Base64 URL safe in shell scripts

The Base64 url safe already supported by basenc command. But it may be not pre-installed on many platfroms.

You can call base64 command but there is a pitfalls:

On most Linux distros the base64 command comes from coreutils package. And it already have a basenc command
On OpenWrt the base64 is not installed by default and comes from coreutils-base64 package and the baseenc command from coreutils-baseenc
You can enable the base64 in BusyBox i.e. if you building the OpenWrt image yourself.
You may use openssl base64 or openssl enc -base64 see docs but it will read by lines so you must also specify -A param.
On MacOS the -d decode flag must be in upper case base64 -D so most users use --decode flag. Previously MacOS used the -d for debug but it's not clear who ever used it. Then Applce just removed the -d param but again it's not clear why they don't changed it to be compatible with coreutils. Don't use any MacOS products. Just don't.
Python also can be usedpython3 -m base64 -d
https://www.yuryoparin.com/2014/05/base64url-in-bash.html pure bash scripts: no any requiteremnts. But works too slowly.
https://gist.github.com/woniuzfb/717dc5dd19374bf9b6cd38078753a9ac wrapper uppon base64 to decode base64 url safe

Shell functions to convert base64 url safe to regular base64:

base64_padding()
{
  local len=$(( ${#1} % 4 ))
  local padded_b64=''
  if [ ${len} = 2 ]; then
    padded_b64="${1}=="
  elif [ ${len} = 3 ]; then
    padded_b64="${1}="
  else
    padded_b64="${1}"
  fi
  echo -n "$padded_b64"
}

base64url_to_b64()
{
  base64_padding "${1}" | tr -- '-_' '+/'
}

I created tickets to fix the discrapanceies:

stokito/b64.md

Base64 URL safe in shell scripts