How to fork()+execv() in Swift

runCommand.swift

import Foundation

func withCStrings(_ strings: [String], scoped: ([UnsafeMutablePointer<CChar>?]) throws -> Void) rethrows {
    let cStrings = strings.map { strdup($0) }
    try scoped(cStrings + [nil])
    cStrings.forEach { free($0) }
}

enum RunCommandError: Error {
    case WaitPIDError
    case POSIXSpawnError(Int32)
}

func runCommand(_ command: String, completion: ((Int32) -> Void)? = nil) throws {
    var pid: pid_t = 0
    let args = ["sh", "-c", command]
    let envs = ProcessInfo().environment.map { k, v in "\(k)=\(v)" }
    try withCStrings(args) { cArgs in
        try withCStrings(envs) { cEnvs in
            var status = posix_spawn(&pid, "/bin/sh", nil, nil, cArgs, cEnvs)
            if status == 0 {
                if (waitpid(pid, &status, 0) != -1) {
                    completion?(status)
                } else {
                    throw RunCommandError.WaitPIDError
                }
            } else {
                throw RunCommandError.POSIXSpawnError(status)
            }
        }
    }
}