Add environment variable access with $NAME syntax

Lexes $NAME as TK_ENVVAR, compiles reads as __getenv("NAME") calls
and writes ($NAME = expr) as __setenv("NAME", expr) calls. Runtime
functions wrap getenv/setenv/unsetenv with automatic tostring coercion.

linit.c

@@ -13,12 +13,14 @@
 
 
 #include <stddef.h>
+#include <stdlib.h>
 
 #include "lua.h"
 
 #include "lualib.h"
 #include "lauxlib.h"
 #include "llimits.h"
+#include "lcmd.h"
 
 
 /*
@@ -40,30 +42,39 @@ static const luaL_Reg stdlibs[] = {
 };
 
 
-/*
-** Stub __command function for backtick command syntax.
-** Returns a table {code=0, stdout=cmdstring, stderr=""}.
-** The real implementation will be provided by issue #03.
-*/
-static int luaB_command (lua_State *L) {
-  const char *cmd = luaL_checkstring(L, 1);
-  lua_createtable(L, 0, 3);
-  lua_pushinteger(L, 0);
-  lua_setfield(L, -2, "code");
-  lua_pushstring(L, cmd);
-  lua_setfield(L, -2, "stdout");
-  lua_pushliteral(L, "");
-  lua_setfield(L, -2, "stderr");
+static int luaB_getenv (lua_State *L) {
+  const char *name = luaL_checkstring(L, 1);
+  const char *val = getenv(name);
+  if (val == NULL)
+    lua_pushnil(L);
+  else
+    lua_pushstring(L, val);
   return 1;
 }
 
 
+static int luaB_setenv (lua_State *L) {
+  const char *name = luaL_checkstring(L, 1);
+  if (lua_isnoneornil(L, 2))
+    unsetenv(name);
+  else {
+    const char *val = luaL_tolstring(L, 2, NULL);
+    setenv(name, val, 1);
+  }
+  return 0;
+}
+
+
 /*
 ** Register shell built-in globals (called after standard libraries).
 */
 static void opencommand (lua_State *L) {
   lua_pushcfunction(L, luaB_command);
   lua_setglobal(L, "__command");
+  lua_pushcfunction(L, luaB_getenv);
+  lua_setglobal(L, "__getenv");
+  lua_pushcfunction(L, luaB_setenv);
+  lua_setglobal(L, "__setenv");
 }