Added all this in pure Lua. ;)

lua/chkgrp-native-min-extended.lua

@@ -0,0 +1,89 @@
+#!/usr/bin/env lua
+
+-- This file is part of chkgrp that is a program that checks if a user is a
+-- member of a group.
+--
+-- Author: Johannes Findeisen <you@hanez.org> - 2025
+-- Homepage: https://git.xw3.org/hanez/chkgrp
+-- License: Apache-2.0 (see LICENSE)
+
+local MAX_NAME = 256
+
+local function read_passwd_gid(username)
+    local f = io.open("/etc/passwd", "r")
+    if not f then return nil end
+
+    for line in f:lines() do
+        local fields = {}
+        for field in string.gmatch(line, "([^:]+)") do
+            table.insert(fields, field)
+        end
+        if fields[1] == username then
+            f:close()
+            return tonumber(fields[4]) -- GID is 4th field
+        end
+    end
+
+    f:close()
+    return nil
+end
+
+local function check_group_membership(username, groupname, user_gid)
+    local f = io.open("/etc/group", "r")
+    if not f then return nil end
+
+    for line in f:lines() do
+        local fields = {}
+        for field in string.gmatch(line, "([^:]+)") do
+            table.insert(fields, field)
+        end
+        if fields[1] == groupname then
+            local group_gid = tonumber(fields[3])
+            if group_gid == user_gid then
+                f:close()
+                return true
+            end
+
+            for member in string.gmatch(fields[4] or "", "[^,]+") do
+                if member == username then
+                    f:close()
+                    return true
+                end
+            end
+
+            f:close()
+            return false
+        end
+    end
+
+    f:close()
+    return nil
+end
+
+local username = arg[1]
+local groupname = arg[2]
+
+if not username or not groupname then
+    os.exit(2)
+end
+
+if #username > MAX_NAME or #groupname > MAX_NAME then
+    os.exit(2)
+end
+
+local user_gid = read_passwd_gid(username)
+if not user_gid then
+    os.exit(2)
+end
+
+local result = check_group_membership(username, groupname, user_gid)
+if result == nil then
+    os.exit(2)
+elseif result == true then
+    print("Yes")
+    os.exit(0)
+else
+    print("No")
+    os.exit(1)
+end
+