Add support for 'while' loop in script

27 files changed, 1102 insertions, 318 deletions

diff --git a/libbuild2/build/script/parser+command-if.test.testscript b/libbuild2/build/script/parser+command-if.test.testscript
index a18a885..8b19186 100644
--- a/libbuild2/build/script/parser+command-if.test.testscript
+++ b/libbuild2/build/script/parser+command-if.test.testscript
@@ -279,7 +279,7 @@
   cmd
   end
   EOI
-  buildfile:12:1: error: 'end' without preceding 'if'
+  buildfile:12:1: error: 'end' without preceding 'if', 'for', or 'while'
   EOE
 
   : before
diff --git a/libbuild2/build/script/parser+while.test.testscript b/libbuild2/build/script/parser+while.test.testscript
new file mode 100644
index 0000000..5587291
--- /dev/null
+++ b/libbuild2/build/script/parser+while.test.testscript
@@ -0,0 +1,133 @@
+# file      : libbuild2/build/script/parser+while.test.testscript
+# license   : MIT; see accompanying LICENSE file
+
+: while
+:
+{
+  : true
+  :
+  $* <<EOI >>EOO
+  while ($v != "aa")
+    cmd "$v"
+    v = "$(v)a"
+  end
+  EOI
+  ? true
+  cmd ''
+  ? true
+  cmd a
+  ? false
+  EOO
+
+  : false
+  :
+  $* <<EOI >>EOO
+  while ($v == "aa")
+    cmd "$v"
+    v = "$(v)a"
+  end
+  EOI
+  ? false
+  EOO
+
+  : without-command
+  :
+  $* <<EOI 2>>EOE != 0
+  while
+    cmd
+  end
+  EOI
+  buildfile:11:6: error: missing program
+  EOE
+}
+
+: end
+:
+{
+  : without-end
+  :
+  $* <<EOI 2>>EOE != 0
+  while true
+    cmd
+  EOI
+  buildfile:13:1: error: expected closing 'end'
+  EOE
+}
+
+: elif
+:
+{
+  : without-if
+  :
+  $* <<EOI 2>>EOE != 0
+  while false
+    elif true
+      cmd
+    end
+  end
+  EOI
+  buildfile:12:3: error: 'elif' without preceding 'if'
+  EOE
+}
+
+: nested
+:
+{
+  $* -l -r <<EOI >>EOO
+  while ($v != "aa")      # 1
+    cmd1 "$v"             # 2
+    if ($v == "a")        # 3
+      cmd2                # 4
+      while ($v2 != "$v") # 5
+        cmd3              # 6
+        v2=$v
+      end
+    else
+      cmd4                # 7
+    end
+    cmd5                  # 8
+    v = "$(v)a"
+  end
+  EOI
+  ? true # 1 i1
+  cmd1 '' # 2 i1
+  ? false # 3 i1
+  cmd4 # 7 i1
+  cmd5 # 8 i1
+  ? true # 1 i2
+  cmd1 a # 2 i2
+  ? true # 3 i2
+  cmd2 # 4 i2
+  ? true # 5 i2 i1
+  cmd3 # 6 i2 i1
+  ? false # 5 i2 i2
+  cmd5 # 8 i2
+  ? false # 1 i3
+  EOO
+}
+
+: contained
+:
+{
+  : eos
+  :
+  $* <<EOI 2>>EOE != 0
+  while
+  EOI
+  buildfile:12:1: error: expected closing 'end'
+  EOE
+}
+
+: var
+:
+$* <<EOI >>EOO
+while ($v1 != "a")
+  v1 = "$(v1)a"
+  v2 = "$v1"
+end
+cmd $v1
+EOI
+? true
+? false
+cmd a
+EOO
diff --git a/libbuild2/build/script/parser.cxx b/libbuild2/build/script/parser.cxx
index 0614f20..1a7f4e1 100644
--- a/libbuild2/build/script/parser.cxx
+++ b/libbuild2/build/script/parser.cxx
@@ -193,9 +193,22 @@ namespace build2
         }
       }
 
+      // Parse a logical line, handling the flow control constructs
+      // recursively.
+      //
+      // If the flow control construct type is specified, then this line is
+      // assumed to belong to such a construct.
+      //
       void parser::
-      pre_parse_line (token& t, type& tt, bool if_line)
+      pre_parse_line (token& t, type& tt, optional<line_type> fct)
       {
+        // enter: next token is peeked at (type in tt)
+        // leave: newline
+
+        assert (!fct                      ||
+                *fct == line_type::cmd_if ||
+                *fct == line_type::cmd_while);
+
         // Determine the line type/start token.
         //
         line_type lt (
@@ -235,19 +248,25 @@ namespace build2
         case line_type::cmd_elif:
         case line_type::cmd_elifn:
         case line_type::cmd_else:
-        case line_type::cmd_end:
           {
-            if (!if_line)
-            {
+            if (!fct || *fct != line_type::cmd_if)
               fail (t) << lt << " without preceding 'if'";
-            }
+          }
+          // Fall through.
+        case line_type::cmd_end:
+          {
+            if (!fct)
+              fail (t) << lt << " without preceding 'if', 'for', or 'while'";
           }
           // Fall through.
         case line_type::cmd_if:
         case line_type::cmd_ifn:
+        case line_type::cmd_while:
           next (t, tt); // Skip to start of command.
 
-          if (lt == line_type::cmd_if || lt == line_type::cmd_ifn)
+          if (lt == line_type::cmd_if  ||
+              lt == line_type::cmd_ifn ||
+              lt == line_type::cmd_while)
             ++level_;
           else if (lt == line_type::cmd_end)
             --level_;
@@ -287,6 +306,50 @@ namespace build2
 
           pre_parse_if_else (t, tt);
         }
+        else if (lt == line_type::cmd_while)
+        {
+          tt = peek (lexer_mode::first_token);
+
+          pre_parse_while (t, tt);
+        }
+      }
+
+      // Pre-parse the flow control construct block line.
+      //
+      void parser::
+      pre_parse_block_line (token& t, type& tt, line_type bt)
+      {
+        // enter: peeked first token of the line (type in tt)
+        // leave: newline
+
+        const location ll (get_location (peeked ()));
+
+        if (tt == type::eos)
+          fail (ll) << "expected closing 'end'";
+
+        line_type fct; // Flow control type the block type relates to.
+
+        switch (bt)
+        {
+        case line_type::cmd_if:
+        case line_type::cmd_ifn:
+        case line_type::cmd_elif:
+        case line_type::cmd_elifn:
+        case line_type::cmd_else:
+          {
+            fct = line_type::cmd_if;
+            break;
+          }
+        case line_type::cmd_while:
+          {
+            fct = line_type::cmd_while;
+            break;
+          }
+        default: assert(false);
+        }
+
+        pre_parse_line (t, tt, fct);
+        assert (tt == type::newline);
       }
 
       void parser::
@@ -295,8 +358,7 @@ namespace build2
         // enter: peeked first token of next line (type in tt)
         // leave: newline
 
-        // Parse lines until we see closing 'end'. Nested if-else blocks are
-        // handled recursively.
+        // Parse lines until we see closing 'end'.
         //
         for (line_type bt (line_type::cmd_if); // Current block.
              ;
@@ -304,25 +366,21 @@ namespace build2
         {
           const location ll (get_location (peeked ()));
 
-          if (tt == type::eos)
-            fail (ll) << "expected closing 'end'";
-
           // Parse one line. Note that this one line can still be multiple
-          // lines in case of if-else. In this case we want to view it as
-          // cmd_if, not cmd_end. Thus remember the start position of the
-          // next logical line.
+          // lines in case of a flow control construct. In this case we want
+          // to view it as cmd_if, not cmd_end. Thus remember the start
+          // position of the next logical line.
           //
           size_t i (script_->body.size ());
 
-          pre_parse_line (t, tt, true /* if_line */);
-          assert (tt == type::newline);
+          pre_parse_block_line (t, tt, bt);
 
           line_type lt (script_->body[i].type);
 
           // First take care of 'end'.
           //
           if (lt == line_type::cmd_end)
-            return;
+            break;
 
           // Check if-else block sequencing.
           //
@@ -346,6 +404,25 @@ namespace build2
         }
       }
 
+      void parser::
+      pre_parse_while (token& t, type& tt)
+      {
+        // enter: peeked first token of next line (type in tt)
+        // leave: newline
+
+        // Parse lines until we see closing 'end'.
+        //
+        for (;; tt = peek (lexer_mode::first_token))
+        {
+          size_t i (script_->body.size ());
+
+          pre_parse_block_line (t, tt, line_type::cmd_while);
+
+          if (script_->body[i].type == line_type::cmd_end)
+            break;
+        }
+      }
+
       command_expr parser::
       parse_command_line (token& t, type& tt)
       {
@@ -946,7 +1023,7 @@ namespace build2
         // Note that we rely on "small function object" optimization here.
         //
         auto exec_cmd = [this] (token& t, build2::script::token_type& tt,
-                                size_t li,
+                                const iteration_index* ii, size_t li,
                                 bool single,
                                 const location& ll)
         {
@@ -958,7 +1035,7 @@ namespace build2
           command_expr ce (
             parse_command_line (t, static_cast<token_type&> (tt)));
 
-          runner_->run (*environment_, ce, li, ll);
+          runner_->run (*environment_, ce, ii, li, ll);
         };
 
         exec_lines (s.body, exec_cmd);
@@ -1009,13 +1086,13 @@ namespace build2
 
         auto exec_cmd = [this, &data] (token& t,
                                        build2::script::token_type& tt,
-                                       size_t li,
+                                       const iteration_index* ii, size_t li,
                                        bool /* single */,
                                        const location& ll)
         {
           // Note that we never reset the line index to zero (as we do in
-          // execute_body()) assuming that there are some script body
-          // commands to follow.
+          // execute_body()) assuming that there are some script body commands
+          // to follow.
           //
           if (tt == type::word && t.value == "depdb")
           {
@@ -1128,7 +1205,7 @@ namespace build2
                 info (rt[0].location ()) << "depdb preamble ends here";
             }
 
-            runner_->run (*environment_, ce, li, ll);
+            runner_->run (*environment_, ce, ii, li, ll);
           }
         };
 
@@ -1191,20 +1268,22 @@ namespace build2
           apply_value_attributes (&var, lhs, move (rhs), kind);
         };
 
-        auto exec_if = [this] (token& t, build2::script::token_type& tt,
-                               size_t li,
-                               const location& ll)
+        auto exec_cond = [this] (token& t, build2::script::token_type& tt,
+                                 const iteration_index* ii, size_t li,
+                                 const location& ll)
         {
           command_expr ce (
             parse_command_line (t, static_cast<token_type&> (tt)));
 
-          // Assume if-else always involves multiple commands.
+          // Assume a flow control construct always involves multiple
+          // commands.
           //
-          return runner_->run_if (*environment_, ce, li, ll);
+          return runner_->run_cond (*environment_, ce, ii, li, ll);
         };
 
         build2::script::parser::exec_lines (begin, end,
-                                            exec_set, exec_cmd, exec_if,
+                                            exec_set, exec_cmd, exec_cond,
+                                            nullptr /* iteration_index */,
                                             environment_->exec_line,
                                             &environment_->var_pool);
       }
@@ -2145,10 +2224,12 @@ namespace build2
             istringstream iss;
             if (prog)
             {
+              // Note: depdb is disallowed inside flow control constructs.
+              //
               string s;
               build2::script::run (*environment_,
                                    cmd,
-                                   li,
+                                   nullptr /* iteration_index */, li,
                                    ll,
                                    !file ? &s : nullptr);
 
diff --git a/libbuild2/build/script/parser.hxx b/libbuild2/build/script/parser.hxx
index 932cbad..987ed1f 100644
--- a/libbuild2/build/script/parser.hxx
+++ b/libbuild2/build/script/parser.hxx
@@ -65,11 +65,18 @@ namespace build2
         pre_parse_script ();
 
         void
-        pre_parse_line (token&, token_type&, bool if_line = false);
+        pre_parse_line (token&, token_type&,
+                        optional<line_type> flow_control_type = nullopt);
+
+        void
+        pre_parse_block_line (token&, token_type&, line_type block_type);
 
         void
         pre_parse_if_else (token&, token_type&);
 
+        void
+        pre_parse_while (token&, token_type&);
+
         command_expr
         parse_command_line (token&, token_type&);
 
@@ -367,13 +374,13 @@ namespace build2
         //
         line* save_line_;
 
-        // The if-else nesting level (and in the future for other flow
-        // control constructs).
+        // The flow control constructs nesting level.
         //
-        // Maintained during pre-parsing and is incremented when the cmd_if or
-        // cmd_ifn lines are encountered, which in particular means that it is
-        // already incremented by the time the if-condition expression is
-        // pre-parsed. Decremented when the cmd_end line is encountered.
+        // Maintained during pre-parsing and is incremented when flow control
+        // construct condition lines are encountered, which in particular
+        // means that it is already incremented by the time the condition
+        // expression is pre-parsed. Decremented when the cmd_end line is
+        // encountered.
         //
         size_t level_ = 0;
 
diff --git a/libbuild2/build/script/parser.test.cxx b/libbuild2/build/script/parser.test.cxx
index 5808015..f8c2696 100644
--- a/libbuild2/build/script/parser.test.cxx
+++ b/libbuild2/build/script/parser.test.cxx
@@ -29,7 +29,9 @@ namespace build2
       class print_runner: public runner
       {
       public:
-        print_runner (bool line): line_ (line) {}
+        print_runner (bool line, bool iterations):
+            line_ (line),
+            iterations_ (iterations) {}
 
         virtual void
         enter (environment&, const location&) override {}
@@ -37,27 +39,27 @@ namespace build2
         virtual void
         run (environment&,
              const command_expr& e,
-             size_t i,
+             const iteration_index* ii, size_t i,
              const location&) override
         {
           cout << e;
 
-          if (line_)
-            cout << " # " << i;
+          if (line_ || iterations_)
+            print_line_info (ii, i);
 
           cout << endl;
         }
 
         virtual bool
-        run_if (environment&,
-                const command_expr& e,
-                size_t i,
-                const location&) override
+        run_cond (environment&,
+                  const command_expr& e,
+                  const iteration_index* ii, size_t i,
+                  const location&) override
         {
           cout << "? " << e;
 
-          if (line_)
-            cout << " # " << i;
+          if (line_ || iterations_)
+            print_line_info (ii, i);
 
           cout << endl;
 
@@ -68,12 +70,32 @@ namespace build2
         leave (environment&, const location&) override {}
 
       private:
+        void
+        print_line_info (const iteration_index* ii, size_t i) const
+        {
+          cout << " #";
+
+          if (line_)
+            cout << ' ' << i;
+
+          if (iterations_ && ii != nullptr)
+          {
+            string s;
+            for (const iteration_index* i (ii); i != nullptr; i = i->prev)
+              s.insert (0, " i" + to_string (i->index));
+
+            cout << s;
+          }
+        }
+
+      private:
         bool line_;
+        bool iterations_;
       };
 
       // Usages:
       //
-      // argv[0] [-l]
+      // argv[0] [-l] [-r]
       // argv[0] -b [-t]
       // argv[0] -d [-t]
       // argv[0] -q
@@ -99,6 +121,9 @@ namespace build2
       // -l
       //    Print the script line number for each executed expression.
       //
+      // -r
+      //    Print the loop iteration numbers for each executed expression.
+      //
       // -b
       //    Dump the parsed script body to stdout.
       //
@@ -136,6 +161,7 @@ namespace build2
         } m (mode::run);
 
         bool print_line (false);
+        bool print_iterations (false);
         optional<string> diag_name;
         bool temp_dir (false);
 
@@ -145,6 +171,8 @@ namespace build2
 
           if (a == "-l")
             print_line = true;
+          else if (a == "-r")
+            print_iterations = true;
           else if (a == "-b")
             m = mode::body;
           else if (a == "-d")
@@ -170,8 +198,9 @@ namespace build2
           }
         }
 
-        assert (!print_line || m == mode::run);
-        assert (!diag_name  || m == mode::diag);
+        assert (!print_line       || m == mode::run);
+        assert (!print_iterations || m == mode::run);
+        assert (!diag_name        || m == mode::diag);
 
         // Fake build system driver, default verbosity.
         //
@@ -223,7 +252,7 @@ namespace build2
           case mode::run:
             {
               environment e (perform_update_id, tt, s.body_temp_dir);
-              print_runner r (print_line);
+              print_runner r (print_line, print_iterations);
               p.execute_body (ctx.global_scope, ctx.global_scope, e, s, r);
               break;
             }
diff --git a/libbuild2/build/script/runner.cxx b/libbuild2/build/script/runner.cxx
index 51139d4..157fc60 100644
--- a/libbuild2/build/script/runner.cxx
+++ b/libbuild2/build/script/runner.cxx
@@ -96,7 +96,7 @@ namespace build2
       void default_runner::
       run (environment& env,
            const command_expr& expr,
-           size_t li,
+           const iteration_index* ii, size_t li,
            const location& ll)
       {
         if (verb >= 3)
@@ -115,20 +115,21 @@ namespace build2
                               (p.recall.string () == "set" ||
                                p.recall.string () == "exit");
                      }) != expr.end ())
-          build2::script::run (env, expr, li, ll);
+          build2::script::run (env, expr, ii, li, ll);
         else if (verb >= 2)
           text << expr;
       }
 
       bool default_runner::
-      run_if (environment& env,
-              const command_expr& expr,
-              size_t li, const location& ll)
+      run_cond (environment& env,
+                const command_expr& expr,
+                const iteration_index* ii, size_t li,
+                const location& ll)
       {
         if (verb >= 3)
           text << ": ?" << expr;
 
-        return build2::script::run_if (env, expr, li, ll);
+        return build2::script::run_cond (env, expr, ii, li, ll);
       }
     }
   }
diff --git a/libbuild2/build/script/runner.hxx b/libbuild2/build/script/runner.hxx
index 558de9b..0652396 100644
--- a/libbuild2/build/script/runner.hxx
+++ b/libbuild2/build/script/runner.hxx
@@ -35,14 +35,14 @@ namespace build2
         virtual void
         run (environment&,
              const command_expr&,
-             size_t index,
+             const iteration_index*, size_t index,
              const location&) = 0;
 
         virtual bool
-        run_if (environment&,
-                const command_expr&,
-                size_t,
-                const location&) = 0;
+        run_cond (environment&,
+                  const command_expr&,
+                  const iteration_index*, size_t,
+                  const location&) = 0;
 
         // Location is the script end location (for diagnostics, etc).
         //
@@ -52,9 +52,9 @@ namespace build2
 
       // Run command expressions.
       //
-      // In dry-run mode don't run the expressions unless they are if-
-      // conditions or execute the set or exit builtins, but print them at
-      // verbosity level 2 and up.
+      // In dry-run mode don't run the expressions unless they are flow
+      // control construct conditions or execute the set or exit builtins, but
+      // print them at verbosity level 2 and up.
       //
       class default_runner: public runner
       {
@@ -65,14 +65,14 @@ namespace build2
         virtual void
         run (environment&,
              const command_expr&,
-             size_t,
+             const iteration_index*, size_t,
              const location&) override;
 
         virtual bool
-        run_if (environment&,
-                const command_expr&,
-                size_t,
-                const location&) override;
+        run_cond (environment&,
+                  const command_expr&,
+                  const iteration_index*, size_t,
+                  const location&) override;
 
         virtual void
         leave (environment&, const location&) override;
diff --git a/libbuild2/build/script/script.hxx b/libbuild2/build/script/script.hxx
index d0ab139..f8df204 100644
--- a/libbuild2/build/script/script.hxx
+++ b/libbuild2/build/script/script.hxx
@@ -20,12 +20,13 @@ namespace build2
     namespace script
     {
       using build2::script::line;
-      using build2::script::lines;
       using build2::script::line_type;
+      using build2::script::lines;
       using build2::script::redirect;
       using build2::script::redirect_type;
       using build2::script::expr_term;
       using build2::script::command_expr;
+      using build2::script::iteration_index;
       using build2::script::deadline;
       using build2::script::timeout;
 
diff --git a/libbuild2/script/parser.cxx b/libbuild2/script/parser.cxx
index c199c0e..c371e5b 100644
--- a/libbuild2/script/parser.cxx
+++ b/libbuild2/script/parser.cxx
@@ -2061,6 +2061,7 @@ namespace build2
         else if (n == "elif")  r = line_type::cmd_elif;
         else if (n == "elif!") r = line_type::cmd_elifn;
         else if (n == "else")  r = line_type::cmd_else;
+        else if (n == "while") r = line_type::cmd_while;
         else if (n == "end")   r = line_type::cmd_end;
         else
         {
@@ -2091,8 +2092,8 @@ namespace build2
     exec_lines (lines::const_iterator i, lines::const_iterator e,
                 const function<exec_set_function>& exec_set,
                 const function<exec_cmd_function>& exec_cmd,
-                const function<exec_if_function>& exec_if,
-                size_t& li,
+                const function<exec_cond_function>& exec_cond,
+                const iteration_index* ii, size_t& li,
                 variable_pool* var_pool)
     {
       try
@@ -2116,6 +2117,69 @@ namespace build2
           next (t, tt);
           const location ll (get_location (t));
 
+          // If end is true, then find the flow control construct's end ('end'
+          // line). Otherwise, find the flow control construct's block end
+          // ('end', 'else', etc). If skip is true then increment the command
+          // line index.
+          //
+          auto fcend = [e, &li] (lines::const_iterator j,
+                                 bool end,
+                                 bool skip) -> lines::const_iterator
+          {
+            // We need to be aware of nested flow control constructs.
+            //
+            size_t n (0);
+
+            for (++j; j != e; ++j)
+            {
+              line_type lt (j->type);
+
+              if (lt == line_type::cmd_if  ||
+                  lt == line_type::cmd_ifn ||
+                  lt == line_type::cmd_while)
+                ++n;
+
+              // If we are nested then we just wait until we get back
+              // to the surface.
+              //
+              if (n == 0)
+              {
+                switch (lt)
+                {
+                case line_type::cmd_elif:
+                case line_type::cmd_elifn:
+                case line_type::cmd_else:
+                  if (end) break;
+                  // Fall through.
+                case line_type::cmd_end:  return j;
+                default: break;
+                }
+              }
+
+              if (lt == line_type::cmd_end)
+                --n;
+
+              if (skip)
+              {
+                // Note that we don't count else and end as commands.
+                //
+                switch (lt)
+                {
+                case line_type::cmd:
+                case line_type::cmd_if:
+                case line_type::cmd_ifn:
+                case line_type::cmd_elif:
+                case line_type::cmd_elifn:
+                case line_type::cmd_while: ++li; break;
+                default:                         break;
+                }
+              }
+            }
+
+            assert (false); // Missing end.
+            return e;
+          };
+
           switch (lt)
           {
           case line_type::var:
@@ -2151,7 +2215,7 @@ namespace build2
                   single = true;
               }
 
-              exec_cmd (t, tt, li++, single, ll);
+              exec_cmd (t, tt, ii, li++, single, ll);
 
               replay_stop ();
               break;
@@ -2167,7 +2231,7 @@ namespace build2
               bool take;
               if (lt != line_type::cmd_else)
               {
-                take = exec_if (t, tt, li++, ll);
+                take = exec_cond (t, tt, ii, li++, ll);
 
                 if (lt == line_type::cmd_ifn || lt == line_type::cmd_elifn)
                   take = !take;
@@ -2180,94 +2244,89 @@ namespace build2
 
               replay_stop ();
 
-              // If end is true, then find the 'end' line. Otherwise, find
-              // the next if-else line. If skip is true then increment the
-              // command line index.
-              //
-              auto next = [e, &li] (lines::const_iterator j,
-                                    bool end,
-                                    bool skip) -> lines::const_iterator
-                {
-                  // We need to be aware of nested if-else chains.
-                  //
-                  size_t n (0);
-
-                  for (++j; j != e; ++j)
-                  {
-                    line_type lt (j->type);
-
-                    if (lt == line_type::cmd_if || lt == line_type::cmd_ifn)
-                      ++n;
-
-                    // If we are nested then we just wait until we get back
-                    // to the surface.
-                    //
-                    if (n == 0)
-                    {
-                      switch (lt)
-                      {
-                      case line_type::cmd_elif:
-                      case line_type::cmd_elifn:
-                      case line_type::cmd_else:
-                        if (end) break;
-                        // Fall through.
-                      case line_type::cmd_end:  return j;
-                      default: break;
-                      }
-                    }
-
-                    if (lt == line_type::cmd_end)
-                      --n;
-
-                    if (skip)
-                    {
-                      // Note that we don't count else and end as commands.
-                      //
-                      switch (lt)
-                      {
-                      case line_type::cmd:
-                      case line_type::cmd_if:
-                      case line_type::cmd_ifn:
-                      case line_type::cmd_elif:
-                      case line_type::cmd_elifn: ++li; break;
-                      default:                         break;
-                      }
-                    }
-                  }
-
-                  assert (false); // Missing end.
-                  return e;
-                };
-
               // If we are taking this branch then we need to parse all the
-              // lines until the next if-else line and then skip all the
-              // lines until the end (unless next is already end).
+              // lines until the next if-else line and then skip all the lines
+              // until the end (unless we are already at the end).
               //
               // Otherwise, we need to skip all the lines until the next
               // if-else line and then continue parsing.
               //
               if (take)
               {
-                // Next if-else.
+                // Find block end.
                 //
-                lines::const_iterator j (next (i, false, false));
+                lines::const_iterator j (fcend (i, false, false));
+
                 if (!exec_lines (i + 1, j,
-                                 exec_set, exec_cmd, exec_if,
-                                 li,
+                                 exec_set, exec_cmd, exec_cond,
+                                 ii, li,
                                  var_pool))
                   return false;
 
-                i = j->type == line_type::cmd_end ? j : next (j, true, true);
+                // Find construct end.
+                //
+                i = j->type == line_type::cmd_end ? j : fcend (j, true, true);
               }
               else
               {
-                i = next (i, false, true);
+                // Find block end.
+                //
+                i = fcend (i, false, true);
+
                 if (i->type != line_type::cmd_end)
                   --i; // Continue with this line (e.g., elif or else).
               }
 
               break;
             }
+          case line_type::cmd_while:
+            {
+              size_t wli (li);
+
+              for (iteration_index wi {1, ii};; wi.index++)
+              {
+                next (t, tt); // Skip to start of command.
+
+                bool exec (exec_cond (t, tt, &wi, li++, ll));
+
+                replay_stop ();
+
+                // If the condition evaluates to true, then we need to parse
+                // all the lines until the end line, prepare for the condition
+                // reevaluation, and re-iterate.
+                //
+                // Otherwise, we need to skip all the lines until the end
+                // line, bail out from the loop, and continue parsing.
+                //
+                if (exec)
+                {
+                  // Find construct end.
+                  //
+                  lines::const_iterator j (fcend (i, true, false));
+
+                  if (!exec_lines (i + 1, j,
+                                   exec_set, exec_cmd, exec_cond,
+                                   &wi, li,
+                                   var_pool))
+                    return false;
+
+                  // Prepare for the condition reevaluation.
+                  //
+                  replay_data (replay_tokens (ln.tokens));
+                  next (t, tt);
+                  li = wli;
+                }
+                else
+                {
+                  // Find construct end.
+                  //
+                  i = fcend (i, true, true);
+                  break; // Bail out from the while-loop.
+                }
+              }
+
+              break;
+            }
           case line_type::cmd_end:
             {
               assert (false);
diff --git a/libbuild2/script/parser.hxx b/libbuild2/script/parser.hxx
index d8e5dbf..9edb6ca 100644
--- a/libbuild2/script/parser.hxx
+++ b/libbuild2/script/parser.hxx
@@ -166,13 +166,13 @@ namespace build2
                                       const location&);
 
       using exec_cmd_function = void (token&, token_type&,
-                                      size_t li,
+                                      const iteration_index*, size_t li,
                                       bool single,
                                       const location&);
 
-      using exec_if_function  = bool (token&, token_type&,
-                                      size_t li,
-                                      const location&);
+      using exec_cond_function  = bool (token&, token_type&,
+                                        const iteration_index*, size_t li,
+                                        const location&);
 
       // If a parser implementation doesn't pre-enter variables into a pool
       // during the pre-parsing phase, then they are entered during the
@@ -183,8 +183,8 @@ namespace build2
       exec_lines (lines::const_iterator b, lines::const_iterator e,
                   const function<exec_set_function>&,
                   const function<exec_cmd_function>&,
-                  const function<exec_if_function>&,
-                  size_t& li,
+                  const function<exec_cond_function>&,
+                  const iteration_index*, size_t& li,
                   variable_pool* = nullptr);
 
       // Customization hooks.
diff --git a/libbuild2/script/run.cxx b/libbuild2/script/run.cxx
index 5b45afd..51a1f92 100644
--- a/libbuild2/script/run.cxx
+++ b/libbuild2/script/run.cxx
@@ -809,7 +809,7 @@ namespace build2
         // regex to file for troubleshooting regardless of whether we print
         // the diagnostics or not. We, however, register it for cleanup in the
         // later case (the expression may still succeed, we can be evaluating
-        // the if condition, etc).
+        // the flow control construct condition, etc).
         //
         optional<path> rp;
         if (env.temp_dir_keep)
@@ -1239,7 +1239,8 @@ namespace build2
               command_pipe::const_iterator bc,
               command_pipe::const_iterator ec,
               auto_fd ifd,
-              size_t ci, size_t li, const location& ll,
+              const iteration_index* ii, size_t li, size_t ci,
+              const location& ll,
               bool diag,
               string* output,
               optional<deadline> dl = nullopt,
@@ -1444,19 +1445,28 @@ namespace build2
 
       // Create a unique path for a command standard stream cache file.
       //
-      auto std_path = [&env, &ci, &li, &ll] (const char* n) -> path
+      auto std_path = [&env, ii, &li, &ci, &ll] (const char* nm) -> path
       {
         using std::to_string;
 
-        path p (n);
+        string s (nm);
+        size_t n (s.size ());
+
+        if (ii != nullptr)
+        {
+          // Note: reverse order (outermost to innermost).
+          //
+          for (const iteration_index* i (ii); i != nullptr; i = i->prev)
+            s.insert (n, "-i" + to_string (i->index));
+        }
 
         // 0 if belongs to a single-line script, otherwise is the command line
         // number (start from one) in the script.
         //
-        if (li > 0)
+        if (li != 0)
         {
-          p += '-';
-          p += to_string (li);
+          s += "-n";
+          s += to_string (li);
         }
 
         // 0 if belongs to a single-command expression, otherwise is the
@@ -1466,13 +1476,13 @@ namespace build2
         // single-line script or to N-th single-command line of multi-line
         // script. These cases are mutually exclusive and so are unambiguous.
         //
-        if (ci > 0)
+        if (ci != 0)
         {
-          p += '-';
-          p += to_string (ci);
+          s += "-c";
+          s += to_string (ci);
         }
 
-        return normalize (move (p), temp_dir (env), ll);
+        return normalize (path (move (s)), temp_dir (env), ll);
       };
 
       // If this is the first pipeline command, then open stdin descriptor
@@ -2206,7 +2216,7 @@ namespace build2
           success = run_pipe (env,
                               nc, ec,
                               move (ofd.in),
-                              ci + 1, li, ll, diag,
+                              ii, li, ci + 1, ll, diag,
                               output,
                               dl, dl_cmd,
                               &pc);
@@ -2333,7 +2343,7 @@ namespace build2
           success = run_pipe (env,
                               nc, ec,
                               move (ofd.in),
-                              ci + 1, li, ll, diag,
+                              ii, li, ci + 1, ll, diag,
                               output,
                               dl, dl_cmd,
                               &pc);
@@ -2471,7 +2481,8 @@ namespace build2
     static bool
     run_expr (environment& env,
               const command_expr& expr,
-              size_t li, const location& ll,
+              const iteration_index* ii, size_t li,
+              const location& ll,
               bool diag,
               string* output)
     {
@@ -2517,7 +2528,7 @@ namespace build2
           r = run_pipe (env,
                         p.begin (), p.end (),
                         auto_fd (),
-                        ci, li, ll, print,
+                        ii, li, ci, ll, print,
                         output);
         }
 
@@ -2530,26 +2541,28 @@ namespace build2
     void
     run (environment& env,
          const command_expr& expr,
-         size_t li, const location& ll,
+         const iteration_index* ii, size_t li,
+         const location& ll,
          string* output)
     {
       // Note that we don't print the expression at any verbosity level
       // assuming that the caller does this, potentially providing some
       // additional information (command type, etc).
       //
-      if (!run_expr (env, expr, li, ll, true /* diag */, output))
+      if (!run_expr (env, expr, ii, li, ll, true /* diag */, output))
         throw failed (); // Assume diagnostics is already printed.
     }
 
     bool
-    run_if (environment& env,
-            const command_expr& expr,
-            size_t li, const location& ll,
-            string* output)
+    run_cond (environment& env,
+              const command_expr& expr,
+              const iteration_index* ii, size_t li,
+              const location& ll,
+              string* output)
     {
       // Note that we don't print the expression here (see above).
       //
-      return run_expr (env, expr, li, ll, false /* diag */, output);
+      return run_expr (env, expr, ii, li, ll, false /* diag */, output);
     }
 
     void
diff --git a/libbuild2/script/run.hxx b/libbuild2/script/run.hxx
index 8bc246c..01b010c 100644
--- a/libbuild2/script/run.hxx
+++ b/libbuild2/script/run.hxx
@@ -44,16 +44,16 @@ namespace build2
     void
     run (environment&,
          const command_expr&,
-         size_t index,
+         const iteration_index*, size_t index,
          const location&,
          string* output = nullptr);
 
     bool
-    run_if (environment&,
-            const command_expr&,
-            size_t index,
-            const location&,
-            string* output = nullptr);
+    run_cond (environment&,
+              const command_expr&,
+              const iteration_index*, size_t index,
+              const location&,
+              string* output = nullptr);
 
     // Perform the registered special file cleanups in the direct order and
     // then the regular cleanups in the reverse order.
diff --git a/libbuild2/script/script.cxx b/libbuild2/script/script.cxx
index 9e6eeed..d4096cf 100644
--- a/libbuild2/script/script.cxx
+++ b/libbuild2/script/script.cxx
@@ -27,6 +27,7 @@ namespace build2
       case line_type::cmd_elif:  s = "'elif'";   break;
       case line_type::cmd_elifn: s = "'elif!'";  break;
       case line_type::cmd_else:  s = "'else'";   break;
+      case line_type::cmd_while: s = "'while'";  break;
       case line_type::cmd_end:   s = "'end'";    break;
       }
 
@@ -186,14 +187,14 @@ namespace build2
     void
     dump (ostream& os, const string& ind, const lines& ls)
     {
-      // Additionally indent the if-branch lines.
+      // Additionally indent the flow control construct block lines.
       //
-      string if_ind;
+      string fc_ind;
 
       for (const line& l: ls)
       {
-        // Before printing indentation, decrease it if the else or end line is
-        // reached.
+        // Before printing indentation, decrease it if the else, end, etc line
+        // is reached.
         //
         switch (l.type)
         {
@@ -202,9 +203,9 @@ namespace build2
         case line_type::cmd_else:
         case line_type::cmd_end:
           {
-            size_t n (if_ind.size ());
+            size_t n (fc_ind.size ());
             assert (n >= 2);
-            if_ind.resize (n - 2);
+            fc_ind.resize (n - 2);
             break;
           }
         default: break;
@@ -212,9 +213,10 @@ namespace build2
 
         // Print indentations.
         //
-        os << ind << if_ind;
+        os << ind << fc_ind;
 
-        // After printing indentation, increase it for if/else branch.
+        // After printing indentation, increase it for the flow control
+        // construct block lines.
         //
         switch (l.type)
         {
@@ -222,7 +224,8 @@ namespace build2
         case line_type::cmd_ifn:
         case line_type::cmd_elif:
         case line_type::cmd_elifn:
-        case line_type::cmd_else:  if_ind += "  "; break;
+        case line_type::cmd_else:
+        case line_type::cmd_while: fc_ind += "  "; break;
         default: break;
         }
 
diff --git a/libbuild2/script/script.hxx b/libbuild2/script/script.hxx
index 5a39659..d6018f0 100644
--- a/libbuild2/script/script.hxx
+++ b/libbuild2/script/script.hxx
@@ -27,6 +27,7 @@ namespace build2
       cmd_elif,
       cmd_elifn,
       cmd_else,
+      cmd_while,
       cmd_end
     };
 
@@ -380,6 +381,15 @@ namespace build2
     ostream&
     operator<< (ostream&, const command_expr&);
 
+    // Stack-allocated linked list of iteration indexes of the nested loops.
+    //
+    struct iteration_index
+    {
+      size_t index; // 1-based.
+
+      const iteration_index* prev; // NULL for the top-most loop.
+    };
+
     struct timeout
     {
       duration value;
diff --git a/libbuild2/test/script/parser+command-if.test.testscript b/libbuild2/test/script/parser+command-if.test.testscript
index 0b72b4a..9e223dd 100644
--- a/libbuild2/test/script/parser+command-if.test.testscript
+++ b/libbuild2/test/script/parser+command-if.test.testscript
@@ -315,6 +315,7 @@
 }
 
 : end
+:
 {
   : without-if
   :
@@ -322,7 +323,7 @@
   cmd
   end
   EOI
-  testscript:2:1: error: 'end' without preceding 'if'
+  testscript:2:1: error: 'end' without preceding 'if', 'for', or 'while'
   EOE
 
   : without-if-semi
@@ -331,10 +332,11 @@
   cmd;
   end
   EOI
-  testscript:2:1: error: 'end' without preceding 'if'
+  testscript:2:1: error: 'end' without preceding 'if', 'for', or 'while'
   EOE
 
   : before
+  :
   {
     : semi
     :
diff --git a/libbuild2/test/script/parser+description.test.testscript b/libbuild2/test/script/parser+description.test.testscript
index cee540f..f656b7d 100644
--- a/libbuild2/test/script/parser+description.test.testscript
+++ b/libbuild2/test/script/parser+description.test.testscript
@@ -313,7 +313,7 @@
     x = y
   end
   EOI
-  testscript:2:1: error: description before/after setup/teardown variable-if
+  testscript:2:1: error: description before/after setup/teardown variable-only 'if'
   EOE
 
   : var-if-after
@@ -323,7 +323,7 @@
     x = y
   end : foo
   EOI
-  testscript:1:1: error: description before/after setup/teardown variable-if
+  testscript:1:1: error: description before/after setup/teardown variable-only 'if'
   EOE
 
   : test
diff --git a/libbuild2/test/script/parser+while.test.testscript b/libbuild2/test/script/parser+while.test.testscript
new file mode 100644
index 0000000..b1a2b44
--- /dev/null
+++ b/libbuild2/test/script/parser+while.test.testscript
@@ -0,0 +1,265 @@
+# file      : libbuild2/test/script/parser+while.test.testscript
+# license   : MIT; see accompanying LICENSE file
+
+: while
+:
+{
+  : true
+  :
+  $* <<EOI >>EOO
+  while ($v != "aa")
+    cmd "$v"
+    v = "$(v)a"
+  end
+  EOI
+  ? true
+  cmd ''
+  ? true
+  cmd a
+  ? false
+  EOO
+
+  : false
+  :
+  $* <<EOI >>EOO
+  while ($v == "aa")
+    cmd "$v"
+    v = "$(v)a"
+  end
+  EOI
+  ? false
+  EOO
+
+  : without-command
+  :
+  $* <<EOI 2>>EOE != 0
+  while
+    cmd
+  end
+  EOI
+  testscript:1:6: error: missing program
+  EOE
+
+  : after-semi
+  :
+  $* -s <<EOI >>EOO
+  cmd1;
+  while ($v != "aa")
+    cmd2 "$v"
+    v = "$(v)a"
+  end
+  EOI
+  {
+    {
+      cmd1
+      ? true
+      cmd2 ''
+      ? true
+      cmd2 a
+      ? false
+    }
+  }
+  EOO
+
+  : setup
+  :
+  $* -s <<EOI >>EOO
+  +while ($v != "aa")
+    cmd2 "$v"
+    v = "$(v)a"
+  end
+  EOI
+  {
+    ? true
+    +cmd2 ''
+    ? true
+    +cmd2 a
+    ? false
+  }
+  EOO
+
+  : tdown
+  :
+  $* -s <<EOI >>EOO
+  -while ($v != "aa")
+    cmd2 "$v"
+    v = "$(v)a"
+  end
+  EOI
+  {
+    ? true
+    -cmd2 ''
+    ? true
+    -cmd2 a
+    ? false
+  }
+  EOO
+}
+
+: end
+:
+{
+  : without-end
+  :
+  $* <<EOI 2>>EOE != 0
+  while true
+    cmd
+  EOI
+  testscript:3:1: error: expected closing 'end'
+  EOE
+}
+
+: elif
+:
+{
+  : without-if
+  :
+  $* <<EOI 2>>EOE != 0
+  while false
+    elif true
+      cmd
+    end
+  end
+  EOI
+  testscript:2:3: error: 'elif' without preceding 'if'
+  EOE
+}
+
+: nested
+:
+{
+  $* -l -r <<EOI >>EOO
+  while ($v != "aa")      # 1
+    cmd1 "$v"             # 2
+    if ($v == "a")        # 3
+      cmd2                # 4
+      while ($v2 != "$v") # 5
+        cmd3              # 6
+        v2=$v
+      end
+    else
+      cmd4                # 7
+    end
+    cmd5                  # 8
+    v = "$(v)a"
+  end;
+  cmd6
+  EOI
+  ? true # 1 i1
+  cmd1 '' # 2 i1
+  ? false # 3 i1
+  cmd4 # 7 i1
+  cmd5 # 8 i1
+  ? true # 1 i2
+  cmd1 a # 2 i2
+  ? true # 3 i2
+  cmd2 # 4 i2
+  ? true # 5 i2 i1
+  cmd3 # 6 i2 i1
+  ? false # 5 i2 i2
+  cmd5 # 8 i2
+  ? false # 1 i3
+  cmd6 # 9
+  EOO
+}
+
+: contained
+:
+{
+  : semi
+  :
+  $* <<EOI 2>>EOE != 0
+  while
+    cmd;
+    cmd
+  end
+  EOI
+  testscript:2:3: error: ';' inside 'while'
+  EOE
+
+  : colon-leading
+  :
+  $* <<EOI 2>>EOE != 0
+  while
+    : foo
+    cmd
+  end
+  EOI
+  testscript:2:3: error: description inside 'while'
+  EOE
+
+  : colon-trailing
+  :
+  $* <<EOI 2>>EOE != 0
+  while
+    cmd : foo
+  end
+  EOI
+  testscript:2:3: error: description inside 'while'
+  EOE
+
+  : eos
+  :
+  $* <<EOI 2>>EOE != 0
+  while
+  EOI
+  testscript:2:1: error: expected closing 'end'
+  EOE
+
+  : scope
+  :
+  $* <<EOI 2>>EOE != 0
+  while
+    cmd
+    {
+    }
+  end
+  EOI
+  testscript:3:3: error: expected closing 'end'
+  EOE
+
+  : setup
+  :
+  $* <<EOI 2>>EOE != 0
+  while
+    +cmd
+  end
+  EOI
+  testscript:2:3: error: setup command inside 'while'
+  EOE
+
+  : tdown
+  :
+  $* <<EOI 2>>EOE != 0
+  while
+    -cmd
+  end
+  EOI
+  testscript:2:3: error: teardown command inside 'while'
+  EOE
+}
+
+: var
+:
+$* <<EOI >>EOO
+while ($v1 != "a")
+  v1 = "$(v1)a"
+  v2 = "$v1"
+end
+cmd $v1
+EOI
+? true
+? false
+cmd a
+EOO
+
+: leading-and-trailing-description
+:
+$* <<EOI 2>>EOE != 0
+: foo
+while false
+  cmd
+end : bar
+EOI
+testscript:4:1: error: both leading and trailing descriptions
+EOE
diff --git a/libbuild2/test/script/parser.cxx b/libbuild2/test/script/parser.cxx
index eb7b140..a99f80a 100644
--- a/libbuild2/test/script/parser.cxx
+++ b/libbuild2/test/script/parser.cxx
@@ -293,22 +293,28 @@ namespace build2
       }
 
       // Parse a logical line (as well as scope-if since the only way to
-      // recognize it is to parse the if line).
+      // recognize it is to parse the if line), handling the flow control
+      // constructs recursively.
       //
       // If one is true then only parse one line returning an indication of
-      // whether the line ended with a semicolon. If if_line is true then this
-      // line can be an if-else construct flow control line (else, end, etc).
+      // whether the line ended with a semicolon. If the flow control
+      // construct type is specified, then this line is assumed to belong to
+      // such construct.
       //
       bool parser::
       pre_parse_line (token& t, type& tt,
                       optional<description>& d,
                       lines* ls,
                       bool one,
-                      bool if_line)
+                      optional<line_type> fct)
       {
         // enter: next token is peeked at (type in tt)
         // leave: newline
 
+        assert (!fct                      ||
+                *fct == line_type::cmd_if ||
+                *fct == line_type::cmd_while);
+
         // Note: token is only peeked at.
         //
         const location ll (get_location (peeked ()));
@@ -364,8 +370,9 @@ namespace build2
             {
               const string& n (t.value);
 
-              if      (n == "if")  lt = line_type::cmd_if;
-              else if (n == "if!") lt = line_type::cmd_ifn;
+              if      (n == "if")    lt = line_type::cmd_if;
+              else if (n == "if!")   lt = line_type::cmd_ifn;
+              else if (n == "while") lt = line_type::cmd_while;
             }
 
             break;
@@ -420,16 +427,20 @@ namespace build2
         case line_type::cmd_elif:
         case line_type::cmd_elifn:
         case line_type::cmd_else:
-        case line_type::cmd_end:
           {
-            if (!if_line)
-            {
+            if (!fct || *fct != line_type::cmd_if)
               fail (t) << lt << " without preceding 'if'";
-            }
+          }
+          // Fall through.
+        case line_type::cmd_end:
+          {
+            if (!fct)
+              fail (t) << lt << " without preceding 'if', 'for', or 'while'";
           }
           // Fall through.
         case line_type::cmd_if:
         case line_type::cmd_ifn:
+        case line_type::cmd_while:
           next (t, tt); // Skip to start of command.
           // Fall through.
         case line_type::cmd:
@@ -440,8 +451,9 @@ namespace build2
               p = parse_command_expr (t, tt, lexer::redirect_aliases);
 
             // Colon and semicolon are only valid in test command lines and
-            // after 'end' in if-else. Note that we still recognize them
-            // lexically, they are just not valid tokens per the grammar.
+            // after 'end' in a flow control construct. Note that we still
+            // recognize them lexically, they are just not valid tokens per
+            // the grammar.
             //
             if (tt != type::newline)
             {
@@ -504,14 +516,17 @@ namespace build2
           if (ls->empty ())
             return semi;
         }
+        else if (lt == line_type::cmd_while)
+          semi = pre_parse_while (t, tt, d, *ls);
 
         // Unless we were told where to put it, decide where it actually goes.
         //
         if (ls == &ls_data)
         {
-          // First pre-check variable and variable-if: by themselves (i.e.,
-          // without a trailing semicolon) they are treated as either setup or
-          // teardown without plus/minus. Also handle illegal line types.
+          // First pre-check variables and variable-only flow control
+          // constructs: by themselves (i.e., without a trailing semicolon)
+          // they are treated as either setup or teardown without
+          // plus/minus. Also handle illegal line types.
           //
           switch (lt)
           {
@@ -524,8 +539,9 @@ namespace build2
             }
           case line_type::cmd_if:
           case line_type::cmd_ifn:
+          case line_type::cmd_while:
             {
-              // See if this is a variable-only command-if.
+              // See if this is a variable-only flow control construct.
               //
               if (find_if (ls_data.begin (), ls_data.end (),
                            [] (const line& l) {
@@ -549,7 +565,7 @@ namespace build2
                     fail (ll) << "description before setup/teardown variable";
                   else
                     fail (ll) << "description before/after setup/teardown "
-                              << "variable-if";
+                              << "variable-only " << lt;
                 }
 
                 // If we don't have any nested scopes or teardown commands,
@@ -793,7 +809,7 @@ namespace build2
                                      td,
                                      &ls,
                                      true /* one */,
-                                     true /* if_line */));
+                                     line_type::cmd_if));
 
           assert (ls.size () == 1 && ls.back ().type == lt);
           assert (tt == type::newline);
@@ -831,6 +847,97 @@ namespace build2
         return false; // We never end with a semi.
       }
 
+      // Pre-parse the flow control construct block line. Fail if the line is
+      // unexpectedly followed with a semicolon or test description.
+      //
+      bool parser::
+      pre_parse_block_line (token& t, type& tt,
+                            line_type bt,
+                            optional<description>& d,
+                            lines& ls)
+      {
+        // enter: peeked first token of the line (type in tt)
+        // leave: newline
+
+        const location ll (get_location (peeked ()));
+
+        switch (tt)
+        {
+        case type::colon:
+          fail (ll) << "description inside " << bt << endf;
+        case type::eos:
+        case type::rcbrace:
+        case type::lcbrace:
+          fail (ll) << "expected closing 'end'" << endf;
+        case type::plus:
+          fail (ll) << "setup command inside " << bt << endf;
+        case type::minus:
+          fail (ll) << "teardown command inside " << bt << endf;
+        }
+
+        // Parse one line. Note that this one line can still be multiple lines
+        // in case of a flow control construct. In this case we want to view
+        // it as, for example, cmd_if, not cmd_end. Thus remember the start
+        // position of the next logical line.
+        //
+        size_t i (ls.size ());
+
+        line_type fct; // Flow control type the block type relates to.
+
+        switch (bt)
+        {
+        case line_type::cmd_if:
+        case line_type::cmd_ifn:
+        case line_type::cmd_elif:
+        case line_type::cmd_elifn:
+        case line_type::cmd_else:
+          {
+            fct = line_type::cmd_if;
+            break;
+          }
+        case line_type::cmd_while:
+          {
+            fct = line_type::cmd_while;
+            break;
+          }
+        default: assert(false);
+        }
+
+        optional<description> td;
+        bool semi (pre_parse_line (t, tt, td, &ls, true /* one */, fct));
+
+        assert (tt == type::newline);
+
+        line_type lt (ls[i].type);
+
+        // First take care of 'end'.
+        //
+        if (lt == line_type::cmd_end)
+        {
+          if (td)
+          {
+            if (d)
+              fail (ll) << "both leading and trailing descriptions";
+
+            d = move (td);
+          }
+
+          return semi;
+        }
+
+        // For any other line trailing semi or description is illegal.
+        //
+        // @@ Not the exact location of semi/colon.
+        //
+        if (semi)
+          fail (ll) << "';' inside " << bt;
+
+        if (td)
+          fail (ll) << "description inside " << bt;
+
+        return false;
+      }
+
       bool parser::
       pre_parse_if_else_command (token& t, type& tt,
                                  optional<description>& d,
@@ -839,70 +946,23 @@ namespace build2
         // enter: peeked first token of next line (type in tt)
         // leave: newline
 
-        // Parse lines until we see closing 'end'. Nested if-else blocks are
-        // handled recursively.
+        // Parse lines until we see closing 'end'.
         //
         for (line_type bt (line_type::cmd_if); // Current block.
              ;
              tt = peek (lexer_mode::first_token))
         {
           const location ll (get_location (peeked ()));
-
-          switch (tt)
-          {
-          case type::colon:
-            fail (ll) << "description inside " << bt << endf;
-          case type::eos:
-          case type::rcbrace:
-          case type::lcbrace:
-            fail (ll) << "expected closing 'end'" << endf;
-          case type::plus:
-            fail (ll) << "setup command inside " << bt << endf;
-          case type::minus:
-            fail (ll) << "teardown command inside " << bt << endf;
-          }
-
-          // Parse one line. Note that this one line can still be multiple
-          // lines in case of if-else. In this case we want to view it as
-          // cmd_if, not cmd_end. Thus remember the start position of the
-          // next logical line.
-          //
           size_t i (ls.size ());
 
-          optional<description> td;
-          bool semi (pre_parse_line (t, tt,
-                                     td,
-                                     &ls,
-                                     true /* one */,
-                                     true /* if_line */));
-          assert (tt == type::newline);
+          bool semi (pre_parse_block_line (t, tt, bt, d, ls));
 
           line_type lt (ls[i].type);
 
           // First take care of 'end'.
           //
           if (lt == line_type::cmd_end)
-          {
-            if (td)
-            {
-              if (d)
-                fail (ll) << "both leading and trailing descriptions";
-
-              d = move (td);
-            }
-
             return semi;
-          }
-
-          // For any other line trailing semi or description is illegal.
-          //
-          // @@ Not the exact location of semi/colon.
-          //
-          if (semi)
-            fail (ll) << "';' inside " << bt;
-
-          if (td)
-            fail (ll) << "description inside " << bt;
 
           // Check if-else block sequencing.
           //
@@ -924,6 +984,36 @@ namespace build2
           default: break;
           }
         }
+
+        assert (false); // Can't be here.
+        return false;
+      }
+
+      bool parser::
+      pre_parse_while (token& t, type& tt,
+                       optional<description>& d,
+                       lines& ls)
+      {
+        // enter: <newline> (previous line)
+        // leave: <newline>
+
+        tt = peek (lexer_mode::first_token);
+
+        // Parse lines until we see closing 'end'.
+        //
+        for (;; tt = peek (lexer_mode::first_token))
+        {
+          size_t i (ls.size ());
+
+          bool semi (
+            pre_parse_block_line (t, tt, line_type::cmd_while, d, ls));
+
+          if (ls[i].type == line_type::cmd_end)
+            return semi;
+        }
+
+        assert (false); // Can't be here.
+        return false;
       }
 
       void parser::
@@ -1424,7 +1514,7 @@ namespace build2
         command_type ct;
 
         auto exec_cmd = [&ct, this] (token& t, build2::script::token_type& tt,
-                                     size_t li,
+                                     const iteration_index* ii, size_t li,
                                      bool single,
                                      const location& ll)
         {
@@ -1437,19 +1527,20 @@ namespace build2
           command_expr ce (
             parse_command_line (t, static_cast<token_type&> (tt)));
 
-          runner_->run (*scope_, ce, ct, li, ll);
+          runner_->run (*scope_, ce, ct, ii, li, ll);
         };
 
-        auto exec_if = [this] (token& t, build2::script::token_type& tt,
-                               size_t li,
-                               const location& ll)
+        auto exec_cond = [this] (token& t, build2::script::token_type& tt,
+                                 const iteration_index* ii, size_t li,
+                                 const location& ll)
         {
           command_expr ce (
             parse_command_line (t, static_cast<token_type&> (tt)));
 
-          // Assume if-else always involves multiple commands.
+          // Assume a flow control construct always involves multiple
+          // commands.
           //
-          return runner_->run_if (*scope_, ce, li, ll);
+          return runner_->run_cond (*scope_, ce, ii, li, ll);
         };
 
         size_t li (1);
@@ -1459,16 +1550,16 @@ namespace build2
           ct = command_type::test;
 
           exec_lines (t->tests_.begin (), t->tests_.end (),
-                      exec_set, exec_cmd, exec_if,
-                      li);
+                      exec_set, exec_cmd, exec_cond,
+                      nullptr /* iteration_index */, li);
         }
         else if (group* g = dynamic_cast<group*> (scope_))
         {
           ct = command_type::setup;
 
           bool exec_scope (exec_lines (g->setup_.begin (), g->setup_.end (),
-                                       exec_set, exec_cmd, exec_if,
-                                       li));
+                                       exec_set, exec_cmd, exec_cond,
+                                       nullptr /* iteration_index */, li));
 
           if (exec_scope)
           {
@@ -1526,7 +1617,8 @@ namespace build2
 
                   try
                   {
-                    take = runner_->run_if (*scope_, ce, li++, ll);
+                    take = runner_->run_cond (
+                      *scope_, ce, nullptr /* iteration_index */, li++, ll);
                   }
                   catch (const exit_scope& e)
                   {
@@ -1637,8 +1729,8 @@ namespace build2
           ct = command_type::teardown;
 
           exec_lines (g->tdown_.begin (), g->tdown_.end (),
-                      exec_set, exec_cmd, exec_if,
-                      li);
+                      exec_set, exec_cmd, exec_cond,
+                      nullptr /* iteration_index */, li);
         }
         else
           assert (false);
diff --git a/libbuild2/test/script/parser.hxx b/libbuild2/test/script/parser.hxx
index 0d15580..31dd41d 100644
--- a/libbuild2/test/script/parser.hxx
+++ b/libbuild2/test/script/parser.hxx
@@ -62,7 +62,13 @@ namespace build2
                         optional<description>&,
                         lines* = nullptr,
                         bool one = false,
-                        bool if_line = false);
+                        optional<line_type> flow_control_type = nullopt);
+
+        bool
+        pre_parse_block_line (token&, token_type&,
+                              line_type block_type,
+                              optional<description>&,
+                              lines&);
 
         bool
         pre_parse_if_else (token&, token_type&,
@@ -79,6 +85,11 @@ namespace build2
                                    optional<description>&,
                                    lines&);
 
+        bool
+        pre_parse_while (token&, token_type&,
+                         optional<description>&,
+                         lines&);
+
         void
         pre_parse_directive (token&, token_type&);
 
diff --git a/libbuild2/test/script/parser.test.cxx b/libbuild2/test/script/parser.test.cxx
index ccd4104..ab0aee9 100644
--- a/libbuild2/test/script/parser.test.cxx
+++ b/libbuild2/test/script/parser.test.cxx
@@ -33,8 +33,11 @@ namespace build2
       class print_runner: public runner
       {
       public:
-        print_runner (bool scope, bool id, bool line)
-            : scope_ (scope), id_ (id), line_ (line) {}
+        print_runner (bool scope, bool id, bool line, bool iterations)
+            : scope_ (scope),
+              id_ (id),
+              line_ (line),
+              iterations_ (iterations) {}
 
         virtual bool
         test (scope&) const override
@@ -99,7 +102,7 @@ namespace build2
         virtual void
         run (scope&,
              const command_expr& e, command_type t,
-             size_t i,
+             const iteration_index* ii, size_t i,
              const location&) override
         {
           const char* s (nullptr);
@@ -113,22 +116,22 @@ namespace build2
 
           cout << ind_ << s << e;
 
-          if (line_)
-            cout << " # " << i;
+          if (line_ || iterations_)
+            print_line_info (ii, i);
 
           cout << endl;
         }
 
         virtual bool
-        run_if (scope&,
-                const command_expr& e,
-                size_t i,
-                const location&) override
+        run_cond (scope&,
+                  const command_expr& e,
+                  const iteration_index* ii, size_t i,
+                  const location&) override
         {
           cout << ind_ << "? " << e;
 
-          if (line_)
-            cout << " # " << i;
+          if (line_ || iterations_)
+            print_line_info (ii, i);
 
           cout << endl;
 
@@ -146,13 +149,33 @@ namespace build2
         }
 
       private:
+        void
+        print_line_info (const iteration_index* ii, size_t i) const
+        {
+          cout << " #";
+
+          if (line_)
+            cout << ' ' << i;
+
+          if (iterations_ && ii != nullptr)
+          {
+            string s;
+            for (const iteration_index* i (ii); i != nullptr; i = i->prev)
+              s.insert (0, " i" + to_string (i->index));
+
+            cout << s;
+          }
+        }
+
+      private:
         bool scope_;
         bool id_;
         bool line_;
+        bool iterations_;
         string ind_;
       };
 
-      // Usage: argv[0] [-s] [-i] [-l] [<testscript-name>]
+      // Usage: argv[0] [-s] [-i] [-l] [-r] [<testscript-name>]
       //
       int
       main (int argc, char* argv[])
@@ -174,6 +197,7 @@ namespace build2
         bool scope (false);
         bool id (false);
         bool line (false);
+        bool iterations (false);
         path name;
 
         for (int i (1); i != argc; ++i)
@@ -186,6 +210,8 @@ namespace build2
             id = true;
           else if (a == "-l")
             line = true;
+          else if (a == "-r")
+            iterations = true;
           else
           {
             name = path (move (a));
@@ -236,7 +262,7 @@ namespace build2
           script s (tt, st, dir_path (work) /= "test-driver");
           p.pre_parse (cin, s);
 
-          print_runner r (scope, id, line);
+          print_runner r (scope, id, line, iterations);
           p.execute (s, r);
         }
         catch (const failed&)
diff --git a/libbuild2/test/script/runner.cxx b/libbuild2/test/script/runner.cxx
index 8054c61..42eef04 100644
--- a/libbuild2/test/script/runner.cxx
+++ b/libbuild2/test/script/runner.cxx
@@ -142,7 +142,8 @@ namespace build2
       void default_runner::
       run (scope& sp,
            const command_expr& expr, command_type ct,
-           size_t li, const location& ll)
+           const iteration_index* ii, size_t li,
+           const location& ll)
       {
         // Noop for teardown commands if keeping tests output is requested.
         //
@@ -175,13 +176,14 @@ namespace build2
             dr << info << "test id: " << sp.id_path.posix_string ();
           });
 
-        build2::script::run (sp, expr, li, ll);
+        build2::script::run (sp, expr, ii, li, ll);
       }
 
       bool default_runner::
-      run_if (scope& sp,
-              const command_expr& expr,
-              size_t li, const location& ll)
+      run_cond (scope& sp,
+                const command_expr& expr,
+                const iteration_index* ii, size_t li,
+                const location& ll)
       {
         if (verb >= 3)
           text << ": ?" << expr;
@@ -197,7 +199,7 @@ namespace build2
             dr << info << "test id: " << sp.id_path.posix_string ();
           });
 
-        return build2::script::run_if (sp, expr, li, ll);
+        return build2::script::run_cond (sp, expr, ii, li, ll);
       }
     }
   }
diff --git a/libbuild2/test/script/runner.hxx b/libbuild2/test/script/runner.hxx
index b6a038d..0309a35 100644
--- a/libbuild2/test/script/runner.hxx
+++ b/libbuild2/test/script/runner.hxx
@@ -51,11 +51,14 @@ namespace build2
         virtual void
         run (scope&,
              const command_expr&, command_type,
-             size_t index,
+             const iteration_index*, size_t index,
              const location&) = 0;
 
         virtual bool
-        run_if (scope&, const command_expr&, size_t, const location&) = 0;
+        run_cond (scope&,
+                  const command_expr&,
+                  const iteration_index*, size_t,
+                  const location&) = 0;
 
         // Location is the scope end location (for diagnostics, etc).
         //
@@ -84,11 +87,14 @@ namespace build2
         virtual void
         run (scope&,
              const command_expr&, command_type,
-             size_t,
+             const iteration_index*, size_t,
              const location&) override;
 
         virtual bool
-        run_if (scope&, const command_expr&, size_t, const location&) override;
+        run_cond (scope&,
+                  const command_expr&,
+                  const iteration_index*, size_t,
+                  const location&) override;
 
         virtual void
         leave (scope&, const location&) override;
diff --git a/libbuild2/test/script/script.hxx b/libbuild2/test/script/script.hxx
index 22f6725..b75f68e 100644
--- a/libbuild2/test/script/script.hxx
+++ b/libbuild2/test/script/script.hxx
@@ -21,13 +21,14 @@ namespace build2
     namespace script
     {
       using build2::script::line;
+      using build2::script::line_type;
       using build2::script::lines;
       using build2::script::redirect;
       using build2::script::redirect_type;
-      using build2::script::line_type;
-      using build2::script::command_expr;
-      using build2::script::expr_term;
       using build2::script::command;
+      using build2::script::expr_term;
+      using build2::script::command_expr;
+      using build2::script::iteration_index;
       using build2::script::environment_vars;
       using build2::script::deadline;
       using build2::script::timeout;
diff --git a/tests/recipe/buildscript/testscript b/tests/recipe/buildscript/testscript
index 0bf752e..54c3bbe 100644
--- a/tests/recipe/buildscript/testscript
+++ b/tests/recipe/buildscript/testscript
@@ -487,7 +487,7 @@ posix = ($cxx.target.class != 'windows')
       : normal
       :
       {
-	cat <<EOI >=bar.h;
+        cat <<EOI >=bar.h;
           bar
           EOI
 
@@ -519,23 +519,23 @@ posix = ($cxx.target.class != 'windows')
           }}
           EOI
 
-	$* 2>>EOE;
+        $* 2>>EOE;
           gen h{baz.h}
           gen h{foo.h}
           EOE
 
-	cat foo.h >>EOO;
-	  bar
-	  baz
-	  EOO
+        cat foo.h >>EOO;
+          bar
+          baz
+          EOO
 
-	$* clean 2>-
+        $* clean 2>-
       }
 
       : byproduct
       :
       {
-	cat <<EOI >=bar.h;
+        cat <<EOI >=bar.h;
           bar
           EOI
 
@@ -562,17 +562,17 @@ posix = ($cxx.target.class != 'windows')
           }}
           EOI
 
-	$* 2>>EOE;
+        $* 2>>EOE;
           gen h{baz.h}
           gen h{foo.h}
           EOE
 
-	cat foo.h >>EOO;
-	  bar
-	  baz
-	  EOO
+        cat foo.h >>EOO;
+          bar
+          baz
+          EOO
 
-	$* clean 2>-
+        $* clean 2>-
       }
     }
   }
@@ -885,3 +885,29 @@ if $posix
     alias{bar} alias{far}
     EOE
 }
+
+: flow-control-construct
+:
+{
+  : while
+  :
+  {
+    echo 'bar' >=bar;
+
+    cat <<EOI >=buildfile;
+      foo: bar
+      {{
+        p = $path($>)
+        while test -f $p != 0
+          cp $path($<) $p
+        end
+      }}
+      EOI
+
+    $* 2>'cp file{foo}';
+
+    cat <<<foo >'bar';
+
+    $* clean 2>-
+  }
+}
diff --git a/tests/test/script/runner/pipe.testscript b/tests/test/script/runner/pipe.testscript
index 205fd55..92ab33e 100644
--- a/tests/test/script/runner/pipe.testscript
+++ b/tests/test/script/runner/pipe.testscript
@@ -17,7 +17,7 @@ $c <'$* -o foo | cat >foo' && $b     : process-to-builtin
   :
   $c <'$* -o foo -s 1 | $* -i 1 >foo -s 2' && $b 2>>/~%EOE% != 0
   %testscript:1:1: error: .+ exited with code 2%
-    info: stdout: test/1/stdout-2
+    info: stdout: test/1/stdout-c2
     info: test id: 1
   EOE
 
@@ -25,9 +25,9 @@ $c <'$* -o foo | cat >foo' && $b     : process-to-builtin
   :
   $c <'$* -o foo -e foo 2>bar | $* -i 2 2>baz' && $b 2>>/~%EOE% != 0
   %testscript:1:1: error: .+ stderr doesn't match expected%
-    info: stderr: test/1/stderr-2
-    info: expected stderr: test/1/stderr-2.orig
-    info: stderr diff: test/1/stderr-2.diff
+    info: stderr: test/1/stderr-c2
+    info: expected stderr: test/1/stderr-c2.orig
+    info: stderr diff: test/1/stderr-c2.diff
   %.{3}
   -baz
   +foo
diff --git a/tests/test/script/runner/redirect.testscript b/tests/test/script/runner/redirect.testscript
index 0fe3aa3..209c4ce 100644
--- a/tests/test/script/runner/redirect.testscript
+++ b/tests/test/script/runner/redirect.testscript
@@ -654,9 +654,9 @@ psr = ($cxx.target.class != 'windows' ? '/' : '\\') # Path separator in regex.
     $* -o bar >?out
     EOI
     %testscript:2: error: ../../../../../driver(.exe)? stdout doesn't match expected%
-      info: stdout: test/1/stdout-2
+      info: stdout: test/1/stdout-n2
       info: expected stdout: test/1/out
-      info: stdout diff: test/1/stdout-2.diff
+      info: stdout diff: test/1/stdout-n2.diff
     %--- \.*%
     %\+\+\+ \.*%
     %@@ \.*%
diff --git a/tests/test/script/runner/while.testscript b/tests/test/script/runner/while.testscript
new file mode 100644
index 0000000..1c58827
--- /dev/null
+++ b/tests/test/script/runner/while.testscript
@@ -0,0 +1,16 @@
+# file      : tests/test/script/runner/while.testscript
+# license   : MIT; see accompanying LICENSE file
+
+.include ../common.testscript
+
+: basics
+:
+$c <<EOI && $b >>EOO
+  while ($v != "aa")
+    echo "$v" >|
+    v = "$(v)a"
+  end
+  EOI
+
+  a
+  EOO