDate in
        print("Hello at \(pubDate.formatted(date: .omitted, time: .standard))")
    }

runLoop.run(mode: .default, before: Date(timeIntervalSinceNow: 8))
timerCancellable.cancel()

and running tail -f /tmp/app_logs.txt in a shell.