Support for --dry-run|-n mode, perform update part

23 files changed, 567 insertions, 367 deletions

diff --git a/build2/algorithm.cxx b/build2/algorithm.cxx
index 0dffb31..06ed922 100644
--- a/build2/algorithm.cxx
+++ b/build2/algorithm.cxx
@@ -1074,14 +1074,15 @@ namespace build2
     {
       // Normally will be there.
       //
-      try_rmbacklink (l, m);
+      if (!dry_run)
+        try_rmbacklink (l, m);
 
       // Skip (ad hoc) targets that don't exist.
       //
       if (!(d ? dir_exists (p) : file_exists (p)))
         return;
 
-      for (;;) // Retry/fallback loop.
+      for (; !dry_run; ) // Retry/fallback loop.
       try
       {
         switch (m)
@@ -1891,6 +1892,8 @@ namespace build2
     // below 3. Note the first extra file/directory that actually got removed
     // for diagnostics below.
     //
+    // Note that dry-run is taken care of by the filesystem functions.
+    //
     target_state er (target_state::unchanged);
     bool ed (false);
     path ep;
diff --git a/build2/b-options.cxx b/build2/b-options.cxx
index 7622682..a7a348f 100644
--- a/build2/b-options.cxx
+++ b/build2/b-options.cxx
@@ -609,14 +609,14 @@ namespace build2
   options ()
   : v_ (),
     V_ (),
-    progress_ (),
-    no_progress_ (),
     quiet_ (),
     verbose_ (1),
     verbose_specified_ (false),
     stat_ (),
     dump_ (),
     dump_specified_ (false),
+    progress_ (),
+    no_progress_ (),
     jobs_ (),
     jobs_specified_ (false),
     max_jobs_ (),
@@ -626,10 +626,11 @@ namespace build2
     max_stack_ (),
     max_stack_specified_ (false),
     serial_stop_ (),
+    dry_run_ (),
+    match_only_ (),
+    structured_result_ (),
     mtime_check_ (),
     no_mtime_check_ (),
-    structured_result_ (),
-    match_only_ (),
     no_column_ (),
     no_line_ (),
     buildfile_ (),
@@ -729,14 +730,6 @@ namespace build2
        << "                     equivalent to \033[1m--verbose 3\033[0m." << ::std::endl;
 
     os << std::endl
-       << "\033[1m--progress\033[0m           Display build progress. If printing to a terminal the" << ::std::endl
-       << "                     progress is displayed by default for low verbosity levels." << ::std::endl
-       << "                     Use \033[1m--no-progress\033[0m to suppress." << ::std::endl;
-
-    os << std::endl
-       << "\033[1m--no-progress\033[0m        Don't display build progress." << ::std::endl;
-
-    os << std::endl
        << "\033[1m--quiet\033[0m|\033[1m-q\033[0m           Run quietly, only printing error messages. This is" << ::std::endl
        << "                     equivalent to \033[1m--verbose 0\033[0m." << ::std::endl;
 
@@ -764,6 +757,14 @@ namespace build2
        << "                     option to dump the state after multiple phases." << ::std::endl;
 
     os << std::endl
+       << "\033[1m--progress\033[0m           Display build progress. If printing to a terminal the" << ::std::endl
+       << "                     progress is displayed by default for low verbosity levels." << ::std::endl
+       << "                     Use \033[1m--no-progress\033[0m to suppress." << ::std::endl;
+
+    os << std::endl
+       << "\033[1m--no-progress\033[0m        Don't display build progress." << ::std::endl;
+
+    os << std::endl
        << "\033[1m--jobs\033[0m|\033[1m-j\033[0m \033[4mnum\033[0m        Number of active jobs to perform in parallel. This" << ::std::endl
        << "                     includes both the number of active threads inside the" << ::std::endl
        << "                     build system as well as the number of external commands" << ::std::endl
@@ -809,13 +810,18 @@ namespace build2
        << "                     default concurrency)." << ::std::endl;
 
     os << std::endl
-       << "\033[1m--mtime-check\033[0m        Perform file modification time sanity checks. These checks" << ::std::endl
-       << "                     can be helpful in diagnosing spurious rebuilds and are" << ::std::endl
-       << "                     enabled by default for the staged version of the build" << ::std::endl
-       << "                     system. Use \033[1m--no-mtime-check\033[0m to disable." << ::std::endl;
+       << "\033[1m--dry-run\033[0m|\033[1m-n\033[0m         Print commands without actually executing them. Note that" << ::std::endl
+       << "                     commands that are required to create an accurate build" << ::std::endl
+       << "                     state will still be executed and the extracted auxiliary" << ::std::endl
+       << "                     dependency information saved. In other words, this is not" << ::std::endl
+       << "                     the \033[4m\"don't touch the filesystem\"\033[0m mode but rather \033[4m\"do" << ::std::endl
+       << "                     minimum amount of work to show what needs to be done\"\033[0m." << ::std::endl
+       << "                     Note also that only the \033[1mperform\033[0m meta-operation supports" << ::std::endl
+       << "                     this mode." << ::std::endl;
 
     os << std::endl
-       << "\033[1m--no-mtime-check\033[0m     Don't perform file modification time sanity checks." << ::std::endl;
+       << "\033[1m--match-only\033[0m         Match the rules but do not execute the operation. This" << ::std::endl
+       << "                     mode is primarily useful for profiling." << ::std::endl;
 
     os << std::endl
        << "\033[1m--structured-result\033[0m  Write the result of execution in a structured form. In" << ::std::endl
@@ -834,12 +840,17 @@ namespace build2
        << "                     unchanged perform update(test) /tmp/dir{hello/}" << ::std::endl
        << "                     changed perform test /tmp/dir{hello/}" << ::std::endl
        << ::std::endl
-       << "                     Currently only the \033[1mperform\033[0m meta-operation supports the" << ::std::endl
+       << "                     Note that only the \033[1mperform\033[0m meta-operation supports the" << ::std::endl
        << "                     structured result output." << ::std::endl;
 
     os << std::endl
-       << "\033[1m--match-only\033[0m         Match the rules but do not execute the operation. This" << ::std::endl
-       << "                     mode is primarily useful for profiling." << ::std::endl;
+       << "\033[1m--mtime-check\033[0m        Perform file modification time sanity checks. These checks" << ::std::endl
+       << "                     can be helpful in diagnosing spurious rebuilds and are" << ::std::endl
+       << "                     enabled by default for the staged version of the build" << ::std::endl
+       << "                     system. Use \033[1m--no-mtime-check\033[0m to disable." << ::std::endl;
+
+    os << std::endl
+       << "\033[1m--no-mtime-check\033[0m     Don't perform file modification time sanity checks." << ::std::endl;
 
     os << std::endl
        << "\033[1m--no-column\033[0m          Don't print column numbers in diagnostics." << ::std::endl;
@@ -911,10 +922,6 @@ namespace build2
       &::build2::cl::thunk< options, bool, &options::v_ >;
       _cli_options_map_["-V"] = 
       &::build2::cl::thunk< options, bool, &options::V_ >;
-      _cli_options_map_["--progress"] = 
-      &::build2::cl::thunk< options, bool, &options::progress_ >;
-      _cli_options_map_["--no-progress"] = 
-      &::build2::cl::thunk< options, bool, &options::no_progress_ >;
       _cli_options_map_["--quiet"] = 
       &::build2::cl::thunk< options, bool, &options::quiet_ >;
       _cli_options_map_["-q"] = 
@@ -927,6 +934,10 @@ namespace build2
       _cli_options_map_["--dump"] = 
       &::build2::cl::thunk< options, std::set<string>, &options::dump_,
         &options::dump_specified_ >;
+      _cli_options_map_["--progress"] = 
+      &::build2::cl::thunk< options, bool, &options::progress_ >;
+      _cli_options_map_["--no-progress"] = 
+      &::build2::cl::thunk< options, bool, &options::no_progress_ >;
       _cli_options_map_["--jobs"] = 
       &::build2::cl::thunk< options, size_t, &options::jobs_,
         &options::jobs_specified_ >;
@@ -952,14 +963,18 @@ namespace build2
       &::build2::cl::thunk< options, bool, &options::serial_stop_ >;
       _cli_options_map_["-s"] = 
       &::build2::cl::thunk< options, bool, &options::serial_stop_ >;
+      _cli_options_map_["--dry-run"] = 
+      &::build2::cl::thunk< options, bool, &options::dry_run_ >;
+      _cli_options_map_["-n"] = 
+      &::build2::cl::thunk< options, bool, &options::dry_run_ >;
+      _cli_options_map_["--match-only"] = 
+      &::build2::cl::thunk< options, bool, &options::match_only_ >;
+      _cli_options_map_["--structured-result"] = 
+      &::build2::cl::thunk< options, bool, &options::structured_result_ >;
       _cli_options_map_["--mtime-check"] = 
       &::build2::cl::thunk< options, bool, &options::mtime_check_ >;
       _cli_options_map_["--no-mtime-check"] = 
       &::build2::cl::thunk< options, bool, &options::no_mtime_check_ >;
-      _cli_options_map_["--structured-result"] = 
-      &::build2::cl::thunk< options, bool, &options::structured_result_ >;
-      _cli_options_map_["--match-only"] = 
-      &::build2::cl::thunk< options, bool, &options::match_only_ >;
       _cli_options_map_["--no-column"] = 
       &::build2::cl::thunk< options, bool, &options::no_column_ >;
       _cli_options_map_["--no-line"] = 
@@ -1180,9 +1195,9 @@ namespace build2
        << ::std::endl
        << "\033[1mb --help\033[0m" << ::std::endl
        << "\033[1mb --version\033[0m" << ::std::endl
-       << "\033[1mb\033[0m [\033[4moptions\033[0m] [\033[4mvariables\033[0m] [\033[4mbuild-spec\033[0m]\033[0m" << ::std::endl
+       << "\033[1mb\033[0m [\033[4moptions\033[0m] [\033[4mvariables\033[0m] [\033[4mbuildspec\033[0m]\033[0m" << ::std::endl
        << ::std::endl
-       << "\033[4mbuild-spec\033[0m = \033[4mmeta-operation\033[0m\033[1m(\033[0m\033[4moperation\033[0m\033[1m(\033[0m\033[4mtarget\033[0m...[\033[1m,\033[0m\033[4mparameters\033[0m]\033[1m)\033[0m...\033[1m)\033[0m...\033[0m" << ::std::endl
+       << "\033[4mbuildspec\033[0m = \033[4mmeta-operation\033[0m\033[1m(\033[0m\033[4moperation\033[0m\033[1m(\033[0m\033[4mtarget\033[0m...[\033[1m,\033[0m\033[4mparameters\033[0m]\033[1m)\033[0m...\033[1m)\033[0m...\033[0m" << ::std::endl
        << ::std::endl
        << "\033[1mDESCRIPTION\033[0m" << ::std::endl
        << ::std::endl
@@ -1190,7 +1205,7 @@ namespace build2
        << "according to the build specification, or \033[4mbuildspec\033[0m for short. This process can" << ::std::endl
        << "be controlled by specifying driver \033[4moptions\033[0m and build system \033[4mvariables\033[0m." << ::std::endl
        << ::std::endl
-       << "Note that \033[4moptions\033[0m, \033[4mvariables\033[0m, and \033[4mbuild-spec\033[0m fragments can be specified in any" << ::std::endl
+       << "Note that \033[4moptions\033[0m, \033[4mvariables\033[0m, and \033[4mbuildspec\033[0m fragments can be specified in any" << ::std::endl
        << "order. To avoid treating an argument that starts with \033[1m'-'\033[0m as an option, add the" << ::std::endl
        << "\033[1m'--'\033[0m separator. To avoid treating an argument that contains \033[1m'='\033[0m as a variable," << ::std::endl
        << "add the second \033[1m'--'\033[0m separator." << ::std::endl;
diff --git a/build2/b-options.hxx b/build2/b-options.hxx
index 94a87ab..45666aa 100644
--- a/build2/b-options.hxx
+++ b/build2/b-options.hxx
@@ -422,12 +422,6 @@ namespace build2
     V () const;
 
     const bool&
-    progress () const;
-
-    const bool&
-    no_progress () const;
-
-    const bool&
     quiet () const;
 
     const uint16_t&
@@ -445,6 +439,12 @@ namespace build2
     bool
     dump_specified () const;
 
+    const bool&
+    progress () const;
+
+    const bool&
+    no_progress () const;
+
     const size_t&
     jobs () const;
 
@@ -473,16 +473,19 @@ namespace build2
     serial_stop () const;
 
     const bool&
-    mtime_check () const;
+    dry_run () const;
 
     const bool&
-    no_mtime_check () const;
+    match_only () const;
 
     const bool&
     structured_result () const;
 
     const bool&
-    match_only () const;
+    mtime_check () const;
+
+    const bool&
+    no_mtime_check () const;
 
     const bool&
     no_column () const;
@@ -547,14 +550,14 @@ namespace build2
     public:
     bool v_;
     bool V_;
-    bool progress_;
-    bool no_progress_;
     bool quiet_;
     uint16_t verbose_;
     bool verbose_specified_;
     bool stat_;
     std::set<string> dump_;
     bool dump_specified_;
+    bool progress_;
+    bool no_progress_;
     size_t jobs_;
     bool jobs_specified_;
     size_t max_jobs_;
@@ -564,10 +567,11 @@ namespace build2
     size_t max_stack_;
     bool max_stack_specified_;
     bool serial_stop_;
+    bool dry_run_;
+    bool match_only_;
+    bool structured_result_;
     bool mtime_check_;
     bool no_mtime_check_;
-    bool structured_result_;
-    bool match_only_;
     bool no_column_;
     bool no_line_;
     path buildfile_;
diff --git a/build2/b-options.ixx b/build2/b-options.ixx
index b05ed66..9b7a8ad 100644
--- a/build2/b-options.ixx
+++ b/build2/b-options.ixx
@@ -244,18 +244,6 @@ namespace build2
   }
 
   inline const bool& options::
-  progress () const
-  {
-    return this->progress_;
-  }
-
-  inline const bool& options::
-  no_progress () const
-  {
-    return this->no_progress_;
-  }
-
-  inline const bool& options::
   quiet () const
   {
     return this->quiet_;
@@ -291,6 +279,18 @@ namespace build2
     return this->dump_specified_;
   }
 
+  inline const bool& options::
+  progress () const
+  {
+    return this->progress_;
+  }
+
+  inline const bool& options::
+  no_progress () const
+  {
+    return this->no_progress_;
+  }
+
   inline const size_t& options::
   jobs () const
   {
@@ -346,15 +346,15 @@ namespace build2
   }
 
   inline const bool& options::
-  mtime_check () const
+  dry_run () const
   {
-    return this->mtime_check_;
+    return this->dry_run_;
   }
 
   inline const bool& options::
-  no_mtime_check () const
+  match_only () const
   {
-    return this->no_mtime_check_;
+    return this->match_only_;
   }
 
   inline const bool& options::
@@ -364,9 +364,15 @@ namespace build2
   }
 
   inline const bool& options::
-  match_only () const
+  mtime_check () const
   {
-    return this->match_only_;
+    return this->mtime_check_;
+  }
+
+  inline const bool& options::
+  no_mtime_check () const
+  {
+    return this->no_mtime_check_;
   }
 
   inline const bool& options::
diff --git a/build2/b.cli b/build2/b.cli
index 35d5e2e..03de044 100644
--- a/build2/b.cli
+++ b/build2/b.cli
@@ -14,15 +14,15 @@ namespace build2
   {
     "<options>
      <variables>
-     <build-spec> <meta-operation> <operation> <target> <parameters>",
+     <buildspec> <meta-operation> <operation> <target> <parameters>",
 
     "\h|SYNOPSIS|
 
      \c{\b{b --help}\n
         \b{b --version}\n
-        \b{b} [<options>] [<variables>] [<build-spec>]}
+        \b{b} [<options>] [<variables>] [<buildspec>]}
 
-     \c{<build-spec> = <meta-operation>\b{(}<operation>\b{(}<target>...[\b{,}<parameters>]\b{)}...\b{)}...}
+     \c{<buildspec> = <meta-operation>\b{(}<operation>\b{(}<target>...[\b{,}<parameters>]\b{)}...\b{)}...}
 
      \h|DESCRIPTION|
 
@@ -31,7 +31,7 @@ namespace build2
      This process can be controlled by specifying driver <options> and build
      system <variables>.
 
-     Note that <options>, <variables>, and <build-spec> fragments can be
+     Note that <options>, <variables>, and <buildspec> fragments can be
      specified in any order. To avoid treating an argument that starts with
      \cb{'-'} as an option, add the \cb{'--'} separator. To avoid treating an
      argument that contains \cb{'='} as a variable, add the second \cb{'--'}
@@ -401,18 +401,6 @@ namespace build2
        \cb{--verbose 3}."
     }
 
-    bool --progress
-    {
-      "Display build progress. If printing to a terminal the progress is
-       displayed by default for low verbosity levels. Use \cb{--no-progress}
-       to suppress."
-    }
-
-    bool --no-progress
-    {
-      "Don't display build progress."
-    }
-
     bool --quiet|-q
     {
       "Run quietly, only printing error messages. This is equivalent to
@@ -456,6 +444,18 @@ namespace build2
        state after multiple phases."
     }
 
+    bool --progress
+    {
+      "Display build progress. If printing to a terminal the progress is
+       displayed by default for low verbosity levels. Use \cb{--no-progress}
+       to suppress."
+    }
+
+    bool --no-progress
+    {
+      "Don't display build progress."
+    }
+
     size_t --jobs|-j
     {
       "<num>",
@@ -508,17 +508,21 @@ namespace build2
        example \cb{-j\ 0} for default concurrency)."
     }
 
-    bool --mtime-check
+    bool --dry-run|-n
     {
-      "Perform file modification time sanity checks. These checks can be
-       helpful in diagnosing spurious rebuilds and are enabled by default
-       for the staged version of the build system. Use \cb{--no-mtime-check}
-       to disable."
+      "Print commands without actually executing them. Note that commands
+       that are required to create an accurate build state will still be
+       executed and the extracted auxiliary dependency information saved. In
+       other words, this is not the \i{\"don't touch the filesystem\"} mode
+       but rather \i{\"do minimum amount of work to show what needs to be
+       done\"}. Note also that only the \cb{perform} meta-operation supports
+       this mode."
     }
 
-    bool --no-mtime-check
+    bool --match-only
     {
-      "Don't perform file modification time sanity checks."
+      "Match the rules but do not execute the operation. This mode is primarily
+       useful for profiling."
     }
 
     bool --structured-result
@@ -540,15 +544,22 @@ namespace build2
        changed perform test /tmp/dir{hello/}
        \
 
-       Currently only the \cb{perform} meta-operation supports the structured
+       Note that only the \cb{perform} meta-operation supports the structured
        result output.
        "
     }
 
-    bool --match-only
+    bool --mtime-check
     {
-      "Match the rules but do not execute the operation. This mode is primarily
-       useful for profiling."
+      "Perform file modification time sanity checks. These checks can be
+       helpful in diagnosing spurious rebuilds and are enabled by default
+       for the staged version of the build system. Use \cb{--no-mtime-check}
+       to disable."
+    }
+
+    bool --no-mtime-check
+    {
+      "Don't perform file modification time sanity checks."
     }
 
     bool --no-column
diff --git a/build2/cc/compile-rule.cxx b/build2/cc/compile-rule.cxx
index 3769b32..3050a37 100644
--- a/build2/cc/compile-rule.cxx
+++ b/build2/cc/compile-rule.cxx
@@ -995,7 +995,15 @@ namespace build2
         // We do need to update the database timestamp, however. Failed that,
         // we will keep re-validating the cached data over and over again.
         //
-        if (u && dd.reading ())
+        // @@ DRYRUN: note that for dry-run we would keep re-touching the
+        // database on every run (because u is true). So for now we suppress
+        // it (the file will be re-validated on the real run anyway). It feels
+        // like support for reusing the (partially) preprocessed output (see
+        // note below) should help solve this properly (i.e., we don't want
+        // to keep re-validating the file on every subsequent dry-run as well
+        // on the real run).
+        //
+        if (u && dd.reading () && !dry_run)
           dd.touch = true;
 
         dd.close ();
@@ -4642,58 +4650,73 @@ namespace build2
       if (verb >= 3)
         print_process (args);
 
-      try
+      // @@ DRYRUN: Currently we discard the (partially) preprocessed file on
+      // dry-run which is a waste. Even if we keep the file around (like we do
+      // for the error case; see above), we currently have no support for
+      // re-using the previously preprocessed output. However, everything
+      // points towards us needing this in the near future since with modules
+      // we may be out of date but not needing to re-preprocess the
+      // translation unit (i.e., one of the imported module's has BMIs
+      // changed).
+      //
+      if (!dry_run)
       {
-        // VC cl.exe sends diagnostics to stdout. It also prints the file name
-        // being compiled as the first line. So for cl.exe we redirect stdout
-        // to a pipe, filter that noise out, and send the rest to stderr.
-        //
-        // For other compilers redirect stdout to stderr, in case any of them
-        // tries to pull off something similar. For sane compilers this should
-        // be harmless.
-        //
-        bool filter (ctype == compiler_type::msvc);
+        try
+        {
+          // VC cl.exe sends diagnostics to stdout. It also prints the file
+          // name being compiled as the first line. So for cl.exe we redirect
+          // stdout to a pipe, filter that noise out, and send the rest to
+          // stderr.
+          //
+          // For other compilers redirect stdout to stderr, in case any of
+          // them tries to pull off something similar. For sane compilers this
+          // should be harmless.
+          //
+          bool filter (ctype == compiler_type::msvc);
 
-        process pr (cpath,
-                    args.data (),
-                    0, (filter ? -1 : 2), 2,
-                    nullptr, // CWD
-                    env.empty () ? nullptr : env.data ());
+          process pr (cpath,
+                      args.data (),
+                      0, (filter ? -1 : 2), 2,
+                      nullptr, // CWD
+                      env.empty () ? nullptr : env.data ());
 
-        if (filter)
-        {
-          try
+          if (filter)
           {
-            ifdstream is (
-              move (pr.in_ofd), fdstream_mode::text, ifdstream::badbit);
+            try
+            {
+              ifdstream is (
+                move (pr.in_ofd), fdstream_mode::text, ifdstream::badbit);
 
-            msvc_filter_cl (is, *sp);
+              msvc_filter_cl (is, *sp);
 
-            // If anything remains in the stream, send it all to stderr. Note
-            // that the eof check is important: if the stream is at eof, this
-            // and all subsequent writes to the diagnostics stream will fail
-            // (and you won't see a thing).
-            //
-            if (is.peek () != ifdstream::traits_type::eof ())
-              diag_stream_lock () << is.rdbuf ();
+              // If anything remains in the stream, send it all to stderr.
+              // Note that the eof check is important: if the stream is at
+              // eof, this and all subsequent writes to the diagnostics stream
+              // will fail (and you won't see a thing).
+              //
+              if (is.peek () != ifdstream::traits_type::eof ())
+                diag_stream_lock () << is.rdbuf ();
 
-            is.close ();
+              is.close ();
+            }
+            catch (const io_error&) {} // Assume exits with error.
           }
-          catch (const io_error&) {} // Assume exits with error.
-        }
 
-        run_finish (args, pr);
-      }
-      catch (const process_error& e)
-      {
-        error << "unable to execute " << args[0] << ": " << e;
+          run_finish (args, pr);
+        }
+        catch (const process_error& e)
+        {
+          error << "unable to execute " << args[0] << ": " << e;
 
-        if (e.child)
-          exit (1);
+          if (e.child)
+            exit (1);
 
-        throw failed ();
+          throw failed ();
+        }
       }
 
+      // Remove preprocessed file (see above).
+      //
       if (pact && verb >= 3)
         md.psrc.active = true;
 
@@ -4702,11 +4725,6 @@ namespace build2
       //
       if (mod && ctype == compiler_type::clang)
       {
-        // Remove the target file if this fails. If we don't do that, we will
-        // end up with a broken build that is up-to-date.
-        //
-        auto_rmfile rm (relm);
-
         // Adjust the command line. First discard everything after -o then
         // build the new "tail".
         //
@@ -4720,35 +4738,46 @@ namespace build2
         if (verb >= 2)
           print_process (args);
 
-        try
+        if (!dry_run)
         {
-          process pr (cpath,
-                      args.data (),
-                      0, 2, 2,
-                      nullptr, // CWD
-                      env.empty () ? nullptr : env.data ());
+          // Remove the target file if this fails. If we don't do that, we
+          // will end up with a broken build that is up-to-date.
+          //
+          auto_rmfile rm (relm);
 
-          run_finish (args, pr);
-        }
-        catch (const process_error& e)
-        {
-          error << "unable to execute " << args[0] << ": " << e;
+          try
+          {
+            process pr (cpath,
+                        args.data (),
+                        0, 2, 2,
+                        nullptr, // CWD
+                        env.empty () ? nullptr : env.data ());
 
-          if (e.child)
-            exit (1);
+            run_finish (args, pr);
+          }
+          catch (const process_error& e)
+          {
+            error << "unable to execute " << args[0] << ": " << e;
 
-          throw failed ();
-        }
+            if (e.child)
+              exit (1);
+
+            throw failed ();
+          }
 
-        rm.cancel ();
+          rm.cancel ();
+        }
       }
 
       timestamp now (system_clock::now ());
-      depdb::check_mtime (start, md.dd, tp, now);
+
+      if (!dry_run)
+        depdb::check_mtime (start, md.dd, tp, now);
 
       // Should we go to the filesystem and get the new mtime? We know the
       // file has been modified, so instead just use the current clock time.
-      // It has the advantage of having the subseconds precision.
+      // It has the advantage of having the subseconds precision. Plus, in
+      // case of dry-run, the file won't be modified.
       //
       t.mtime (now);
       return target_state::changed;
diff --git a/build2/cc/link-rule.cxx b/build2/cc/link-rule.cxx
index 8b4d3ee..7d5eb83 100644
--- a/build2/cc/link-rule.cxx
+++ b/build2/cc/link-rule.cxx
@@ -1687,9 +1687,7 @@ namespace build2
 
         auto p (windows_manifest (t, rpath_timestamp != timestamp_nonexistent));
         path& mf (p.first);
-        bool mf_cf (p.second); // Changed flag (timestamp resolution).
-
-        timestamp mf_mt (mtime (mf));
+        timestamp mf_mt (p.second);
 
         if (tsys == "mingw32")
         {
@@ -1699,7 +1697,7 @@ namespace build2
           //
           manifest = mf + ".o";
 
-          if (mf_mt > mtime (manifest) || mf_cf)
+          if (mf_mt == timestamp_nonexistent || mf_mt > mtime (manifest))
           {
             path of (relative (manifest));
 
@@ -1717,53 +1715,59 @@ namespace build2
             if (verb >= 3)
               print_process (args);
 
-            try
+            if (!dry_run)
             {
-              process pr (rc, args, -1);
+              auto_rmfile rm (of);
 
               try
               {
-                ofdstream os (move (pr.out_fd));
+                process pr (rc, args, -1);
 
-                // 1 is resource ID, 24 is RT_MANIFEST. We also need to escape
-                // Windows path backslashes.
-                //
-                os << "1 24 \"";
-
-                const string& s (mf.string ());
-                for (size_t i (0), j;; i = j + 1)
+                try
                 {
-                  j = s.find ('\\', i);
-                  os.write (s.c_str () + i,
-                            (j == string::npos ? s.size () : j) - i);
+                  ofdstream os (move (pr.out_fd));
+
+                  // 1 is resource ID, 24 is RT_MANIFEST. We also need to
+                  // escape Windows path backslashes.
+                  //
+                  os << "1 24 \"";
 
-                  if (j == string::npos)
-                    break;
+                  const string& s (mf.string ());
+                  for (size_t i (0), j;; i = j + 1)
+                  {
+                    j = s.find ('\\', i);
+                    os.write (s.c_str () + i,
+                              (j == string::npos ? s.size () : j) - i);
 
-                  os.write ("\\\\", 2);
-                }
+                    if (j == string::npos)
+                      break;
+
+                    os.write ("\\\\", 2);
+                  }
 
-                os << "\"" << endl;
+                  os << "\"" << endl;
 
-                os.close ();
+                  os.close ();
+                  rm.cancel ();
+                }
+                catch (const io_error& e)
+                {
+                  if (pr.wait ()) // Ignore if child failed.
+                    fail << "unable to pipe resource file to " << args[0]
+                         << ": " << e;
+                }
+
+                run_finish (args, pr);
               }
-              catch (const io_error& e)
+              catch (const process_error& e)
               {
-                if (pr.wait ()) // Ignore if child failed.
-                  fail << "unable to pipe resource file to " << args[0]
-                       << ": " << e;
-              }
+                error << "unable to execute " << args[0] << ": " << e;
 
-              run_finish (args, pr);
-            }
-            catch (const process_error& e)
-            {
-              error << "unable to execute " << args[0] << ": " << e;
+                if (e.child)
+                  exit (1);
 
-              if (e.child)
-                exit (1);
-
-              throw failed ();
+                throw failed ();
+              }
             }
 
             update = true; // Manifest changed, force update.
@@ -1773,7 +1777,7 @@ namespace build2
         {
           manifest = move (mf); // Save for link.exe's /MANIFESTINPUT.
 
-          if (mf_mt > mt || mf_cf)
+          if (mf_mt == timestamp_nonexistent || mf_mt > mt)
             update = true; // Manifest changed, force update.
         }
       }
@@ -2382,7 +2386,8 @@ namespace build2
 
       args.push_back (nullptr);
 
-      // Cleanup old (versioned) libraries.
+      // Cleanup old (versioned) libraries. Let's do it even for dry-run to
+      // keep things simple.
       //
       if (lt.shared_library ())
       {
@@ -2437,20 +2442,13 @@ namespace build2
         // We use relative paths to the object files which means we may end
         // up with different ones depending on CWD and some implementation
         // treat them as different archive members. So remote the file to
-        // be sure. Note that we ignore errors leaving it to the achiever
+        // be sure. Note that we ignore errors leaving it to the archiever
         // to complain.
         //
         if (mt != timestamp_nonexistent)
           try_rmfile (relt, true);
       }
 
-      // Remove the target file if any of the subsequent (after the linker)
-      // actions fail or if the linker fails but does not clean up its mess
-      // (like link.exe). If we don't do that, then we will end up with a
-      // broken build that is up-to-date.
-      //
-      auto_rmfile rm (relt);
-
       if (verb == 1)
         text << (lt.static_library () ? "ar " : "ld ") << t;
       else if (verb == 2)
@@ -2562,84 +2560,96 @@ namespace build2
       if (verb > 2)
         print_process (args);
 
-      try
-      {
-        // VC tools (both lib.exe and link.exe) send diagnostics to stdout.
-        // Also, link.exe likes to print various gratuitous messages. So for
-        // link.exe we redirect stdout to a pipe, filter that noise out, and
-        // send the rest to stderr.
-        //
-        // For lib.exe (and any other insane compiler that may try to pull off
-        // something like this) we are going to redirect stdout to stderr. For
-        // sane compilers this should be harmless.
-        //
-        bool filter (tsys == "win32-msvc" && !lt.static_library ());
+      // Remove the target file if any of the subsequent (after the linker)
+      // actions fail or if the linker fails but does not clean up its mess
+      // (like link.exe). If we don't do that, then we will end up with a
+      // broken build that is up-to-date.
+      //
+      auto_rmfile rm;
 
-        process pr (*ld, args.data (), 0, (filter ? -1 : 2));
+      if (!dry_run)
+      {
+        rm = auto_rmfile (relt);
 
-        if (filter)
+        try
         {
-          try
+          // VC tools (both lib.exe and link.exe) send diagnostics to stdout.
+          // Also, link.exe likes to print various gratuitous messages. So for
+          // link.exe we redirect stdout to a pipe, filter that noise out, and
+          // send the rest to stderr.
+          //
+          // For lib.exe (and any other insane linker that may try to pull off
+          // something like this) we are going to redirect stdout to stderr.
+          // For sane compilers this should be harmless.
+          //
+          bool filter (tsys == "win32-msvc" && !lt.static_library ());
+
+          process pr (*ld, args.data (), 0, (filter ? -1 : 2));
+
+          if (filter)
           {
-            ifdstream is (
-              move (pr.in_ofd), fdstream_mode::text, ifdstream::badbit);
+            try
+            {
+              ifdstream is (
+                move (pr.in_ofd), fdstream_mode::text, ifdstream::badbit);
 
-            msvc_filter_link (is, t, ot);
+              msvc_filter_link (is, t, ot);
 
-            // If anything remains in the stream, send it all to stderr. Note
-            // that the eof check is important: if the stream is at eof, this
-            // and all subsequent writes to the diagnostics stream will fail
-            // (and you won't see a thing).
-            //
-            if (is.peek () != ifdstream::traits_type::eof ())
-              diag_stream_lock () << is.rdbuf ();
+              // If anything remains in the stream, send it all to stderr.
+              // Note that the eof check is important: if the stream is at
+              // eof, this and all subsequent writes to the diagnostics stream
+              // will fail (and you won't see a thing).
+              //
+              if (is.peek () != ifdstream::traits_type::eof ())
+                diag_stream_lock () << is.rdbuf ();
 
-            is.close ();
+              is.close ();
+            }
+            catch (const io_error&) {} // Assume exits with error.
           }
-          catch (const io_error&) {} // Assume exits with error.
-        }
 
-        run_finish (args, pr);
-      }
-      catch (const process_error& e)
-      {
-        error << "unable to execute " << args[0] << ": " << e;
-
-        // In a multi-threaded program that fork()'ed but did not exec(),
-        // it is unwise to try to do any kind of cleanup (like unwinding
-        // the stack and running destructors).
-        //
-        if (e.child)
+          run_finish (args, pr);
+        }
+        catch (const process_error& e)
         {
-          rm.cancel ();
+          error << "unable to execute " << args[0] << ": " << e;
+
+          // In a multi-threaded program that fork()'ed but did not exec(), it
+          // is unwise to try to do any kind of cleanup (like unwinding the
+          // stack and running destructors).
+          //
+          if (e.child)
+          {
+            rm.cancel ();
 #ifdef _WIN32
-          trm.cancel ();
+            trm.cancel ();
 #endif
-          exit (1);
-        }
+            exit (1);
+          }
 
-        throw failed ();
-      }
+          throw failed ();
+        }
 
-      // VC link.exe creates an import library and .exp file for an executable
-      // if any of its object files export any symbols (think a unit test
-      // linking libus{}). And, no, there is no way to suppress it. Well,
-      // there is a way: create a .def file with an empty EXPORTS section,
-      // pass it to lib.exe to create a dummy .exp (and .lib), and then pass
-      // this empty .exp to link.exe. Wanna go this way? Didn't think so.
-      // Having no way to disable this, the next simplest thing seems to be
-      // just cleaning the mess up.
-      //
-      // Note also that if at some point we decide to support such "shared
-      // executables" (-rdynamic, etc), then it will probably have to be a
-      // different target type (exes{}?) since it will need a different set
-      // of object files (-fPIC so probably objs{}), etc.
-      //
-      if (lt.executable () && tsys == "win32-msvc")
-      {
-        path b (relt.base ());
-        try_rmfile (b + ".lib", true /* ignore_errors */);
-        try_rmfile (b + ".exp", true /* ignore_errors */);
+        // VC link.exe creates an import library and .exp file for an
+        // executable if any of its object files export any symbols (think a
+        // unit test linking libus{}). And, no, there is no way to suppress
+        // it. Well, there is a way: create a .def file with an empty EXPORTS
+        // section, pass it to lib.exe to create a dummy .exp (and .lib), and
+        // then pass this empty .exp to link.exe. Wanna go this way? Didn't
+        // think so. Having no way to disable this, the next simplest thing
+        // seems to be just cleaning the mess up.
+        //
+        // Note also that if at some point we decide to support such "shared
+        // executables" (-rdynamic, etc), then it will probably have to be a
+        // different target type (exes{}?) since it will need a different set
+        // of object files (-fPIC so probably objs{}), etc.
+        //
+        if (lt.executable () && tsys == "win32-msvc")
+        {
+          path b (relt.base ());
+          try_rmfile (b + ".lib", true /* ignore_errors */);
+          try_rmfile (b + ".exp", true /* ignore_errors */);
+        }
       }
 
       if (ranlib)
@@ -2654,7 +2664,8 @@ namespace build2
         if (verb >= 2)
           print_process (args);
 
-        run (rl, args);
+        if (!dry_run)
+          run (rl, args);
       }
 
       if (tclass == "windows")
@@ -2676,6 +2687,9 @@ namespace build2
           if (verb >= 3)
             text << "ln -sf " << f << ' ' << l;
 
+          if (dry_run)
+            return;
+
           try
           {
             if (file_exists (l, false /* follow_symlinks */)) // The -f part.
@@ -2701,29 +2715,35 @@ namespace build2
         if (!so.empty ()) {ln (f->leaf (), so); f = &so;}
         if (!lk.empty ()) {ln (f->leaf (), lk);}
       }
-
-      // Apple ar (from cctools) for some reason truncates fractional seconds
-      // when running on APFS (HFS has a second resolution so it's not an
-      // issue there). This can lead to object files being newer than the
-      // archive, which is naturally bad news. Filed as bug 49604334.
-      //
-      // Note that this block is not inside #ifdef __APPLE__ because we could
-      // be cross-compiling, theoretically. We also make sure we use Apple's
-      // ar (which is (un)recognized as 'generic') instead of, say, llvm-ar.
-      //
-      if (lt.static_library ()                      &&
-          tsys == "darwin"                          &&
-          cast<string> (rs["bin.ar.id"]) == "generic")
+      else if (lt.static_library ())
       {
-        touch (tp, false /* create */, verb_never);
+        // Apple ar (from cctools) for some reason truncates fractional
+        // seconds when running on APFS (HFS has a second resolution so it's
+        // not an issue there). This can lead to object files being newer than
+        // the archive, which is naturally bad news. Filed as bug 49604334.
+        //
+        // Note that this block is not inside #ifdef __APPLE__ because we
+        // could be cross-compiling, theoretically. We also make sure we use
+        // Apple's ar (which is (un)recognized as 'generic') instead of, say,
+        // llvm-ar.
+        //
+        if (tsys == "darwin" && cast<string> (rs["bin.ar.id"]) == "generic")
+        {
+          if (!dry_run)
+            touch (tp, false /* create */, verb_never);
+        }
       }
 
-      rm.cancel ();
-      dd.check_mtime (tp);
+      if (!dry_run)
+      {
+        rm.cancel ();
+        dd.check_mtime (tp);
+      }
 
       // Should we go to the filesystem and get the new mtime? We know the
       // file has been modified, so instead just use the current clock time.
-      // It has the advantage of having the subseconds precision.
+      // It has the advantage of having the subseconds precision. Plus, in
+      // case of dry-run, the file won't be modified.
       //
       t.mtime (system_clock::now ());
       return target_state::changed;
diff --git a/build2/cc/link-rule.hxx b/build2/cc/link-rule.hxx
index b239dee..1a77aef 100644
--- a/build2/cc/link-rule.hxx
+++ b/build2/cc/link-rule.hxx
@@ -151,7 +151,7 @@ namespace build2
 
       // Windows-specific (windows-manifest.cxx).
       //
-      pair<path, bool>
+      pair<path, timestamp>
       windows_manifest (const file&, bool rpath_assembly) const;
 
       // pkg-config's .pc file generation (pkgconfig.cxx).
diff --git a/build2/cc/pkgconfig.cxx b/build2/cc/pkgconfig.cxx
index eef1271..6f30dc9 100644
--- a/build2/cc/pkgconfig.cxx
+++ b/build2/cc/pkgconfig.cxx
@@ -1234,9 +1234,6 @@ namespace build2
       auto* t (find_adhoc_member<pc> (l));
       assert (t != nullptr);
 
-      const path& p (t->path ());
-      auto_rmfile arm (p);
-
       // By default we assume things go into install.{include, lib}.
       //
       using install::resolve_dir;
@@ -1244,9 +1241,16 @@ namespace build2
       dir_path idir (resolve_dir (l, cast<dir_path> (l["install.include"])));
       dir_path ldir (resolve_dir (l, cast<dir_path> (l["install.lib"])));
 
+      const path& p (t->path ());
+
       if (verb >= 2)
         text << "cat >" << p;
 
+      if (dry_run)
+        return;
+
+      auto_rmfile arm (p);
+
       try
       {
         ofdstream os (p);
diff --git a/build2/cc/windows-manifest.cxx b/build2/cc/windows-manifest.cxx
index 268e8c7..f890ad5 100644
--- a/build2/cc/windows-manifest.cxx
+++ b/build2/cc/windows-manifest.cxx
@@ -37,9 +37,9 @@ namespace build2
 
     // Generate a Windows manifest and if necessary create/update the manifest
     // file corresponding to the exe{} target. Return the manifest file path
-    // as well as whether it was changed.
+    // and its timestamp if unchanged or timestamp_nonexistent otherwise.
     //
-    pair<path, bool> link_rule::
+    pair<path, timestamp> link_rule::
     windows_manifest (const file& t, bool rpath_assembly) const
     {
       tracer trace (x, "link_rule::windows_manifest");
@@ -100,13 +100,15 @@ namespace build2
       //
       path mf (t.path () + ".manifest");
 
-      if (exists (mf))
+      timestamp mt (mtime (mf));
+
+      if (mt != timestamp_nonexistent)
       {
         try
         {
-          ifdstream ifs (mf);
-          if (ifs.read_text () == m)
-            return make_pair (move (mf), false);
+          ifdstream is (mf);
+          if (is.read_text () == m)
+            return make_pair (move (mf), mt);
         }
         catch (const io_error&)
         {
@@ -117,18 +119,25 @@ namespace build2
       if (verb >= 3)
         text << "cat >" << mf;
 
-      try
-      {
-        ofdstream ofs (mf);
-        ofs << m;
-        ofs.close ();
-      }
-      catch (const io_error& e)
+      if (!dry_run)
       {
-        fail << "unable to write to " << mf << ": " << e;
+        auto_rmfile rm (mf);
+
+        try
+        {
+          ofdstream os (mf);
+          os << m;
+          os.close ();
+          rm.cancel ();
+
+        }
+        catch (const io_error& e)
+        {
+          fail << "unable to write to " << mf << ": " << e;
+        }
       }
 
-      return make_pair (move (mf), true);
+      return make_pair (move (mf), timestamp_nonexistent);
     }
   }
 }
diff --git a/build2/cc/windows-rpath.cxx b/build2/cc/windows-rpath.cxx
index 46fe75b..0a19db2 100644
--- a/build2/cc/windows-rpath.cxx
+++ b/build2/cc/windows-rpath.cxx
@@ -290,23 +290,9 @@ namespace build2
           mkdir (ad, 3);
       }
 
-      const char* pa (windows_manifest_arch (tcpu));
-
-      if (verb >= 3)
-        text << "cat >" << am;
-
-      try
+      // Symlink or copy the DLLs.
+      //
       {
-        ofdstream ofs (am);
-
-        ofs << "<?xml version='1.0' encoding='UTF-8' standalone='yes'?>\n"
-            << "<assembly xmlns='urn:schemas-microsoft-com:asm.v1'\n"
-            << "          manifestVersion='1.0'>\n"
-            << "  <assemblyIdentity name='" << an << "'\n"
-            << "                    type='win32'\n"
-            << "                    processorArchitecture='" << pa << "'\n"
-            << "                    version='0.0.0.0'/>\n";
-
         const scope& as (*t.root_scope ().weak_scope ()); // Amalgamation.
 
         auto link = [&as, &ad] (const path& f, const path& l)
@@ -328,15 +314,20 @@ namespace build2
             // part of the same amalgamation. This way if the amalgamation is
             // moved as a whole, the links will remain valid.
             //
-            if (f.sub (as.out_path ()))
-              mksymlink (f.relative (ad), l);
-            else
-              mksymlink (f, l);
+            if (!dry_run)
+            {
+              if (f.sub (as.out_path ()))
+                mksymlink (f.relative (ad), l);
+              else
+                mksymlink (f, l);
+            }
 
             print ("ln -s");
           }
           catch (const system_error& e)
           {
+            // Note: can never end up here on dry-run.
+
             // Note that we are not guaranteed (here and below) that the
             // system_error exception is of the generic category.
             //
@@ -378,7 +369,6 @@ namespace build2
               }
             }
           }
-
         };
 
         for (const windows_dll& wd: dlls)
@@ -398,13 +388,40 @@ namespace build2
             path pp (*wd.pdb);
             link (pp, ad / pp.leaf ());
           }
-
-          ofs << "  <file name='" << dn.string () << "'/>\n";
         }
+      }
+
+      if (verb >= 3)
+        text << "cat >" << am;
+
+      if (dry_run)
+        return;
+
+      auto_rmfile rm (am);
+
+      try
+      {
+        ofdstream os (am);
+
+        const char* pa (windows_manifest_arch (tcpu));
+
+        os << "<?xml version='1.0' encoding='UTF-8' standalone='yes'?>\n"
+           << "<assembly xmlns='urn:schemas-microsoft-com:asm.v1'\n"
+           << "          manifestVersion='1.0'>\n"
+           << "  <assemblyIdentity name='" << an << "'\n"
+           << "                    type='win32'\n"
+           << "                    processorArchitecture='" << pa << "'\n"
+           << "                    version='0.0.0.0'/>\n";
+
+
+
+        for (const windows_dll& wd: dlls)
+          os << "  <file name='" << path (wd.dll).leaf () << "'/>\n";
 
-        ofs << "</assembly>\n";
+        os << "</assembly>\n";
 
-        ofs.close ();
+        os.close ();
+        rm.cancel ();
       }
       catch (const io_error& e)
       {
diff --git a/build2/cli/rule.cxx b/build2/cli/rule.cxx
index cc3e5dd..2da5802 100644
--- a/build2/cli/rule.cxx
+++ b/build2/cli/rule.cxx
@@ -322,9 +322,11 @@ namespace build2
       else if (verb)
         text << "cli " << s;
 
-      run (cli, args);
-
-      dd.check_mtime (tp);
+      if (!dry_run)
+      {
+        run (cli, args);
+        dd.check_mtime (tp);
+      }
 
       t.mtime (system_clock::now ());
       return target_state::changed;
diff --git a/build2/context.cxx b/build2/context.cxx
index e1aaeee..aa75b6c 100644
--- a/build2/context.cxx
+++ b/build2/context.cxx
@@ -364,6 +364,7 @@ namespace build2
   atomic_count skip_count;
 
   bool keep_going = false;
+  bool dry_run = false;
 
   variable_overrides
   reset (const strings& cmd_vars)
diff --git a/build2/context.hxx b/build2/context.hxx
index 74cfa5f..567786d 100644
--- a/build2/context.hxx
+++ b/build2/context.hxx
@@ -402,6 +402,37 @@ namespace build2
   //
   extern bool keep_going;
 
+  // Dry run flag (see --dry-run|-n).
+  //
+  // This flag is set only for the final execute phase (as opposed to those
+  // that interrupt match) by the perform meta operation's execute() callback.
+  //
+  // Note that for this mode to function properly we have to use fake mtimes.
+  // Specifically, a rule that pretends to update a target must set its mtime
+  // to system_clock::now() and everyone else must use this cached value. In
+  // other words, there should be no mtime re-query from the filesystem.
+  //
+  // At first, it may seem like we should also "dry-run" changes to depdb. But
+  // that would be both problematic (some rules update it in apply() during
+  // the match phase) and wasteful (why discard information). Also, depdb may
+  // serve as an input to some commands (for example, to provide C++ module
+  // mapping) which means that without updating it the commands we print might
+  // not be runnable (think of the compilation database).
+  //
+  // One thing we need to be careful about if we are updating depdb is to not
+  // render the target up-to-date. But in this case the depdb file will be
+  // older than the target which in our model is treated as an interrupted
+  // update (see depdb for details).
+  //
+  // Note also that sometimes it makes sense to do a bit more than absolutely
+  // necessary or to discard information in order to keep the rule logic sane.
+  // And some rules may choose to ignore this flag altogether. In this case,
+  // however, the rule should be careful not to rely on functions (notably
+  // from filesystem) that respect this flag in order not to end up with a
+  // job half done.
+  //
+  extern bool dry_run;
+
   // Reset the build state. In particular, this removes all the targets,
   // scopes, and variables.
   //
diff --git a/build2/depdb.hxx b/build2/depdb.hxx
index 64ea627..ceb58ac 100644
--- a/build2/depdb.hxx
+++ b/build2/depdb.hxx
@@ -54,7 +54,9 @@ namespace build2
   //
   // If we assume that an update of the database also means an update of the
   // target, then this "interrupted update" situation can be easily detected
-  // by comparing the database and target modification timestamps.
+  // by comparing the database and target modification timestamps. This is
+  // also used to handle the dry-run mode where we essentially do the
+  // interruption ourselves.
   //
   struct depdb_base
   {
diff --git a/build2/filesystem.cxx b/build2/filesystem.cxx
index 5e6df5f..7242347 100644
--- a/build2/filesystem.cxx
+++ b/build2/filesystem.cxx
@@ -4,6 +4,7 @@
 
 #include <build2/filesystem.hxx>
 
+#include <build2/context.hxx>
 #include <build2/diagnostics.hxx>
 
 using namespace std;
@@ -11,15 +12,18 @@ using namespace butl;
 
 namespace build2
 {
-  bool
+  void
   touch (const path& p, bool create, uint16_t v)
   {
     if (verb >= v)
       text << "touch " << p;
 
+    if (dry_run)
+      return;
+
     try
     {
-      return touch_file (p, create);
+      touch_file (p, create);
     }
     catch (const system_error& e)
     {
@@ -112,7 +116,11 @@ namespace build2
 
     try
     {
-      rs = try_rmsymlink (p, d);
+      rs = dry_run
+        ? (butl::entry_exists (p)
+           ? rmfile_status::success
+           : rmfile_status::not_exist)
+        : try_rmsymlink (p, d);
     }
     catch (const system_error& e)
     {
@@ -140,13 +148,16 @@ namespace build2
     if (verb >= v)
       text << "rmdir -r " << d;
 
-    try
-    {
-      butl::rmdir_r (d, dir);
-    }
-    catch (const system_error& e)
+    if (!dry_run)
     {
-      fail << "unable to remove directory " << d << ": " << e;
+      try
+      {
+        butl::rmdir_r (d, dir);
+      }
+      catch (const system_error& e)
+      {
+        fail << "unable to remove directory " << d << ": " << e;
+      }
     }
 
     return rmdir_status::success;
diff --git a/build2/filesystem.hxx b/build2/filesystem.hxx
index ed99685..2044141 100644
--- a/build2/filesystem.hxx
+++ b/build2/filesystem.hxx
@@ -12,6 +12,11 @@
 
 // Higher-level filesystem utilities built on top of <libbutl/filesystem.mxx>.
 //
+// Compared to the libbutl's versions, these handle errors and issue
+// diagnostics. Some of them also print the corresponding command line
+// equivalent at the specified verbosity level. Note that most of such
+// functions also handle the dry_run flag.
+//
 namespace build2
 {
   using butl::auto_rmfile;
@@ -30,12 +35,12 @@ namespace build2
     explicit operator bool () const {return v == T::success;}
   };
 
-  // Set the file access and modification times to the current time printing
-  // the standard diagnostics starting from the specified verbosity level. If
-  // the file does not exist and create is true, create it and fail otherwise.
-  // Return true if the file was created and false otherwise.
+  // Set the file access and modification times (unless dry-run) to the
+  // current time printing the standard diagnostics starting from the
+  // specified verbosity level. If the file does not exist and create is true,
+  // create it and fail otherwise.
   //
-  bool
+  void
   touch (const path&, bool create, uint16_t verbosity = 1);
 
   // Return the modification time for an existing regular file and
@@ -51,12 +56,16 @@ namespace build2
     return mtime (p.string ().c_str ());
   }
 
-  // Create the directory and print the standard diagnostics starting from
-  // the specified verbosity level.
+  // Create the directory and print the standard diagnostics starting from the
+  // specified verbosity level.
   //
-  // Note that this implementation is not suitable if it is expected that the
-  // directory will exist in the majority of cases and performance is
-  // important. See the fsdir{} rule for details.
+  // Note that these functions ignore the dry_run flag (we might need to save
+  // something in such a directory, such as depdb, ignoring dry_run). Overall,
+  // it feels like we should establish the structure even for dry-run.
+  //
+  // Note that the implementation may not be suitable if the performance is
+  // important and it is expected that the directory will exist in most cases.
+  // See the fsdir{} rule for details.
   //
   using mkdir_status = butl::mkdir_status;
 
@@ -66,10 +75,10 @@ namespace build2
   fs_status<mkdir_status>
   mkdir_p (const dir_path&, uint16_t verbosity = 1);
 
-  // Remove the file and print the standard diagnostics starting from the
-  // specified verbosity level. The second argument is only used in
-  // diagnostics, to print the target name. Passing the path for target will
-  // result in the relative path being printed.
+  // Remove the file (unless dry-run) and print the standard diagnostics
+  // starting from the specified verbosity level. The second argument is only
+  // used in diagnostics, to print the target name. Passing the path for
+  // target will result in the relative path being printed.
   //
   using rmfile_status = butl::rmfile_status;
 
@@ -89,6 +98,8 @@ namespace build2
     return rmfile (f, f, verbosity);
   }
 
+  // Similar to rmfile() but for symlinks.
+  //
   fs_status<rmfile_status>
   rmsymlink (const path&, bool dir, uint16_t verbosity);
 
@@ -112,10 +123,10 @@ namespace build2
     return rmdir (d, d, verbosity);
   }
 
-  // Remove the directory recursively and print the standard diagnostics
-  // starting from the specified verbosity level. Note that this function
-  // returns not_empty if we try to remove a working directory. If the dir
-  // argument is false, then the directory itself is not removed.
+  // Remove the directory recursively (unless dry-run) and print the standard
+  // diagnostics starting from the specified verbosity level. Note that this
+  // function returns not_empty if we try to remove a working directory. If
+  // the dir argument is false, then the directory itself is not removed.
   //
   // @@ Collides (via ADL) with butl::rmdir_r(), which sucks.
   //
diff --git a/build2/filesystem.txx b/build2/filesystem.txx
index fb224b6..919a26e 100644
--- a/build2/filesystem.txx
+++ b/build2/filesystem.txx
@@ -4,7 +4,7 @@
 
 #include <type_traits> // is_base_of
 
-#include <build2/context.hxx>     // work
+#include <build2/context.hxx>
 #include <build2/diagnostics.hxx>
 
 namespace build2
@@ -34,7 +34,9 @@ namespace build2
 
     try
     {
-      rs = try_rmfile (f);
+      rs = dry_run
+        ? file_exists (f) ? rmfile_status::success : rmfile_status::not_exist
+        : try_rmfile (f);
     }
     catch (const system_error& e)
     {
@@ -54,9 +56,6 @@ namespace build2
   {
     using namespace butl;
 
-    bool w (work.sub (d)); // Don't try to remove working directory.
-    rmdir_status rs;
-
     // We don't want to print the command if we couldn't remove the directory
     // because it does not exist (just like we don't print mkdir if it already
     // exists) or if it is not empty. This makes the below code a bit ugly.
@@ -72,9 +71,13 @@ namespace build2
       }
     };
 
+    bool w (false); // Don't try to remove working directory.
+    rmdir_status rs;
     try
     {
-      rs = !w ? try_rmdir (d) : rmdir_status::not_empty;
+      rs = dry_run
+        ? dir_exists (d) ? rmdir_status::success : rmdir_status::not_exist
+        : !(w = work.sub (d)) ? try_rmdir (d) : rmdir_status::not_empty;
     }
     catch (const system_error& e)
     {
diff --git a/build2/in/rule.cxx b/build2/in/rule.cxx
index 2f8094d..a6d4f2c 100644
--- a/build2/in/rule.cxx
+++ b/build2/in/rule.cxx
@@ -179,6 +179,9 @@ namespace build2
       // For now we assume this is ok since this is probably not very common
       // and it makes the overall logic simpler.
       //
+      // Note also that because updating the depdb essentially requires
+      // performing the substitutions, this rule ignored the dry-run mode.
+      //
       size_t dd_skip (0); // Number of "good" variable lines.
 
       if (update)
@@ -260,7 +263,7 @@ namespace build2
       else if (verb)
         text << program_ << ' ' << ip;
 
-      // Read and process the file, one line at a time.
+      // Read and process the file, one line at a time, while updating depdb.
       //
       const char* what;
       const path* whom;
@@ -278,9 +281,11 @@ namespace build2
         if (t.is_a<exe> ())
           prm |= permissions::xu | permissions::xg | permissions::xo;
 
-        // Remove the existing file to make sure permissions take effect.
+        // Remove the existing file to make sure permissions take effect. If
+        // this fails then presumable writing to it will fail as well and we
+        // will complain there.
         //
-        rmfile (tp, 3 /* verbosity */);
+        try_rmfile (tp, true /* ignore_error */);
 
         what = "open"; whom = &tp;
         ofdstream ofs (fdopen (tp,
diff --git a/build2/in/rule.hxx b/build2/in/rule.hxx
index 001fc66..b3430c5 100644
--- a/build2/in/rule.hxx
+++ b/build2/in/rule.hxx
@@ -19,6 +19,9 @@ namespace build2
     // Note that a derived rule can use the target data pad to cache data
     // (e.g., in match()) to be used in substitute/lookup() calls.
     //
+    // Note also that currently this rule ignores the dry-run mode (see
+    // perform_update() for the rationale).
+    //
     class rule: public build2::rule
     {
     public:
diff --git a/build2/operation.cxx b/build2/operation.cxx
index 74db2f3..9bc8a4e 100644
--- a/build2/operation.cxx
+++ b/build2/operation.cxx
@@ -269,6 +269,10 @@ namespace build2
 
     phase_lock pl (run_phase::execute); // Never switched.
 
+    // Set the dry-run flag.
+    //
+    dry_run = ops.dry_run ();
+
     // Setup progress reporting if requested.
     //
     string what; // Note: must outlive monitor_guard.
@@ -338,6 +342,10 @@ namespace build2
 
     sched.tune (0); // Restore original scheduler settings.
 
+    // Clear the dry-run flag.
+    //
+    dry_run = false;
+
     // Clear the progress if present.
     //
     if (mg)
diff --git a/build2/rule.cxx b/build2/rule.cxx
index 2a10a90..79b91b3 100644
--- a/build2/rule.cxx
+++ b/build2/rule.cxx
@@ -196,6 +196,8 @@ namespace build2
         text << "mkdir " << t;
     };
 
+    // Note: ignoring the dry_run flag.
+    //
     mkdir_status ms;
 
     try
diff --git a/build2/rule.hxx b/build2/rule.hxx
index b6c0154..a9bc178 100644
--- a/build2/rule.hxx
+++ b/build2/rule.hxx
@@ -58,6 +58,9 @@ namespace build2
     static const alias_rule instance;
   };
 
+  // Note that this rule ignores the dry_run flag; see mkdir() in filesystem
+  // for the rationale.
+  //
   class fsdir_rule: public rule
   {
   public: