#!/bin/bash
# Emits {"swap":<pct>} — swap usage percent, read straight from /proc/meminfo.
# Emits {"swap":null} when no swap is configured.
set -uo pipefail

swap_total=0
swap_free=0
while read -r key value _; do
    case "$key" in
    SwapTotal:) swap_total=$value ;;
    SwapFree:) swap_free=$value ;;
    esac
done </proc/meminfo

swap_pct="null"
if ((swap_total > 0)); then
    swap_pct=$(awk -v t="$swap_total" -v f="$swap_free" 'BEGIN { printf "%.0f", (t-f)/t*100 }')
fi

printf '{"swap":%s}\n' "$swap_pct"