IP validation fails after switching to mapfile

Question

I have a shell function for validating IPv4 address strings:

is_ipv4_addr() {
  IFS='.'
  declare -a octets=( ${1:-} )
  if [ "${#octets[*]}" -eq 4 ]; then
    for octet in "${octets[@]}"; do
      if [[ ! "$octet" =~ ^[0-9]+$ ]] || [ "$octet" -lt 0 ] || [ "$octet" -gt 255 ]; then
        return 2
      fi
    done
    return 0
  fi
  return 1
}

However, shellcheck complains about the unquoted ${1:-} and adding quotes around it would break splitting the string at dots. I know I could just disable this particular check with a comment, but that always seems like an ugly workaround to me, so I decided to use mapfile instead:

is_ipv4_addr() {
  mapfile -d. -t octets <<<"${1:-}"
  if [ "${#octets[*]}" -eq 4 ]; then
    for octet in "${octets[@]}"; do
      if [[ ! "$octet" =~ ^[0-9]+$ ]] || [ "$octet" -lt 0 ] || [ "$octet" -gt 255 ]; then
        return 2
      fi
    done
    return 0
  fi
  return 1
}

But now validation always fails, even for valid IPv4 addresses like 8.8.8.8. The returned status code is 2, so the function thinks that one of the octets is incorrect. Why?

Answer 1

The issue is caused by the here-string, which implicitly adds a trailing newline to the string. If you add an echo statement before the return

if [[ ! "$octet" =~ ^[0-9]+$ ]] || [ "$octet" -lt 0 ] || [ "$octet" -gt 255 ]; then
  echo "-${octet}-"
  return 2
fi

you'll see output like this:

$ is_ipv4_addr '1.2.3.4'
-1-
-2-
-3-
-4
-

To fix the issue use process substitution instead of the here-doc. Make sure the command you use for echoing the string in that substituted statement also does not add a trailing newline, e.g.

mapfile -d. -t octets < <(echo -n "${1:-}")

or

mapfile -d. -t octets < <(printf "%s" "${1:-}")

Note that the parameter -d is not available prior to Bash 4.4. For older versions you need to use something like this instead:

mapfile -t octets < <(echo -n "${1:-}" | tr '.' "\n")