ps: Add `--ppid` option to filter by parent PID

Base/usr/share/man/man1/ps.md

@@ -5,7 +5,7 @@ ps - list currently running processes
 ## Synopsis
 
 ```**sh
-$ ps [--version] [-a] [-A] [-e] [-f] [-p pid-list] [-q pid-list] [-t tty-list] [-u user-list]
+$ ps [--version] [-a] [-A] [-e] [-f] [-p pid-list] [--ppid pid-list] [-q pid-list] [-t tty-list] [-u user-list]
 ```
 
 ## Description
@@ -19,6 +19,7 @@ For each process, print its PID (process ID), to which TTY it belongs, and invok
 * `-A` or `-e`: Consider all processes, not just those in the current TTY.
 * `-f`: Also print for each process: UID (as resolved username), PPID (parent PID), and STATE (Runnable, Sleeping, Selecting, Reading, etc.)
 * `-p pid-list`: Select processes matching any of the given PIDs. `pid-list` is a list of PIDs, separated by commas or spaces.
+* `--ppid pid-list`: Select processes whose PPID matches any of the given PIDs. `pid-list` is a list of PIDs, separated by commas or spaces.
 * `-q pid-list`: Only consider the given PIDs, if they exist. Output the processes in the order provided by `pid-list`. `pid-list` is a list of PIDs, separated by commas or spaces.
 * `-t tty-list`: Select processes associated with any of the given terminals. `tty-list` is a list of short TTY names (e.g: `pts:0`) or the full TTY device paths, separated by commas or spaces.
 * `-u user-list`: Select processes matching any of the given UIDs. `user-list` is a list of UIDs or login names, separated by commas or spaces.

Userland/Utilities/ps.cpp

@@ -121,6 +121,7 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
     bool provided_filtering_option = false;
     bool provided_quick_pid_list = false;
     Vector<pid_t> pid_list;
+    Vector<pid_t> parent_pid_list;
     Vector<DeprecatedString> tty_list;
     Vector<uid_t> uid_list;
 
@@ -136,6 +137,13 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
             warnln("Could not parse '{}' as a PID.", pid_string);
         return pid;
     }));
+    args_parser.add_option(make_list_option(parent_pid_list, "Show processes with a matching PPID. (Comma- or space-separated list.)", "ppid", {}, "pid-list", [&](StringView pid_string) {
+        provided_filtering_option = true;
+        auto pid = pid_string.to_int();
+        if (!pid.has_value())
+            warnln("Could not parse '{}' as a PID.", pid_string);
+        return pid;
+    }));
     args_parser.add_option(make_list_option(pid_list, "Show processes with a matching PID. (Comma- or space-separated list.) Processes will be listed in the order given.", nullptr, 'q', "pid-list", [&](StringView pid_string) {
         provided_quick_pid_list = true;
         auto pid = pid_string.to_int();
@@ -217,6 +225,7 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
             // Default is to show processes from the current TTY
             if ((!provided_filtering_option && process.tty == this_pseudo_tty_name.bytes_as_string_view())
                 || (!pid_list.is_empty() && pid_list.contains_slow(process.pid))
+                || (!parent_pid_list.is_empty() && parent_pid_list.contains_slow(process.ppid))
                 || (!uid_list.is_empty() && uid_list.contains_slow(process.uid))
                 || (!tty_list.is_empty() && tty_list.contains_slow(process.tty))
                 || (every_terminal_process_flag && !process.tty.is_empty())) {