Bash script to parse JSON Web Tokens and pretty-print their contents

jwt-parse.sh

#!/bin/bash

# Parses JSON Web Tokens (https://en.wikipedia.org/wiki/JSON_Web_Token)
# and pretty-prints their content.
#
# © 2021 Daniel Lenski <[email protected]>, MIT License

jq=$(which jq || echo cat)

for jwt in "$@"; do
    IFS=. read -r -a bits <<< "$jwt"

    # Convert from URL-safe b64 to "standard" b64, and fix padding
    for i in $(seq 0 2); do
        bits[$i]=$(tr '_-' '/+' <<< "${bits[$i]}")
        case $(( ${#bits[$i]} % 4 )) in
            1) bits[$i]+="===" ;;
            2) bits[$i]+="==" ;;
            3) bits[$i]+="=" ;;
        esac
    done

    echo "Header:"
    echo "======="
    echo
    echo "${bits[0]}" | base64 -d | $jq
    echo

    echo "Claims:"
    echo "======="
    echo
    echo "${bits[1]}" | base64 -d | $jq
    echo

    echo "Signature:"
    echo "=========="
    echo
    echo "${bits[2]}" | base64 -d | hexdump -v -e '30/1 "%02X" 1/30 "\n"'
done