Running a shell command in scala native capturing output

Shell.scala

import scalanative.native._, stdlib._, stdio._

@extern
object Pipe {
  def popen(cmd: CString, mode: CString): Ptr[FILE]  = extern
  def pclose(file: Ptr[FILE]): Unit = extern
}

object Shell {
  def runCommand(cmd: String, cwd: Option[String] = None): Option[String] =
    Zone { implicit z =>
      val buffSize = 100
      val buff = alloc[CChar](buffSize)
      val s = new StringBuilder()
      val fullCmd = cwd.map(dir => s"cd $dir && ").getOrElse("") + cmd
      Pipe.popen(toCString(fullCmd), toCString("r")) match {
        case null =>
          None
        case filePtr =>
          while (fgets(buff, buffSize, filePtr) != null) {
            s.++=(fromCString(buff))
          }
          Pipe.pclose(filePtr)
          Some(s.result())
      }
    }
}

object HelloShell extends App {
  import Shell._
  println(runCommand("ls -l").getOrElse(""))
  println("-" * 50)
  println(runCommand("ls -l", Some("/")).getOrElse(""))
}