#!/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. Defaults to $$.

        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

        Get the PID of the first ancestor of the current process, if there
        is no ancestor named "not_a_process."

                rpid $$ not_a_process

        Get the PID of the current process itself, if it is named
        "current_process."

                rpid $$ current_process
'

# Validate the arguments
if [[ "$#" -gt 2 ]]; then
  printf '%s' "${USAGE}" 1>&2
  exit 64
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}"

prev_pid="${pid}"
current_pid="${pid}"
while [[ "${name}" != "${root_name}" && -r "/proc/${current_pid}/status" ]]; do
  mapfile info < \
    <(grep --null -E -m 2 '^(Name|PPid):' "/proc/${current_pid}/status" \
      | sort | cut -f 2)
  name="${info[0]##[[:space:]]}"
  name="${name%%[[:space:]]}"
  prev_pid="${current_pid}"
  current_pid="${info[1]##[[:space:]]}"
  current_pid="${current_pid%%[[:space:]]}"
done

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