#!/bin/sh
set -eu

NAME="${0##*/}"
INTERVAL="${INTERVAL:-0.1}"
PID="${1:-}"

[ -n "$PID" ] || {
  printf "usage: %s <PID>\n" "$NAME"
  exit 1
} >&2

while sleep "$INTERVAL"; do
  out="$(pstree -g 3 "$1")"
  clear
  printf "%s\n" "$out"
done