unix/main: Add command-line -h option for printing help text.

This adds a -h option to print the usage help text and adds a new, shorter
error message that is printed when invalid arguments are given.  This
behaviour follows CPython (and other tools) more closely.

docs/unix/quickref.rst

@@ -8,7 +8,7 @@ Command line options
 
 Usage::
 
-    micropython [ -i ] [ -O<level> ] [ -v ] [ -X <option> ] [ -c <command> | -m <module> | <script> ] [ <args> ]
+    micropython [ -h ] [ -i ] [ -O<level> ] [ -v ] [ -X <option> ] [ -c <command> | -m <module> | <script> ] [ <args> ]
 
 
 Invocation options:
@@ -38,6 +38,10 @@ interactive REPL mode.
 
 General options:
 
+.. option:: -h
+
+    Prints a help message containing the command line usage and exits.
+
 .. option:: -i
 
     Enables inspection. When this flag is set, MicroPython will enter the

ports/unix/main.c

@@ -303,10 +303,11 @@ STATIC int do_str(const char *str) {
     return execute_from_lexer(LEX_SRC_STR, str, MP_PARSE_FILE_INPUT, false);
 }
 
-STATIC int usage(char **argv) {
+STATIC void print_help(char **argv) {
     printf(
 "usage: %s [<opts>] [-X <implopt>] [-c <command> | -m <module> | <filename>]\n"
 "Options:\n"
+"-h : print this help message\n"
 "-i : enable inspection via REPL after running command/module/file\n"
 #if MICROPY_DEBUG_PRINTERS
 "-v : verbose (trace various operations); can be multiple\n"
@@ -335,7 +336,10 @@ STATIC int usage(char **argv) {
     if (impl_opts_cnt == 0) {
         printf("  (none)\n");
     }
+}
 
+STATIC int invalid_args(void) {
+    fprintf(stderr, "Invalid command line arguments. Use -h option for help.\n");
     return 1;
 }
 
@@ -343,9 +347,13 @@ STATIC int usage(char **argv) {
 STATIC void pre_process_options(int argc, char **argv) {
     for (int a = 1; a < argc; a++) {
         if (argv[a][0] == '-') {
+            if (strcmp(argv[a], "-h") == 0) {
+                print_help(argv);
+                exit(0);
+            }
             if (strcmp(argv[a], "-X") == 0) {
                 if (a + 1 >= argc) {
-                    exit(usage(argv));
+                    exit(invalid_args());
                 }
                 if (0) {
                 } else if (strcmp(argv[a + 1], "compile-only") == 0) {
@@ -394,8 +402,7 @@ STATIC void pre_process_options(int argc, char **argv) {
 #endif
                 } else {
 invalid_arg:
-                    fprintf(stderr, "Invalid option\n");
-                    exit(usage(argv));
+                    exit(invalid_args());
                 }
                 a++;
             }
@@ -563,7 +570,7 @@ MP_NOINLINE int main_(int argc, char **argv) {
                 inspect = true;
             } else if (strcmp(argv[a], "-c") == 0) {
                 if (a + 1 >= argc) {
-                    return usage(argv);
+                    return invalid_args();
                 }
                 ret = do_str(argv[a + 1]);
                 if (ret & FORCED_EXIT) {
@@ -572,7 +579,7 @@ MP_NOINLINE int main_(int argc, char **argv) {
                 a += 1;
             } else if (strcmp(argv[a], "-m") == 0) {
                 if (a + 1 >= argc) {
-                    return usage(argv);
+                    return invalid_args();
                 }
                 mp_obj_t import_args[4];
                 import_args[0] = mp_obj_new_str(argv[a + 1], strlen(argv[a + 1]));
@@ -627,7 +634,7 @@ MP_NOINLINE int main_(int argc, char **argv) {
                     for (char *p = argv[a] + 1; *p && *p == 'O'; p++, MP_STATE_VM(mp_optimise_value)++);
                 }
             } else {
-                return usage(argv);
+                return invalid_args();
             }
         } else {
             char *pathbuf = malloc(PATH_MAX);