diff --git a/src/Pure/Admin/build_jdk.scala b/src/Pure/Admin/build_jdk.scala
--- a/src/Pure/Admin/build_jdk.scala
+++ b/src/Pure/Admin/build_jdk.scala
@@ -1,241 +1,240 @@
 /*  Title:      Pure/Admin/build_jdk.scala
     Author:     Makarius
 
 Build Isabelle jdk component from original platform installations.
 */
 
 package isabelle
 
 
 import java.io.{File => JFile}
 import java.nio.file.Files
 import java.nio.file.attribute.PosixFilePermission
 
 import scala.util.matching.Regex
 
 
 object Build_JDK
 {
   /* platform and version information */
 
   sealed case class JDK_Platform(
     platform_name: String,
     platform_regex: Regex,
     exe: String = "java",
     macos_home: Boolean = false,
     jdk_version: String = "")
   {
     override def toString: String = platform_name
 
     def platform_path: Path = Path.explode(platform_name)
 
     def detect(jdk_dir: Path): Option[JDK_Platform] =
     {
       val major_version =
       {
         val Major_Version = """.*jdk(\d+).*$""".r
         val jdk_name = jdk_dir.file.getName
         jdk_name match {
           case Major_Version(s) => s
           case _ => error("Cannot determine major version from " + quote(jdk_name))
         }
       }
 
       val path = jdk_dir + Path.explode("bin") + Path.explode(exe)
       if (path.is_file) {
         val file_descr = Isabelle_System.bash("file -b " + File.bash_path(path)).check.out
         if (platform_regex.pattern.matcher(file_descr).matches) {
           val Version = ("^(" + major_version + """\.[0-9.]+\+\d+)(?:-LTS)?$""").r
           val version_lines =
             Isabelle_System.bash("strings " + File.bash_path(path)).check
               .out_lines.flatMap({ case Version(s) => Some(s) case _ => None })
           version_lines match {
             case List(jdk_version) => Some(copy(jdk_version = jdk_version))
             case _ => error("Expected unique version within executable " + path)
           }
         }
         else None
       }
       else None
     }
   }
 
   val templates: List[JDK_Platform] =
     List(
       JDK_Platform("arm64-darwin", """.*Mach-O 64-bit.*arm64.*""".r, macos_home = true),
       JDK_Platform("arm64-linux", """.*ELF 64-bit.*ARM aarch64.*""".r),
       JDK_Platform("x86_64-darwin", """.*Mach-O 64-bit.*x86[-_]64.*""".r, macos_home = true),
       JDK_Platform("x86_64-linux", """.*ELF 64-bit.*x86[-_]64.*""".r),
       JDK_Platform("x86_64-windows", """.*PE32\+ executable.*x86[-_]64.*""".r, exe = "java.exe"))
 
 
   /* README */
 
   def readme(jdk_version: String): String =
 """This is OpenJDK """ + jdk_version + """ based on downloads by Azul, see also
 https://www.azul.com/downloads/zulu-community/?package=jdk
 
 The main license is GPL2, but some modules are covered by other (more liberal)
 licenses, see legal/* for details.
 
 Linux, Windows, macOS all work uniformly, depending on platform-specific
 subdirectories.
 """
 
 
   /* settings */
 
   val settings: String =
 """# -*- shell-script -*- :mode=shellscript:
 
 case "$ISABELLE_PLATFORM_FAMILY" in
   linux)
     ISABELLE_JAVA_PLATFORM="$ISABELLE_PLATFORM64"
     ISABELLE_JDK_HOME="$COMPONENT/$ISABELLE_JAVA_PLATFORM"
     ;;
   windows)
     ISABELLE_JAVA_PLATFORM="$ISABELLE_WINDOWS_PLATFORM64"
     ISABELLE_JDK_HOME="$COMPONENT/$ISABELLE_JAVA_PLATFORM"
     ;;
   macos)
     if [ -n "$ISABELLE_APPLE_PLATFORM64" -a -d "$COMPONENT/$ISABELLE_APPLE_PLATFORM64" ]
     then
       ISABELLE_JAVA_PLATFORM="$ISABELLE_APPLE_PLATFORM64"
     else
       ISABELLE_JAVA_PLATFORM="$ISABELLE_PLATFORM64"
     fi
     ISABELLE_JDK_HOME="$COMPONENT/$ISABELLE_JAVA_PLATFORM"
     ;;
 esac
 """
 
 
   /* extract archive */
 
   def extract_archive(dir: Path, archive: Path): JDK_Platform =
   {
     try {
       val tmp_dir = Isabelle_System.make_directory(dir + Path.explode("tmp"))
 
       if (archive.get_ext == "zip") {
         Isabelle_System.bash(
           "unzip -x " + File.bash_path(archive.absolute), cwd = tmp_dir.file).check
       }
       else {
         Isabelle_System.gnutar("-xzf " + File.bash_path(archive), dir = tmp_dir).check
       }
 
       val dir_entry =
         File.read_dir(tmp_dir) match {
           case List(s) => s
           case _ => error("Archive contains multiple directories")
         }
 
       val jdk_dir = tmp_dir + Path.explode(dir_entry)
       val platform =
         templates.view.flatMap(_.detect(jdk_dir))
           .headOption.getOrElse(error("Failed to detect JDK platform"))
 
       val platform_dir = dir + platform.platform_path
       if (platform_dir.is_dir) error("Directory already exists: " + platform_dir)
 
       Isabelle_System.move_file(jdk_dir, platform_dir)
 
       platform
     }
     catch { case ERROR(msg) => cat_error(msg, "The error(s) above occurred for " + archive) }
   }
 
 
   /* build jdk */
 
   def build_jdk(
     archives: List[Path],
     progress: Progress = new Progress,
     target_dir: Path = Path.current): Unit =
   {
     if (Platform.is_windows) error("Cannot build jdk on Windows")
 
     Isabelle_System.with_tmp_dir("jdk")(dir =>
       {
         progress.echo("Extracting ...")
         val platforms = archives.map(extract_archive(dir, _))
 
         val jdk_version =
           platforms.map(_.jdk_version).distinct match {
             case List(version) => version
             case Nil => error("No archives")
             case versions =>
               error("Archives contain multiple JDK versions: " + commas_quote(versions))
           }
 
         templates.filterNot(p1 => platforms.exists(p2 => p1.platform_name == p2.platform_name))
         match {
           case Nil =>
           case missing => error("Missing platforms: " + commas_quote(missing.map(_.platform_name)))
         }
 
         val jdk_name = "jdk-" + jdk_version
         val jdk_path = Path.explode(jdk_name)
         val component_dir = dir + jdk_path
 
         Isabelle_System.make_directory(component_dir + Path.explode("etc"))
         File.write(Components.settings(component_dir), settings)
         File.write(component_dir + Path.explode("README"), readme(jdk_version))
 
         for (platform <- platforms) {
           Isabelle_System.move_file(dir + platform.platform_path, component_dir)
         }
 
         for (file <- File.find_files(component_dir.file, include_dirs = true)) {
           val path = file.toPath
           val perms = Files.getPosixFilePermissions(path)
           perms.add(PosixFilePermission.OWNER_READ)
           perms.add(PosixFilePermission.GROUP_READ)
           perms.add(PosixFilePermission.OTHERS_READ)
           perms.add(PosixFilePermission.OWNER_WRITE)
           if (file.isDirectory) {
             perms.add(PosixFilePermission.OWNER_WRITE)
             perms.add(PosixFilePermission.OWNER_EXECUTE)
             perms.add(PosixFilePermission.GROUP_EXECUTE)
             perms.add(PosixFilePermission.OTHERS_EXECUTE)
           }
           Files.setPosixFilePermissions(path, perms)
         }
 
         progress.echo("Archiving ...")
         Isabelle_System.gnutar(
-          "-czf " + File.bash_path(target_dir + jdk_path.ext("tar.gz")) + " " + jdk_name,
-          dir = dir).check
+          "-czf " + File.bash_path(target_dir + jdk_path.tar.gz) + " " + jdk_name, dir = dir).check
       })
   }
 
 
   /* Isabelle tool wrapper */
 
   val isabelle_tool =
     Isabelle_Tool("build_jdk", "build Isabelle jdk component from original archives",
       Scala_Project.here, args =>
     {
       var target_dir = Path.current
 
       val getopts = Getopts("""
 Usage: isabelle build_jdk [OPTIONS] ARCHIVES...
 
   Options are:
     -D DIR       target directory (default ".")
 
   Build jdk component from tar.gz archives, with original jdk archives
   for Linux, Windows, and macOS.
 """,
         "D:" -> (arg => target_dir = Path.explode(arg)))
 
       val more_args = getopts(args)
       if (more_args.isEmpty) getopts.usage()
 
       val archives = more_args.map(Path.explode)
       val progress = new Console_Progress()
 
       build_jdk(archives = archives, progress = progress, target_dir = target_dir)
     })
 }
diff --git a/src/Pure/General/path.scala b/src/Pure/General/path.scala
--- a/src/Pure/General/path.scala
+++ b/src/Pure/General/path.scala
@@ -1,309 +1,311 @@
 /*  Title:      Pure/General/path.scala
     Author:     Makarius
 
 Algebra of file-system paths: basic POSIX notation, extended by named
 roots (e.g. //foo) and variables (e.g. $BAR).
 */
 
 package isabelle
 
 
 import java.io.{File => JFile}
 
 import scala.util.matching.Regex
 
 
 object Path
 {
   /* path elements */
 
   sealed abstract class Elem
   private case class Root(name: String) extends Elem
   private case class Basic(name: String) extends Elem
   private case class Variable(name: String) extends Elem
   private case object Parent extends Elem
 
   private def err_elem(msg: String, s: String): Nothing =
     error(msg + " path element " + quote(s))
 
   private val illegal_elem = Set("", "~", "~~", ".", "..")
   private val illegal_char = "/\\$:\"'<>|?*"
 
   private def check_elem(s: String): String =
     if (illegal_elem.contains(s)) err_elem("Illegal", s)
     else {
       for (c <- s) {
         if (c.toInt < 32)
           err_elem("Illegal control character " + c.toInt + " in", s)
         if (illegal_char.contains(c))
           err_elem("Illegal character " + quote(c.toString) + " in", s)
       }
       s
     }
 
   private def root_elem(s: String): Elem = Root(check_elem(s))
   private def basic_elem(s: String): Elem = Basic(check_elem(s))
   private def variable_elem(s: String): Elem = Variable(check_elem(s))
 
   private def apply_elem(y: Elem, xs: List[Elem]): List[Elem] =
     (y, xs) match {
       case (Root(_), _) => List(y)
       case (Parent, Root(_) :: _) => xs
       case (Parent, Basic(_) :: rest) => rest
       case _ => y :: xs
     }
 
   private def norm_elems(elems: List[Elem]): List[Elem] =
     elems.foldRight(List.empty[Elem])(apply_elem)
 
   private def implode_elem(elem: Elem, short: Boolean): String =
     elem match {
       case Root("") => ""
       case Root(s) => "//" + s
       case Basic(s) => s
       case Variable("USER_HOME") if short => "~"
       case Variable("ISABELLE_HOME") if short => "~~"
       case Variable(s) => "$" + s
       case Parent => ".."
     }
 
   private def squash_elem(elem: Elem): String =
     elem match {
       case Root("") => "ROOT"
       case Root(s) => "SERVER_" + s
       case Basic(s) => s
       case Variable(s) => s
       case Parent => "PARENT"
     }
 
 
   /* path constructors */
 
   val current: Path = new Path(Nil)
   val root: Path = new Path(List(Root("")))
   def named_root(s: String): Path = new Path(List(root_elem(s)))
   def make(elems: List[String]): Path = new Path(elems.reverse.map(basic_elem))
   def basic(s: String): Path = new Path(List(basic_elem(s)))
   def variable(s: String): Path = new Path(List(variable_elem(s)))
   val parent: Path = new Path(List(Parent))
 
   val USER_HOME: Path = variable("USER_HOME")
   val ISABELLE_HOME: Path = variable("ISABELLE_HOME")
 
 
   /* explode */
 
   def explode(str: String): Path =
   {
     def explode_elem(s: String): Elem =
       try {
         if (s == "..") Parent
         else if (s == "~") Variable("USER_HOME")
         else if (s == "~~") Variable("ISABELLE_HOME")
         else if (s.startsWith("$")) variable_elem(s.substring(1))
         else basic_elem(s)
       }
       catch { case ERROR(msg) => cat_error(msg, "The error(s) above occurred in " + quote(str)) }
   
     val ss = space_explode('/', str)
     val r = ss.takeWhile(_.isEmpty).length
     val es = ss.dropWhile(_.isEmpty)
     val (roots, raw_elems) =
       if (r == 0) (Nil, es)
       else if (r == 1) (List(Root("")), es)
       else if (es.isEmpty) (List(Root("")), Nil)
       else (List(root_elem(es.head)), es.tail)
     val elems = raw_elems.filterNot(s => s.isEmpty || s == ".").map(explode_elem)
 
     new Path(norm_elems(elems reverse_::: roots))
   }
 
   def is_wellformed(str: String): Boolean =
     try { explode(str); true } catch { case ERROR(_) => false }
 
   def is_valid(str: String): Boolean =
     try { explode(str).expand; true } catch { case ERROR(_) => false }
 
   def split(str: String): List[Path] =
     space_explode(':', str).filterNot(_.isEmpty).map(explode)
 
 
   /* encode */
 
   val encode: XML.Encode.T[Path] = (path => XML.Encode.string(path.implode))
 
 
   /* reserved names */
 
   private val reserved_windows: Set[String] =
     Set("CON", "PRN", "AUX", "NUL",
       "COM1", "COM2", "COM3", "COM4", "COM5", "COM6", "COM7", "COM8", "COM9",
       "LPT1", "LPT2", "LPT3", "LPT4", "LPT5", "LPT6", "LPT7", "LPT8", "LPT9")
 
   def is_reserved(name: String): Boolean =
     Long_Name.explode(name).exists(a => reserved_windows.contains(Word.uppercase(a)))
 
 
   /* case-insensitive names */
 
   def check_case_insensitive(paths: List[Path]): Unit =
   {
     val table =
       paths.foldLeft(Multi_Map.empty[String, String]) { case (tab, path) =>
         val name = path.expand.implode
         tab.insert(Word.lowercase(name), name)
       }
     val collisions =
       (for { (_, coll) <- table.iterator_list if coll.length > 1 } yield coll).toList.flatten
     if (collisions.nonEmpty) {
       error(("Collision of file names due case-insensitivity:" :: collisions).mkString("\n  "))
     }
   }
 }
 
 
 final class Path private(protected val elems: List[Path.Elem]) // reversed elements
 {
   override def hashCode: Int = elems.hashCode
   override def equals(that: Any): Boolean =
     that match {
       case other: Path => elems == other.elems
       case _ => false
     }
 
   def is_current: Boolean = elems.isEmpty
   def is_absolute: Boolean = elems.nonEmpty && elems.last.isInstanceOf[Path.Root]
   def is_root: Boolean = elems match { case List(Path.Root(_)) => true case _ => false }
   def is_basic: Boolean = elems match { case List(Path.Basic(_)) => true case _ => false }
   def starts_basic: Boolean = elems.nonEmpty && elems.last.isInstanceOf[Path.Basic]
 
   def +(other: Path): Path = new Path(other.elems.foldRight(elems)(Path.apply_elem))
 
 
   /* implode */
 
   private def gen_implode(short: Boolean): String =
     elems match {
       case Nil => "."
       case List(Path.Root("")) => "/"
       case _ => elems.map(Path.implode_elem(_, short)).reverse.mkString("/")
     }
   def implode: String = gen_implode(false)
   def implode_short: String = gen_implode(true)
 
   override def toString: String = quote(implode)
 
 
   /* base element */
 
   private def split_path: (Path, String) =
     elems match {
       case Path.Basic(s) :: xs => (new Path(xs), s)
       case _ => error("Cannot split path into dir/base: " + toString)
     }
 
   def dir: Path = split_path._1
   def base: Path = new Path(List(Path.Basic(split_path._2)))
 
   def ext(e: String): Path =
     if (e == "") this
     else {
       val (prfx, s) = split_path
       prfx + Path.basic(s + "." + e)
     }
 
   def xz: Path = ext("xz")
   def html: Path = ext("html")
   def tex: Path = ext("tex")
   def pdf: Path = ext("pdf")
   def thy: Path = ext("thy")
+  def tar: Path = ext("tar")
+  def gz: Path = ext("gz")
 
   def backup: Path =
   {
     val (prfx, s) = split_path
     prfx + Path.basic(s + "~")
   }
 
   def backup2: Path =
   {
     val (prfx, s) = split_path
     prfx + Path.basic(s + "~~")
   }
 
   def platform_exe: Path =
     if (Platform.is_windows) ext("exe") else this
 
   private val Ext = new Regex("(.*)\\.([^.]*)")
 
   def split_ext: (Path, String) =
   {
     val (prefix, base) = split_path
     base match {
       case Ext(b, e) => (prefix + Path.basic(b), e)
       case _ => (prefix + Path.basic(base), "")
     }
   }
 
   def drop_ext: Path = split_ext._1
   def get_ext: String = split_ext._2
 
   def squash: Path = new Path(elems.map(elem => Path.Basic(Path.squash_elem(elem))))
 
 
   /* expand */
 
   def expand_env(env: Map[String, String]): Path =
   {
     def eval(elem: Path.Elem): List[Path.Elem] =
       elem match {
         case Path.Variable(s) =>
           val path = Path.explode(Isabelle_System.getenv_strict(s, env))
           if (path.elems.exists(_.isInstanceOf[Path.Variable]))
             error("Illegal path variable nesting: " + s + "=" + path.toString)
           else path.elems
         case x => List(x)
       }
 
     new Path(Path.norm_elems(elems.flatMap(eval)))
   }
 
   def expand: Path = expand_env(Isabelle_System.settings())
 
   def file_name: String = expand.base.implode
 
 
   /* implode wrt. given directories */
 
   def implode_symbolic: String =
   {
     val directories =
       Library.space_explode(':', Isabelle_System.getenv("ISABELLE_DIRECTORIES")).reverse
     val full_name = expand.implode
     directories.view.flatMap(a =>
       try {
         val b = Path.explode(a).expand.implode
         if (full_name == b) Some(a)
         else {
           Library.try_unprefix(b + "/", full_name) match {
             case Some(name) => Some(a + "/" + name)
             case None => None
           }
         }
       } catch { case ERROR(_) => None }).headOption.getOrElse(implode)
   }
 
   def position: Position.T = Position.File(implode_symbolic)
 
 
   /* platform files */
 
   def file: JFile = File.platform_file(this)
   def is_file: Boolean = file.isFile
   def is_dir: Boolean = file.isDirectory
 
   def absolute_file: JFile = File.absolute(file)
   def canonical_file: JFile = File.canonical(file)
 
   def absolute: Path = File.path(absolute_file)
   def canonical: Path = File.path(canonical_file)
 }