#!/bin/bash

USAGE='
USAGE: rpid <pid|-h|-[-]help> [<root_name>=login]

        pid     The PID of the process for which you want to find the root PID.

        root_name
                The name (not command line) of the root process, for which
                to find the PID. Defaults to login.

EXAMPLES:

        Any of these will display this help message.

                rpid -h
                rpid -help
                rpid --help

        Get the PID of the login process that is the ancestor of the current
        process.

                rpid $$
'

# Validate the arguments
if [[ "$#" -lt 1 || "$#" -gt 2 ]]; then
  printf '%s' "${USAGE}" 1>&2
  exit 128
fi

pid="$1"

# Check if the user needs help
if [[ "${pid}" =~ ^(-h|-(-)?help)$ ]]; then
  printf '%s' "${USAGE}" 1>&2
  exit 0
fi

root_name="${2:-login}"

get_info() {
  local -ri current_pid="$1"
  if [[ ! -r "/proc/${current_pid}/status" ]]; then exit 1; fi
  mapfile info < \
    <(grep --null -E -m 2 '^(Name|PPid):' "/proc/${current_pid}/status" \
      | sort | cut -f 2)
  name="${info[0]##[[:space:]]}"
  name="${name%%[[:space:]]}"
  ppid="${info[1]##[[:space:]]}"
  ppid="${ppid%%[[:space:]]}"
}

next_pid="${pid}"
while [[ "${name}" != "${root_name}" && "${ppid}" -ne 1 ]]; do
  get_info "${next_pid}";
  name_pid="${next_pid}"
  next_pid="${ppid}"
done

if [[ "${name}" != "${root_name}" ]]; then exit 1; fi
printf '%s\n' "${name_pid}"