add simple id implementation

programs/CMakeLists.txt

@@ -1,5 +1,6 @@
 # List of programs.
 set(PROGRAM_LIST
+    id.c
     stat.c
     false.c
     nice.c

programs/id.c

@@ -0,0 +1,30 @@
+/// @file id.c
+/// @brief
+/// @copyright (c) 2024 This file is distributed under the MIT License.
+/// See LICENSE.md for details.
+
+#include <sys/unistd.h>
+#include <stdio.h>
+#include <string.h>
+
+int main(int argc, char **argv)
+{
+    if (argc == 1) {
+        printf("uid=%d gid=%d\n", getuid(), getgid());
+    } else if (strncmp(argv[1], "--help", 6) == 0) {
+        printf("Usage: %s [OPTION]\n", argv[0]);
+        printf("Print user and group information\n\n");
+        printf("  -g, --group  print only the effective group ID");
+        printf("  -u, --user   print only the effective user ID");
+        printf("      --help   display this help and exit");
+    } else if (strncmp(argv[1], "-u", 2) == 0 || strncmp(argv[1], "--user", 6) == 0) {
+        printf("%d\n", getuid());
+    } else if (strncmp(argv[1], "-g", 2) == 0 || strncmp(argv[1], "--group", 7) == 0) {
+        printf("%d\n", getgid());
+    } else {
+        printf("%s: invalid option '%s'\n", argv[0], argv[1]);
+        printf("Try '%s --help' for more information\n", argv[0]);
+    }
+
+    return 0;
+}