clarified 'export_files' in session ROOT: require explicit "isabelle build -e";
tuned messages;

--- a/NEWS	Fri Feb 15 12:34:29 2019 +0100
+++ b/NEWS	Fri Feb 15 17:00:21 2019 +0100
@@ -104,10 +104,10 @@
 
 * Code generation: command 'export_code' without file keyword exports
 code as regular theory export, which can be materialized using the
-command-line tool "isabelle export" or 'export_files' in the session
-ROOT, or browsed in Isabelle/jEdit via the "isabelle-export:"
-file-system. To get generated code dumped into output, use "file \<open>\<close>".
-Minor INCOMPATIBILITY.
+command-line tools "isabelle export" or "isabelle build -e" (with
+'export_files' in the session ROOT), or browsed in Isabelle/jEdit via
+the "isabelle-export:" file-system. To get generated code dumped into
+output, use "file \<open>\<close>". Minor INCOMPATIBILITY.
 
 * Code generation for OCaml: proper strings are used for literals.
 Minor INCOMPATIBILITY.
@@ -721,7 +721,9 @@
 *** ML ***
 
 * Operation Export.export emits theory exports (arbitrary blobs), which
-are stored persistently in the session build database.
+are stored persistently in the session build database. Command-line
+tools "isabelle export" and "isabelle build -e" allow to materialize
+exports in the physical file-system.
 
 * Command 'ML_export' exports ML toplevel bindings to the global
 bootstrap environment of the ML process. This allows ML evaluation
@@ -777,8 +779,9 @@
 * Command-line tool "isabelle imports -I" also reports actual session
 imports. This helps to minimize the session dependency graph.
 
-* The command-line tool "export" and 'export_files' in session ROOT
-entries retrieve theory exports from the session build database.
+* The command-line tool "export" and "isabelle build -e" (with
+'export_files' in session ROOT entries) retrieve theory exports from the
+session build database.
 
 * The command-line tools "isabelle server" and "isabelle client" provide
 access to the Isabelle Server: it supports responsive session management

--- a/src/Doc/System/Sessions.thy	Fri Feb 15 12:34:29 2019 +0100
+++ b/src/Doc/System/Sessions.thy	Fri Feb 15 17:00:21 2019 +0100
@@ -145,13 +145,14 @@
   \<^verbatim>\<open>document\<close> within the session root directory.
 
   \<^descr> \isakeyword{export_files}~\<open>(\<close>\isakeyword{in}~\<open>target_dir) [number]
-  patterns\<close> writes theory exports to the file-system: the \<open>target_dir\<close>
-  specification is relative to the session root directory; its default is
-  \<^verbatim>\<open>export\<close>. Exports are selected via \<open>patterns\<close> as in @{tool_ref export}
-  (\secref{sec:tool-export}). The number given in brackets (default: 0)
-  specifies elements that should be pruned from each name: it allows to reduce
-  the resulting directory hierarchy at the danger of overwriting files due to
-  loss of uniqueness.
+  patterns\<close> specifies theory exports that may get written to the file-system,
+  e.g. via @{tool_ref build} with option \<^verbatim>\<open>-e\<close> (\secref{sec:tool-build}). The
+  \<open>target_dir\<close> specification is relative to the session root directory; its
+  default is \<^verbatim>\<open>export\<close>. Exports are selected via \<open>patterns\<close> as in @{tool_ref
+  export} (\secref{sec:tool-export}). The number given in brackets (default:
+  0) specifies elements that should be pruned from each name: it allows to
+  reduce the resulting directory hierarchy at the danger of overwriting files
+  due to loss of uniqueness.
 \<close>
 
 
@@ -288,6 +289,7 @@
     -b           build heap images
     -c           clean build
     -d DIR       include session directory
+    -e           export files from session specification into file-system
     -f           fresh build
     -g NAME      select session group NAME
     -j INT       maximum number of parallel jobs (default 1)
@@ -374,6 +376,15 @@
   parent or import graph) before performing the specified build operation.
 
   \<^medskip>
+  Option \<^verbatim>\<open>-e\<close> executes the \isakeyword{export_files} directives from the ROOT
+  specification of all explicitly selected sessions: the status of the session
+  build database needs to be OK, but the session could have been built
+  earlier. Using \isakeyword{export_files}, a session may serve as abstract
+  interface for add-on build artefacts, but these are only materialized on
+  explicit request: without option \<^verbatim>\<open>-e\<close> there is no effect on the physical
+  file-system yet.
+
+  \<^medskip>
   Option \<^verbatim>\<open>-f\<close> forces a fresh build of all selected sessions and their
   requirements.
 

--- a/src/Pure/Thy/export.scala	Fri Feb 15 12:34:29 2019 +0100
+++ b/src/Pure/Thy/export.scala	Fri Feb 15 17:00:21 2019 +0100
@@ -264,8 +264,7 @@
     progress: Progress = No_Progress,
     export_prune: Int = 0,
     export_list: Boolean = false,
-    export_patterns: List[String] = Nil,
-    export_prefix: String = "")
+    export_patterns: List[String] = Nil)
   {
     using(store.open_database(session_name))(db =>
     {
@@ -298,7 +297,7 @@
               }
               else export_dir + Path.make(elems.drop(export_prune))
 
-            progress.echo(export_prefix + "export " + path)
+            progress.echo("export " + path + (if (entry.executable) " (executable)" else ""))
             Isabelle_System.mkdirs(path.dir)
             Bytes.write(path, entry.uncompressed(cache = store.xz_cache))
             File.set_executable(path, entry.executable)

--- a/src/Pure/Tools/build.scala	Fri Feb 15 12:34:29 2019 +0100
+++ b/src/Pure/Tools/build.scala	Fri Feb 15 17:00:21 2019 +0100
@@ -334,16 +334,8 @@
 
       Isabelle_System.rm_tree(export_tmp_dir)
 
-      if (result1.ok) {
-        for ((dir, prune, pats) <- info.export_files) {
-          Export.export_files(store, name, info.dir + dir,
-            progress = if (verbose) progress else No_Progress,
-            export_prune = prune,
-            export_patterns = pats,
-            export_prefix = name + ": ")
-        }
+      if (result1.ok)
         Present.finish(progress, store.browser_info, graph_file, info, name)
-      }
 
       graph_file.delete
 
@@ -405,6 +397,7 @@
     soft_build: Boolean = false,
     system_mode: Boolean = false,
     verbose: Boolean = false,
+    export_files: Boolean = false,
     pide: Boolean = false,
     requirements: Boolean = false,
     all_sessions: Boolean = false,
@@ -640,8 +633,8 @@
 
                   val numa_node = numa_nodes.next(used_node(_))
                   val job =
-                    new Job(progress, name, info, deps, store, do_output,
-                      verbose, pide, numa_node, queue.command_timings(name))
+                    new Job(progress, name, info, deps, store, do_output, verbose, pide = pide,
+                      numa_node, queue.command_timings(name))
                   loop(pending, running + (name -> (ancestor_heaps, job)), results)
                 }
                 else {
@@ -672,6 +665,21 @@
         (for ((name, result) <- results0.iterator)
           yield (name, (result.process, result.info))).toMap)
 
+    if (export_files) {
+      for (name <- full_sessions.imports_selection(selection1).iterator if results(name).ok) {
+        val info = results.info(name)
+        if (info.export_files.nonEmpty) {
+          progress.echo("Exporting " + info.name + " ...")
+          for ((dir, prune, pats) <- info.export_files) {
+            Export.export_files(store, name, info.dir + dir,
+              progress = if (verbose) progress else No_Progress,
+              export_prune = prune,
+              export_patterns = pats)
+          }
+        }
+      }
+    }
+
     if (results.rc != 0 && (verbose || !no_build)) {
       val unfinished =
         (for {
@@ -721,6 +729,7 @@
     var build_heap = false
     var clean_build = false
     var dirs: List[Path] = Nil
+    var export_files = false
     var fresh_build = false
     var session_groups: List[String] = Nil
     var max_jobs = 1
@@ -747,6 +756,7 @@
     -b           build heap images
     -c           clean build
     -d DIR       include session directory
+    -e           export files from session specification into file-system
     -f           fresh build
     -g NAME      select session group NAME
     -j INT       maximum number of parallel jobs (default 1)
@@ -772,6 +782,7 @@
       "b" -> (_ => build_heap = true),
       "c" -> (_ => clean_build = true),
       "d:" -> (arg => dirs = dirs ::: List(Path.explode(arg))),
+      "e" -> (_ => export_files = true),
       "f" -> (_ => fresh_build = true),
       "g:" -> (arg => session_groups = session_groups ::: List(arg)),
       "j:" -> (arg => max_jobs = Value.Int.parse(arg)),
@@ -814,6 +825,7 @@
           soft_build = soft_build,
           system_mode = system_mode,
           verbose = verbose,
+          export_files = export_files,
           pide = pide,
           requirements = requirements,
           all_sessions = all_sessions,