Quickly decode JWTs on the command line. Does not check signature. DO NOT use in production. Created for debbugging purposes.

jwtdecode

#!/bin/bash

function usage
{
  cat <<EOH
usage:
  <command which outputs JWT> | ${0##*/}
    example: echo -n "<jwt>" | ${0##*/}

  If you have the JWT on your pasteboard you can just call: ${0##*/}
EOH
}

if [ -p /dev/stdin ]; then
  read jwt
else
  pb=$(pbpaste)
  jwt="${1:-$pb}"
fi

if [[ -z $jwt ]]; then
  echo -e "\033[31mNo input to decode\033[0m" 1>&2
  usage 1>&2
  exit 1
fi

OIFS=$IFS
IFS=.

for part in $jwt; do
  while :; do
    if [[ $(echo "${#part} * 6 % 8" | bc) -eq 6 ]]; then
      echo "VERY invalid Base64 input, EXITING" 1>&2
      exit 1
    fi
    if [[ $(echo "${#part} * 6 % 8" | bc) -eq 0 ]]; then
      break
    fi
    part="${part}="
  done
  decoded=$(echo -n $part | tr "-" "/" | base64 -d 2> /dev/null)
  if [[ "${decoded:0:1}" == "{" ]]; then
    echo $decoded | jq .
    echo
  fi
done

IFS=$OIFS