Zeroed free space of vfat filesystem

zeroes.sh

#!/bin/sh

set -eu

usage() {
  printf "usage: %s <filesystem>\n\n" "$(basename "$0")"
}

if [ $# != 1 ]; then usage >&2; exit 1; fi
fs="${1%/}"
test -d "$fs" || {
  echo "** ERROR: \"$fs\" does not exists"
  return 1
}


debug=${debug:-}

size=$((1024 * $(df --output=avail "$fs" | tail -n 1)))
split=$((2 * 1024 * 1024 * 1024))

# just for quick test
if [ -n "$debug" ]; then
  size=$((1024 * 1024 * 10))
  split=$((1 * 1024 * 1024))
fi


# Pretty print a number
ppn() {
  local n s sufx
  n="$1"
  sufx=""
  s="$n$sufx"
  while [ ${#s} -gt 5 ] && [ "$sufx" != "Yi" ]; do
    case "$sufx" in
      (Zi) sufx="Yi" ;;
      (Ei) sufx="Zi" ;;
      (Pi) sufx="Ei" ;;
      (Ti) sufx="Pi" ;;
      (Gi) sufx="Ti" ;;
      (Mi) sufx="Gi" ;;
      (Ki) sufx="Mi" ;;
      ("") sufx="Ki" ;;
    esac
    n=$((n / 1024))
    s="$n$sufx"
  done
  echo "$s"
}

zero() {
  dd if=/dev/zero bs=1M count="$(((size + 1024*1024 - 1) /(1024*1024)))" status=none
}

fseq() {
  local offset i remain
  i=0
  remain="$size"
  for offset in $(seq 0 "$split" "$size"); do
    if [ "$remain" -lt "$split" ]; then
      echo "$i" "$offset" "$remain"
    else
      echo "$i" "$offset" "$split"
    fi
    i=$((i+1))
    remain=$((remain - split))
    test "$remain" -eq "0" && break
  done
}

wipe() {
  local wd i offset n filename count
  wd="$1"
  exec 3<&0
  exec <&-
  fseq | while read -r i offset n; do
    filename="$(printf "%s/_wipe%04d" "$wd" "$i")"
    if [ -n "$debug" ]; then
      printf>&2 "# segment %d/%d (%s, %s remaining)\n" \
        "$((i + 1))" "$segments" "$(ppn "$n")" "$(ppn $((size - offset)))"
      printf>&2 "# Creating %s\n" "$filename"
    fi
    count=$((n /1024 /64))
    dd <&3 "of=$filename" oflag=sync,noatime status=none ibs=64K obs=1M "count=$count"
  done
}

target="$fs/_wipe"
test -d "$target" && rm -fr "$target"
test -d "$target" && {
  echo "** ERROR: \"$target\" already exists and is not a directory"
  exit 1
}
mkdir "$target"

if [ -n "$debug" ]; then
  segments=$(( (size + split - 1) / split ))
  printf>&2 "# %d segments will be created\n" "$segments"
fi

zero | pv -s "$size" -B 64K | wipe "$target"
[ -n "$debug" ] && ls -lh "$target"
rm -vfr "$target"