Make os.pwd modifiable via the os.pwd0 dynamic variable

Readme.adoc

@@ -1815,8 +1815,8 @@ wd / os.up
 wd / os.up / os.up
 ----
 
-Note that there are no in-built operations to change the `os.pwd`. In general,
-you should not need to: simply defining a new path, e.g.
+`os.pwd` can be modified in certain scopes via the `os.dynamicPwd` dynamic variable, but
+best practice is not to change it. Instead simply define a new path, e.g.
 
 [source,scala]
 ----

os/src-jvm/package.scala

@@ -2,6 +2,7 @@ import scala.language.implicitConversions
 import java.nio.file.FileSystem
 import java.nio.file.FileSystems
 import java.nio.file.Paths
+import scala.util.DynamicVariable
 
 package object os {
   type Generator[+T] = geny.Generator[T]
@@ -38,7 +39,9 @@ package object os {
   /**
    * The current working directory for this process.
    */
-  val pwd: Path = os.Path(java.nio.file.Paths.get(".").toAbsolutePath)
+  def pwd: Path = dynamicPwd.value
+  val dynamicPwd: DynamicVariable[Path] =
+    new DynamicVariable(os.Path(java.nio.file.Paths.get(".").toAbsolutePath))
 
   val up: RelPath = RelPath.up
 

os/src-native/package.scala

@@ -1,5 +1,6 @@
 import java.nio.file.FileSystem
 import java.nio.file.FileSystems
+import scala.util.DynamicVariable
 package object os {
   type Generator[+T] = geny.Generator[T]
   val Generator = geny.Generator
@@ -31,7 +32,9 @@ package object os {
   /**
    * The current working directory for this process.
    */
-  val pwd: Path = os.Path(java.nio.file.Paths.get(".").toAbsolutePath)
+  def pwd: Path = dynamicPwd.value
+  val dynamicPwd: DynamicVariable[Path] =
+    new DynamicVariable(os.Path(java.nio.file.Paths.get(".").toAbsolutePath))
 
   val up: RelPath = RelPath.up
 

os/test/src/PathTests.scala

@@ -434,6 +434,14 @@ object PathTests extends TestSuite {
       System.err.printf("p[%s]\n", posix(p))
       assert(posix(p) contains "/omg")
     }
+    test("dynamicPwd") {
+      val x = os.pwd
+      val y = os.dynamicPwd.withValue(os.pwd / "hello") {
+        os.pwd
+      }
+
+      assert(x / "hello" == y)
+    }
   }
   // compare absolute paths
   def sameFile(a: java.nio.file.Path, b: java.nio.file.Path): Boolean = {

os/test/src/SubprocessTests.scala

@@ -203,5 +203,24 @@ object SubprocessTests extends TestSuite {
         assert(output.out.lines() == Seq("HELLO /usr"))
       }
     }
+    test("dynamicPwd") {
+      // Windows doesnt have bash installed so a bit inconvenient
+      // to run these subprocesses for testing
+      if (!scala.util.Properties.isWin) {
+        val outsidePwd = os.pwd
+        val tmp0 = os.temp.dir()
+        val tmp = os.followLink(tmp0).getOrElse(tmp0)
+        val x = proc("bash", "-c", "pwd").call()
+        val y = os.dynamicPwd.withValue(tmp) {
+          proc("bash", "-c", "pwd").call()
+        }
+
+        val z = proc("bash", "-c", "pwd").call()
+        assert(outsidePwd.toString != tmp.toString)
+        assert(x.out.trim() == outsidePwd.toString)
+        assert(y.out.trim() == tmp.toString)
+        assert(z.out.trim() == outsidePwd.toString)
+      }
+    }
   }
 }