Implement $VAR expansion in commands (issue #25)

Extend read_command_body() to detect $NAME and trigger interpolation
using the same fragment-split mechanism as ${expr}. The lexer collects
the identifier into cmd_envvar; the parser's unified parseinterp()
branches on it to emit tostring(getenv(NAME)).

lparser.c

@@ -553,23 +553,35 @@ static void singlevar (LexState *ls, expdesc *var) {
 }
 
 
+static void codeenvget (LexState *ls, expdesc *v, TString *name);
+
 /*
-** Helper: parse a single ${expr} interpolation fragment.
-** Emits tostring(expr), reads the next command continuation fragment,
-** and pushes it as a string constant.  Returns with the continuation
-** token already set (TK_COMMAND or TK_INTERACTIVE).
+** Parse a single interpolation fragment in a command: either ${expr}
+** or $NAME.  Emits tostring(<value>), reads the continuation fragment,
+** and pushes it as a string constant.
+** For ${expr}: advances past the command token, parses expr, checks '}'.
+** For $NAME:  the identifier was already collected by the lexer into
+**             cmd_envvar — no tokens to consume, just emit getenv(name).
 */
 static void parseinterp (LexState *ls, TString *tsname, int *nconcat) {
   FuncState *fs = ls->fs;
   expdesc interp, tostrfn;
   int tostr_base;
-  luaX_next(ls);  /* advance past TK_COMMAND / TK_INTERACTIVE */
-  /* emit tostring(expr) */
+  TString *envname = ls->cmd_envvar;
+  ls->cmd_envvar = NULL;
   buildglobal(ls, tsname, &tostrfn);
   luaK_exp2nextreg(fs, &tostrfn);
   tostr_base = tostrfn.u.info;
-  expr(ls, &interp);
-  check(ls, '}');
+  if (envname != NULL) {
+    /* $NAME — emit getenv(name) */
+    codeenvget(ls, &interp, envname);
+  }
+  else {
+    /* ${expr} — parse the expression */
+    luaX_next(ls);  /* advance past TK_COMMAND / TK_INTERACTIVE */
+    expr(ls, &interp);
+    check(ls, '}');
+  }
   luaK_exp2nextreg(fs, &interp);
   luaK_codeABC(fs, OP_CALL, tostr_base, 2, 2);
   fs->freereg = cast_byte(tostr_base + 1);