implement -restart2info command line flag and document it

doc/lammps.1

@@ -226,6 +226,20 @@ arguments of the "dump" command. See the
 .B LAMMPS
 manual for details on either of the two commands.
 .TP
+\fB\-r2info <restart file> <keyword> ...\fR or
+\fB\-restart2info <restart file> <keyword> ...\fR
+Write infomation about the <restart file> previously written by
+.B LAMMPS
+to the screen and immediately exit.  Following <restart file>
+argument, additional keywords for the
+.B LAMMPS
+"info" command may be added to increase the amount of information
+written. By default "system" "group" "fix" "compute" are already
+set. See the
+.B LAMMPS
+manual for details on either of the two commands.
+.TP
+.TP
 \fB\-sc <file name>\fR or \fB\-screen <file name>\fR
 Specify a file for
 .B LAMMPS

doc/src/Run_options.rst

@@ -22,6 +22,7 @@ letter abbreviation can be used:
 * :ref:`-ro or -reorder <reorder>`
 * :ref:`-r2data or -restart2data <restart2data>`
 * :ref:`-r2dump or -restart2dump <restart2dump>`
+* :ref:`-r2info or -restart2info <restart2info>`
 * :ref:`-sc or -screen <screen>`
 * :ref:`-sr or skiprun <skiprun>`
 * :ref:`-sf or -suffix <suffix>`
@@ -503,6 +504,36 @@ e.g. the *nfile* and *fileper* keywords.  See the
 
 ----------
 
+.. _restart2info:
+
+**-restart2info restartfile keyword ...**
+
+.. versionadded:: TBD
+
+Write out some info about the restart file and and immediately exit.  This
+is the same operation as if the following 2-line input script were
+run:
+
+.. code-block:: LAMMPS
+
+   read_restart restartfile
+   info system group compute fix keyword ...
+
+The specified restartfile name may contain the wild-card character "\*".
+The restartfile name may also contain the wild-card character "%".  The
+meaning of these characters is explained on the :doc:`read_restart
+<read_restart>` documentation.  The use of "%" means that a parallel
+restart file can be read.  Note that a filename such as file.\* may need
+to be enclosed in quotes or the "\*" character prefixed with a backslash
+("\") to avoid shell expansion of the "\*" character.
+
+The :doc:`info command <info>` is already followed by the most useful
+keywords, but additional keywords like *coeffs* or *comm* may be
+appended to write out more details about the information stored in the
+restart file.
+
+----------
+
 .. _screen:
 
 **-screen file**

doc/src/read_restart.rst

@@ -47,6 +47,12 @@ Restart files are saved in binary format to enable exact restarts,
 meaning that the trajectories of a restarted run will precisely match
 those produced by the original run had it continued on.
 
+Some information about a restart file can be gathered directly from the
+command line when using LAMMPS with the :ref:`-restart2info
+<restart2info>` command line flag.  On Unix-like operating systems (like
+Linux or macOS), one can also :ref:`configure the "file" command line
+program <magic>` to display basic information about a restart file
+
 The binary restart file format was not designed with backward, forward,
 or cross-platform compatibility in mind, so the files are only expected
 to be read correctly by the same LAMMPS executable on the same platform.

src/info.cpp

@@ -261,7 +261,7 @@ void Info::command(int narg, char **arg)
         ++idx;
       }
     } else {
-      error->warning(FLERR,"Ignoring unknown or incorrect info command flag");
+      error->warning(FLERR,"Ignoring unknown or incorrect info command flag: {}",arg[idx]);
       ++idx;
     }
   }

src/lammps.cpp

@@ -210,6 +210,7 @@ LAMMPS::LAMMPS(int narg, char **arg, MPI_Comm communicator) :
   int kokkosflag = 0;
   int restart2data = 0;
   int restart2dump = 0;
+  int restart2info = 0;
   int restartremap = 0;
   int citeflag = 1;
   int citescreen = CiteMe::TERSE;
@@ -383,6 +384,8 @@ LAMMPS::LAMMPS(int narg, char **arg, MPI_Comm communicator) :
         error->universe_all(FLERR,"Invalid command-line argument");
       if (restart2dump)
         error->universe_all(FLERR, "Cannot use both -restart2data and -restart2dump");
+      if (restart2info)
+        error->universe_all(FLERR, "Cannot use both -restart2data and -restart2info");
       restart2data = 1;
       restartfile = arg[iarg+1];
       inflag = -1;               // skip inflag check
@@ -405,6 +408,8 @@ LAMMPS::LAMMPS(int narg, char **arg, MPI_Comm communicator) :
         error->universe_all(FLERR,"Invalid command-line argument");
       if (restart2data)
         error->universe_all(FLERR, "Cannot use both -restart2data and -restart2dump");
+      if (restart2info)
+        error->universe_all(FLERR, "Cannot use both -restart2dump and -restart2info");
       restart2dump = 1;
       restartfile = arg[iarg+1];
       inflag = -1;               // skip inflag check
@@ -421,6 +426,23 @@ LAMMPS::LAMMPS(int narg, char **arg, MPI_Comm communicator) :
       while (iarg < narg && arg[iarg][0] != '-') iarg++;
       wlast = iarg;
 
+    } else if (strcmp(arg[iarg],"-restart2info") == 0 ||
+               strcmp(arg[iarg],"-r2info") == 0) {
+      if (iarg+2 > narg)
+        error->universe_all(FLERR,"Invalid command-line argument");
+      if (restart2data)
+        error->universe_all(FLERR, "Cannot use both -restart2data and -restart2info");
+      if (restart2dump)
+        error->universe_all(FLERR, "Cannot use both -restart2dump and -restart2info");
+      restart2info = 1;
+      restartfile = arg[iarg+1];
+      inflag = -1;               // skip inflag check
+      iarg += 2;
+      // delimit args for the write_data command
+      wfirst = iarg;
+      while (iarg < narg && arg[iarg][0] != '-') iarg++;
+      wlast = iarg;
+
     } else if (strcmp(arg[iarg],"-screen") == 0 ||
                strcmp(arg[iarg],"-sc") == 0) {
       if (iarg+2 > narg)
@@ -727,15 +749,17 @@ LAMMPS::LAMMPS(int narg, char **arg, MPI_Comm communicator) :
   // add args between wfirst and wlast to write_data or write_data command
   // add "noinit" to write_data to prevent a system init
 
-  if (restart2data || restart2dump) {
+  if (restart2data || restart2dump || restart2info) {
     std::string cmd = fmt::format("read_restart {}",restartfile);
     if (restartremap) cmd += " remap\n";
     input->one(cmd);
     if (restart2data) cmd = "write_data ";
-    else cmd = "write_dump";
+    else if (restart2dump) cmd = "write_dump";
+    else cmd = "info system group compute fix";
     for (iarg = wfirst; iarg < wlast; iarg++)
        cmd += fmt::format(" {}", arg[iarg]);
-    cmd += " noinit";
+    if (restart2data || restart2dump)
+      cmd += " noinit";
     input->one(cmd);
     error->done(0);
   }
@@ -1260,6 +1284,7 @@ void _noopt LAMMPS::help()
           "-restart2data rfile dfile ... : convert restart to data file (-r2data)\n"
           "-restart2dump rfile dgroup dstyle dfile ... \n"
           "                            : convert restart to dump file (-r2dump)\n"
+          "-restart2info rfile         : print info about restart rfile (-r2info)\n"
           "-reorder topology-specs     : processor reordering (-r)\n"
           "-screen none/filename       : where to send screen output (-sc)\n"
           "-skiprun                    : skip loops in run and minimize (-sr)\n"