swap-scan.sh

#!/usr/bin/env bash
# Scan per-process swap usage (VmSwap from /proc).
# Usage: swap-scan.sh [-n N] [-a]

set -euo pipefail

TOP_N=20
SHOW_ALL=0

while getopts "n:ah" opt; do
  case "$opt" in
    n) TOP_N="$OPTARG" ;;
    a) SHOW_ALL=1; TOP_N=0 ;;
    h)
      cat <<'USAGE'
Usage: swap-scan.sh [-n N] [-a]
  -n N   show top N processes (default 20)
  -a     show all processes with non-zero swap
  -h     help
USAGE
      exit 0
      ;;
    *) exit 2 ;;
  esac
done

human() {
  local kb=$1
  if (( kb >= 1048576 )); then
    awk -v k="$kb" 'BEGIN { printf "%.1f GiB", k/1048576 }'
  elif (( kb >= 1024 )); then
    awk -v k="$kb" 'BEGIN { printf "%.1f MiB", k/1024 }'
  else
    printf "%d kB" "$kb"
  fi
}

tmpdir=
cleanup() { [[ -n "${tmpdir:-}" && -d "$tmpdir" ]] && rm -rf "$tmpdir"; }
trap cleanup EXIT
tmpdir=$(mktemp -d)
rows="$tmpdir/rows"
: >"$rows"

for status in /proc/[0-9]*/status; do
  [[ -r "$status" ]] || continue
  pid=${status#/proc/}
  pid=${pid%/status}

  # awk splits on whitespace, so leading spaces before numbers are fine
  read -r name rss swap < <(awk '
    /^Name:/   { name=$2 }
    /^VmRSS:/  { rss=$2+0 }
    /^VmSwap:/ { swap=$2+0 }
    END { printf "%s %d %d\n", (name==""?"?":name), rss+0, swap+0 }
  ' "$status" 2>/dev/null) || continue

  (( swap > 0 )) || continue

  cmd=
  if [[ -r "/proc/$pid/cmdline" ]]; then
    cmd=$(tr '\0' ' ' <"/proc/$pid/cmdline" 2>/dev/null || true)
  fi
  # trim and collapse spaces; fallback to [comm]
  cmd=$(printf '%s' "$cmd" | sed 's/^[[:space:]]*//;s/[[:space:]]*$//;s/[[:space:]]\{1,\}/ /g' | cut -c1-100)
  [[ -z "$cmd" ]] && cmd="[$name]"

  printf '%s\1%s\1%s\1%s\1%s\n' "$swap" "$rss" "$pid" "$name" "$cmd" >>"$rows"
done

total_swap_kb=0
count=0
if [[ -s "$rows" ]]; then
  while IFS=$'\1' read -r swap _r _p _n _c; do
    total_swap_kb=$((total_swap_kb + swap))
    count=$((count + 1))
  done <"$rows"
fi

echo "=== Swap summary ==="
if command -v free >/dev/null 2>&1; then
  free -h | grep -iE 'Mem|Swap' || free -h
fi
if command -v swapon >/dev/null 2>&1; then
  swapon --show 2>/dev/null || true
fi
echo "Processes with swap > 0: $count"
echo "Sum of process VmSwap:   $(human "$total_swap_kb") (${total_swap_kb} kB)"
echo

echo "=== Top by process (PID) ==="
printf '%-8s %-10s %-10s %s\n' "PID" "SWAP" "RSS" "COMMAND"
if [[ ! -s "$rows" ]]; then
  echo "(none)"
else
  sort -t$'\1' -k1,1nr "$rows" | {
    if (( SHOW_ALL )); then cat; else head -n "$TOP_N"; fi
  } | while IFS=$'\1' read -r swap rss pid name cmd; do
    printf '%-8s %-10s %-10s %s\n' "$pid" "$(human "$swap")" "$(human "$rss")" "$cmd"
  done
fi
echo

echo "=== Aggregated by process name ==="
printf '%-10s %-6s %s\n' "SWAP" "COUNT" "NAME"
if [[ ! -s "$rows" ]]; then
  echo "(none)"
else
  awk -F'\1' '{ a[$4]+=$1; c[$4]++ }
    END { for (n in a) printf "%d\1%d\1%s\n", a[n], c[n], n }
  ' "$rows" | sort -t$'\1' -k1,1nr | {
    if (( SHOW_ALL )); then cat; else head -n "$TOP_N"; fi
  } | while IFS=$'\1' read -r swap cnt name; do
    printf '%-10s x%-5s %s\n' "$(human "$swap")" "$cnt" "$name"
  done
fi