Add root to Path & root constructor

os/src-jvm/package.scala

@@ -10,6 +10,12 @@ package object os {
    */
   val root: Path = Path(java.nio.file.Paths.get(".").toAbsolutePath.getRoot)
 
+  def root(root: String): Path = {
+    val path = Path(root)
+    assert(path.root == root, s"$root is not a root path")
+    path
+  }
+
   def resource(implicit resRoot: ResourceRoot = Thread.currentThread().getContextClassLoader) = {
     os.ResourcePath.resource(resRoot)
   }

os/src-native/package.scala

@@ -8,6 +8,12 @@ package object os {
    */
   val root: Path = Path(java.nio.file.Paths.get(".").toAbsolutePath.getRoot)
 
+  def root(root: String): Path = {
+    val path = Path(root)
+    assert(path.root == root, s"$root is not a root path")
+    path
+  }
+
   /**
    * The user's home directory
    */

os/src/Path.scala

@@ -484,6 +484,7 @@ class Path private[os] (val wrapped: java.nio.file.Path)
     new SeekableSource.ChannelSource(java.nio.file.Files.newByteChannel(wrapped))
 
   require(wrapped.isAbsolute || Path.driveRelative(wrapped), s"$wrapped is not an absolute path")
+  def root = Option(wrapped.getRoot).map(_.toString).getOrElse("")
   def segments: Iterator[String] = wrapped.iterator().asScala.map(_.toString)
   def getSegment(i: Int): String = wrapped.getName(i).toString
   def segmentCount = wrapped.getNameCount

os/test/src/PathTests.scala

@@ -1,6 +1,7 @@
 package test.os
 
 import java.nio.file.Paths
+import java.io.File
 
 import os._
 import os.Path.{driveRoot}
@@ -420,6 +421,14 @@ object PathTests extends TestSuite {
       assert(result1 == expected)
       assert(result2 == expected)
     }
+    test("custom root") {
+      assert(os.root == os.root(os.root.root))
+      File.listRoots().foreach { root =>
+        val path = os.root(root.toPath().toString) / "test" / "dir"
+        assert(path.root == root.toString)
+        assert(path.relativeTo(os.root(root.toPath().toString)) == rel / "test" / "dir")
+      }
+    }
     test("issue201") {
       val p = Path("/omg") // driveRelative path does not throw exception.
       System.err.printf("p[%s]\n", posix(p))