I implemented a 100% BASH Base32 decoder according to Sectio 6 of RFC4648.

Download:

• base32d.v1.1.sh (sig) (sha1: cdf9100662efd6d9b31d087f98197a385d54df7a) – updated to take advantage of BASH return values and bitwise operations
• base32d.v1.0.sh (sig) (sha1: 314f0cb9b395c8cbad323163ef5adc246cdea66c)

#!/bin/bash
# base32d.sh v1.1 September 21, 2014
# Copyright (c) 2014 Kenji Yoshino https://www.tidgubi.com
# This script is released under the Version 3 of the GNU General Public
# License https://www.gnu.org/licenses/gpl-3.0.txt
#
# This script implements 100% BASH base32 decoding as specified in section 6 of
# https://tools.ietf.org/html/rfc4648

function base32dErrorMsg {
   printf 'Invalid character encountered.\n'
}

# decodeChar $ch
# converts Encoded char to Value according to Table 3 of RFC4648 and treat
# lowercase as valid characters.
function decodeChar {
   local ch="${1:0:1}"
   # if this is an = or empty, return 0, but exit 2 to flag that this is the
   # end of the input
   if [[ "$ch" = '=' || "${#ch}" -eq 0 ]]; then
      printf '0'
      exit 2
   fi
   # convert the char to its ASCII Value
   local val=$(printf '%d' "'$ch")
   
   # shift lowercase to upper case
   if [[ $val -gt 96 ]]; then
      (( val -= 32 ))
   fi
   # shift the ASCII value to convert it to the 0-31 value
   if [[ $val -gt 55 ]]; then
      (( val -= 65))
   else
      (( val -= 24))
   fi
   
   # Verify that the decoded value is valid
   if [[ $val -gt 31 || $val -lt 0 ]]; then
      base32dErrorMsg
      exit 1
   fi
   printf "$val"
}

function base32dUsage {
   printf 'Usage: base32d.sh [ | -f ] [-h]\n'
   printf '\n'
   exit 1
}

function base32dexit {
   if [[ "$1" -eq 2 ]]; then
      printf '\n'
      exit 0
   elif [[ "$1" -eq 1 ]]; then
      base32dErrorMsg
      exit 1
   fi
}

function base32d {
   # verify that any command line parameters are valid.
   local input
   local prefix='\x'
   if [[ "$#" -gt 0 ]]; then
      if [[ "$1" = '-f' ]]; then
         if [[ "$2" = '-' ]]; then
            input=$(cat)
         elif [[ -r "$2" ]]; then
            input=$(cat "$2")
         else
            printf 'Cannot access the specified file.\n'
            exit 1
         fi
         if [[ "$3" = '-h' ]]; then
            prefix=''
         fi
      elif [[ "$1" = '-h' ]]; then
         input=$(cat)
         prefix=''
      else
         input="$1"
         if [[ "$2" = '-h' ]]; then
            prefix=''
         fi
      fi
   else
      input=$(cat)
   fi
   
   local ndx=0
   local len=${#input}
   local buffer
   local term=0
   local val

   while [ $ndx -lt $len ]; do
      # parse the next quantum (40 bits) of characters out of the input
      buffer=$(decodeChar "${input:$ndx:1}") || base32dexit $? #1
      (( buffer <<= 5 ))
      (( ndx++ ))
      val=$(decodeChar "${input:$ndx:1}"); term=$? #2
      (( buffer |= val ))
      # print the first 5 + 3 bits
      printf "$prefix$(printf '%02x' $(( buffer >> 2 )) )" # 1st byte
      base32dexit $term
      (( buffer <<= 5 ))
      (( ndx++ ))
      val=$(decodeChar "${input:$ndx:1}") || base32dexit $? #3
      (( buffer |= val ))
      (( buffer <<= 5 ))
      (( ndx++ ))
      val=$(decodeChar "${input:$ndx:1}"); term=$? #4
      (( buffer |= val ))
      (( buffer &= 0xFFF ))
      # print 2 + 5 + 1 bits
      printf "$prefix$(printf '%02x' $(( buffer >> 4 )) )" # 2nd byte
      base32dexit $term
      (( buffer <<= 5 ))
      (( ndx++ ))
      val=$(decodeChar "${input:$ndx:1}") || base32dexit $? #5
      (( buffer |= val ))
      (( buffer &= 0x1FF ))
      printf "$prefix$(printf '%02x' $(( buffer >> 1 )) )" #3rd byte
      base32dexit $term
      (( buffer <<= 5 ))
      (( ndx++ ))
      val=$(decodeChar "${input:$ndx:1}") || base32dexit $? #6
      (( buffer |= val ))
      (( buffer <<= 5 ))
      (( ndx++ ))
      val=$(decodeChar "${input:$ndx:1}"); term=$? #7
      (( buffer |= val ))
      (( buffer &= 0x7FF ))
      printf "$prefix$(printf '%02x' $(( buffer >> 3 )) )" # 4th byte
      base32dexit $term
      (( buffer <<= 5 ))
      (( ndx++ ))
      val=$(decodeChar "${input:$ndx:1}") || base32dexit $? #8
      (( buffer |= val ))
      printf "$prefix$(printf '%02x' $(( buffer & 0xFF )) )" # 5th byte
      (( ndx++ ))
   done
   
   printf '\n'
   exit 0
}

case $# in
   0)
      base32d
      ;;
   1)
      base32d "$1"
      ;;
   2)
      base32d "$1" "$2"
      ;;
   *)
      base32d "$1" "$2" "$3"
      ;;
esac