Add shell aliases via lush.aliases table (issue #28)

Aliases rewrite the command string before pipeline splitting, so alias
values may contain pipes (e.g. lush.aliases.foo = "echo hi |"). Expansion
happens once per dispatch to prevent recursion.

lcmd.c

@@ -897,6 +897,67 @@ static int try_builtin (lua_State *L, ParsedArgs *pa) {
 }
 
 
+/* ===== alias expansion ===== */
+
+/*
+** Extract the first whitespace-delimited word from cmd, look it up in
+** lush.aliases. If found, replace the command string on the Lua stack
+** (position 1) with alias_value + remaining_text, and update *cmd.
+** Expansion happens on the raw string, before pipeline splitting,
+** so alias values may contain pipes.
+** Returns 1 if expanded, 0 if no alias.
+*/
+static int expand_alias (lua_State *L, const char **cmd) {
+  const char *s = *cmd;
+  const char *rest;
+  size_t wordlen;
+  const char *alias;
+  size_t alias_len, rest_len, total;
+  char *expanded;
+
+  /* skip leading whitespace */
+  while (*s == ' ' || *s == '\t') s++;
+  if (*s == '\0') return 0;
+
+  /* find end of first word */
+  rest = s;
+  while (*rest && *rest != ' ' && *rest != '\t') rest++;
+  wordlen = (size_t)(rest - s);
+
+  /* look up in lush.aliases */
+  lua_rawgeti(L, LUA_REGISTRYINDEX, LUA_RIDX_LUSH);
+  lua_getfield(L, -1, "aliases");
+  lua_pushlstring(L, s, wordlen);
+  lua_gettable(L, -2);
+
+  if (!lua_isstring(L, -1)) {
+    lua_pop(L, 3);
+    return 0;
+  }
+
+  alias = lua_tolstring(L, -1, &alias_len);
+  rest_len = strlen(rest);  /* includes leading space if any */
+
+  total = alias_len + rest_len;
+  expanded = (char *)malloc(total + 1);
+  if (expanded == NULL) {
+    lua_pop(L, 3);
+    return 0;
+  }
+  memcpy(expanded, alias, alias_len);
+  memcpy(expanded + alias_len, rest, rest_len);
+  expanded[total] = '\0';
+  lua_pop(L, 3);
+
+  /* replace command on Lua stack */
+  lua_pushstring(L, expanded);
+  lua_replace(L, 1);
+  free(expanded);
+  *cmd = lua_tostring(L, 1);
+  return 1;
+}
+
+
 /* ===== lushCmd_command ===== */
 
 int lushCmd_command (lua_State *L) {
@@ -910,6 +971,9 @@ int lushCmd_command (lua_State *L) {
   DynBuf buf_out, buf_err;
   struct sigaction sa_old, sa_new;
 
+  /* expand alias before pipeline splitting */
+  expand_alias(L, &cmd);
+
   /* try to split into pipeline stages */
   if (split_pipeline(cmd, stages, &nstages) != 0)
     return luaL_error(L, "invalid pipeline syntax in command");
@@ -1076,6 +1140,9 @@ int lushCmd_interactive (lua_State *L) {
   struct sigaction sa_old_pipe, sa_old_int, sa_old_quit, sa_new;
   DynBuf buf_out, buf_err;
 
+  /* expand alias before pipeline splitting */
+  expand_alias(L, &cmd);
+
   /* try to split into pipeline stages */
   if (split_pipeline(cmd, stages, &nstages) != 0)
     return luaL_error(L, "invalid pipeline syntax in command");
@@ -1217,6 +1284,9 @@ static const luaL_Reg lushlib[] = {
 
 LUAMOD_API int luaopen_lush (lua_State *L) {
   luaL_newlib(L, lushlib);
+  /* create aliases subtable */
+  lua_createtable(L, 0, 4);
+  lua_setfield(L, -2, "aliases");
   /* intern function name strings for OP_LUSH VM access */
   lushname[LUSH_OP_COMMAND]     = luaS_new(L, "command");
   lushname[LUSH_OP_INTERACTIVE] = luaS_new(L, "interactive");