usermod: Add `-a` and `-r` options to append or remove extra groups

Base/usr/share/man/man8/usermod.md

@@ -5,7 +5,7 @@ usermod - modify a user account
 ## Synopsis
 
 ```sh
-$ usermod [--uid uid] [--gid group] [--groups groups] [--lock] [--unlock] [--home new-home] [--move] [--shell path-to-shell] [--gecos general-info] <username>
+$ usermod [--append] [--uid uid] [--gid group] [--groups groups] [--lock] [--remove] [--unlock] [--home new-home] [--move] [--shell path-to-shell] [--gecos general-info] <username>
 ```
 
 ## Description
@@ -17,10 +17,12 @@ This program must be run as root.
 
 * `--help`: Display help message and exit
 * `--version`: Print version
+* `-a`, `--append`: Append the supplementary groups specified with the -G option to the user
 * `-u uid`, `--uid uid`: The new numerical value of the user's ID
 * `-g group`, `--gid group`: The group name or number of the user's new initial login group
 * `-G groups`, `--groups groups`: Set the user's supplementary groups. Groups are specified with a comma-separated list. Group names or numbers may be used
 * `-L`, `--lock`: Lock password
+* `-r`, `--remove`: Remove the supplementary groups specified with the -G option from the user
 * `-U`, `--unlock`: Unlock password
 * `-d new-home`, `--home new-home`: The user's new login directory
 * `-m`, `--move`: Move the content of the user's home directory to the new location

Userland/Utilities/usermod.cpp

@@ -49,8 +49,10 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
     TRY(Core::System::unveil("/etc", "rwc"));
 
     uid_t uid = 0;
+    bool append_extra_gids = false;
     Optional<gid_t> gid;
     bool lock = false;
+    bool remove_extra_gids = false;
     bool unlock = false;
     StringView new_home_directory;
     bool move_home = false;
@@ -61,6 +63,7 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
 
     auto args_parser = Core::ArgsParser();
     args_parser.set_general_help("Modify a user account");
+    args_parser.add_option(append_extra_gids, "Append the supplementary groups specified with the -G option to the user", "append", 'a');
     args_parser.add_option(uid, "The new numerical value of the user's ID", "uid", 'u', "uid");
     args_parser.add_option(Core::ArgsParser::Option {
         .argument_mode = Core::ArgsParser::OptionArgumentMode::Required,
@@ -91,6 +94,7 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
         },
     });
     args_parser.add_option(lock, "Lock password", "lock", 'L');
+    args_parser.add_option(remove_extra_gids, "Remove the supplementary groups specified with the -G option from the user", "remove", 'r');
     args_parser.add_option(unlock, "Unlock password", "unlock", 'U');
     args_parser.add_option(new_home_directory, "The user's new login directory", "home", 'd', "new-home");
     args_parser.add_option(move_home, "Move the content of the user's home directory to the new location", "move", 'm');
@@ -100,6 +104,18 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
 
     args_parser.parse(arguments);
 
+    if (extra_gids.is_empty() && (append_extra_gids || remove_extra_gids)) {
+        warnln("The -a and -r options can only be used with the -G option");
+        args_parser.print_usage(stderr, arguments.strings[0]);
+        return 1;
+    }
+
+    if (append_extra_gids && remove_extra_gids) {
+        warnln("The -a and -r options are mutually exclusive");
+        args_parser.print_usage(stderr, arguments.strings[0]);
+        return 1;
+    }
+
     if (lock && unlock) {
         warnln("The -L and -U options are mutually exclusive");
         args_parser.print_usage(stderr, arguments.strings[0]);
@@ -178,7 +194,20 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
         target_account.set_gecos(gecos);
     }
 
-    if (!extra_gids.is_empty()) {
+    if (append_extra_gids) {
+        for (auto gid : target_account.extra_gids())
+            extra_gids.append(gid);
+    }
+
+    if (remove_extra_gids) {
+        Vector<gid_t> current_extra_gids = target_account.extra_gids();
+        for (auto gid : extra_gids)
+            current_extra_gids.remove_all_matching([gid](auto current_gid) { return current_gid == gid; });
+
+        extra_gids = move(current_extra_gids);
+    }
+
+    if (!extra_gids.is_empty() || remove_extra_gids) {
         target_account.set_extra_gids(extra_gids);
     }