Userland: Added flags to id command (#253)

Fixes #158.

Userland/id.cpp

@@ -1,18 +1,102 @@
 #include <alloca.h>
+#include <getopt.h>
 #include <grp.h>
 #include <pwd.h>
 #include <stdio.h>
 #include <unistd.h>
 
-extern "C" int main(int, char**);
+static int print_id_objects();
+
+static bool flag_print_uid = false;
+static bool flag_print_gid = false;
+static bool flag_print_name = false;
+static bool flag_print_gid_all = false;
 
 int main(int argc, char** argv)
 {
-    (void)argc;
+    static const char* valid_option_characters = "ugGn";
-    (void)argv;
+    int opt;
+    while ((opt = getopt(argc, argv, valid_option_characters)) != -1) {
+        switch (opt) {
+        case 'u':
+            flag_print_uid = true;
+            break;
+        case 'g':
+            flag_print_gid = true;
+            break;
+        case 'G':
+            flag_print_gid_all = true;
+            break;
+        case 'n':
+            flag_print_name = true;
+            break;
+
+        default:
+            fprintf(stderr, "usage: ls [-%s] [paths...]\n", valid_option_characters);
+            return 1;
+        }
+    }
+
+    if (flag_print_uid + flag_print_gid + flag_print_gid_all > 1) {
+        fprintf(stderr, "cannot print \"only\" of more than one choice\n");
+        return 1;
+    }
+
+    int status = print_id_objects();
+    return status;
+}
+
+bool print_uid_object(uid_t uid)
+{
+    if (flag_print_name) {
+        struct passwd* pw = getpwuid(uid);
+        printf("%s", pw ? pw->pw_name : "n/a");
+    } else
+        printf("%u", uid);
+
+    return true;
+}
+
+bool print_gid_object(gid_t gid)
+{
+    if (flag_print_name) {
+        struct group* gr = getgrgid(gid);
+        printf("%s", gr ? gr->gr_name : "n/a");
+    } else
+        printf("%u", gid);
+    return true;
+}
+
+bool print_gid_list()
+{
+    int extra_gid_count = getgroups(0, nullptr);
+    if (extra_gid_count) {
+        auto* extra_gids = (gid_t*)alloca(extra_gid_count * sizeof(gid_t));
+        int rc = getgroups(extra_gid_count, extra_gids);
+
+        if (rc < 0) {
+            perror("\ngetgroups");
+            return false;
+        }
+
+        for (int g = 0; g < extra_gid_count; ++g) {
+            auto* gr = getgrgid(extra_gids[g]);
+            if (flag_print_name && gr)
+                printf("%s", gr->gr_name);
+            else
+                printf("%u", extra_gids[g]);
+            if (g != extra_gid_count - 1)
+                printf(" ");
+        }
+    }
+    return true;
+}
+
+bool print_full_id_list()
+{
+
     uid_t uid = getuid();
     gid_t gid = getgid();
-
     struct passwd* pw = getpwuid(uid);
     struct group* gr = getgrgid(gid);
 
@@ -24,7 +108,7 @@ int main(int argc, char** argv)
         int rc = getgroups(extra_gid_count, extra_gids);
         if (rc < 0) {
             perror("\ngetgroups");
-            return 1;
+            return false;
         }
         printf(", groups=");
         for (int g = 0; g < extra_gid_count; ++g) {
@@ -37,6 +121,25 @@ int main(int argc, char** argv)
                 printf(",");
         }
     }
+    return true;
+}
+
+int print_id_objects()
+{
+    if (flag_print_uid) {
+        if (!print_uid_object(getuid()))
+            return 1;
+    } else if (flag_print_gid) {
+        if (!print_gid_object(getgid()))
+            return 1;
+    } else if (flag_print_gid_all) {
+        if (!print_gid_list())
+            return 1;
+    } else {
+        if (!print_full_id_list())
+            return 1;
+    }
+
     printf("\n");
     return 0;
 }