Read Base64 encoded JWT

read-jwt.sh

#!/bin/bash

# Split the JWT into parts (header.payload.signature)
# TOKEN="eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvbiBEb2UiLCJpYXQiOjE1MTYyMzkwMjJ9.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c"

# Name of the required environment variable
REQUIRED_VAR="TOKEN"

# Check if the variable is not set or empty
if [[ -z "${!REQUIRED_VAR}" ]]; then
  echo "❌ Error: Environment variable '$REQUIRED_VAR' is required but not set."
  echo ""
  echo "👉 Please set it before running this script. Example:"
  echo ""
  echo "   export $REQUIRED_VAR=your_base64_jwt_token_here"
  echo "   ./$(basename "$0")"
  echo ""
  exit 1
fi

# Extract the header
header=$(echo "$TOKEN" | cut -d '.' -f1)

# Convert Base64URL to standard Base64
header_b64=$(echo "$header" | tr '_-' '/+')

# Add padding if needed
remainder=$((${#header_b64} % 4))
if [ $remainder -eq 2 ]; then
  header_b64="${header_b64}=="
elif [ $remainder -eq 3 ]; then
  header_b64="${header_b64}="
fi

# Decode Header
echo "$header_b64" | base64 -d | jq

# Extract the body
body=$(echo "$TOKEN" | cut -d "." -f2)

# Convert Base64URL to standard Base64
body_b64=$(echo "$body" | tr '_-' '/+')

# Add padding if needed
remainder=$((${#body_b64} % 4))
if [ $remainder -eq 2 ]; then
  body_b64="${body_b64}=="
elif [ $remainder -eq 3 ]; then
  body_b64="${body_b64}="
fi

# Decode body
echo "$body_b64" | base64 -d | jq