src/Pure/System/isabelle_system.scala

 
     if (gnutar_check) bash("tar " + options + args, redirect = redirect)
     else error("Expected to find GNU tar executable")
   }
 
+  def require_command(cmds: String*)
+  {
+    for (cmd <- cmds) {
+      if (!bash(Bash.string(cmd) + " --version").ok) error("Missing command: " + quote(cmd))
+    }
+  }
+
   private lazy val curl_check: Unit =
-    try { bash("curl --version").check }
-    catch { case ERROR(_) => error("Cannot download files: missing curl") }
+    try { require_command("curl") }
+    catch { case ERROR(msg) => error(msg + " --- cannot download files") }
 
   def download(url: String, file: Path, progress: Progress = new Progress): Unit =
   {
     curl_check
     progress.echo("Getting " + quote(url))