#!/usr/bin/env bash
set -euo pipefail

if [[ $# -lt 1 || $# -gt 2 || ! $1 =~ ^[0-9]+$ ]]; then
    printf 'usage: %s PID [SECONDS]\n' "$0" >&2
    exit 64
fi

sample_pid=$1
duration=${2:-600}
if [[ ! ${duration} =~ ^[0-9]+$ || ${duration} -lt 1 ]]; then
    printf 'SECONDS must be a positive integer\n' >&2
    exit 64
fi
if [[ ! -r "/proc/${sample_pid}/stat" ]]; then
    printf 'PID %s is not readable\n' "${sample_pid}" >&2
    exit 66
fi

printf 'elapsed_s\tcpu_percent\trss_kib\n'
clock_ticks=$(getconf CLK_TCK)
read_ticks() {
    local stat_line stat_fields
    IFS= read -r stat_line < "/proc/${sample_pid}/stat" || return 1
    stat_fields=${stat_line##*) }
    read -r -a stat_fields <<<"${stat_fields}"
    printf '%s\n' "$((stat_fields[11] + stat_fields[12]))"
}

previous_ticks=$(read_ticks)
for ((elapsed = 1; elapsed <= duration; elapsed++)); do
    sleep 1
    if [[ ! -r "/proc/${sample_pid}/stat" ]] || ! current_ticks=$(read_ticks); then
        printf 'process %s exited after %s seconds\n' "${sample_pid}" "${elapsed}" >&2
        exit 1
    fi
    delta_ticks=$((current_ticks - previous_ticks))
    cpu=$(awk -v ticks="${delta_ticks}" -v hz="${clock_ticks}" \
        'BEGIN { printf "%.2f", ticks * 100 / hz }')
    rss=$(awk '/^VmRSS:/ { print $2; exit }' "/proc/${sample_pid}/status")
    printf '%s\t%s\t%s\n' "${elapsed}" "${cpu}" "${rss}"
    previous_ticks=${current_ticks}
done