Bash script to encode and decode to base64url

base64url

#!/usr/bin/env bash

# Encodes/decodes a string to/from base64url.

print_help() {
    echo "Usage: $(basename "$0") [-d] STRING"
    echo
    echo "Without -d encodes the string to base64url."
    echo
    echo "  -d  decodes the string from base64url (or base64)"
    echo "  -h  shows this help"
    exit 0
}

if [ "$#" -gt 0 ] && [ "$#" -le 2 ]
then
    decode=false

    while getopts "dh:" flag
    do
        case "${flag}" in
            d) decode=true;;
            h) print_help;;
            *) print_help;;
        esac
    done

    if [ "$decode" = false ]
    then
        if [ "$#" -eq 1 ]
        then
            echo "$1" | base64 | tr '/+' '_+' && exit 0
        fi
    else
        if [ "$#" -eq 2 ]
        then
            # Avoid base64 "invalid input": https://unix.stackexchange.com/questions/631501/base64-d-decodes-but-says-invalid-input
            # "fold -w 4" split every 4 characters into separate lines
            # "sed '$ d'" deletes the last line (the extraneous padding)
            # "tr -d '\n'" joins all lines
            # " sed '$a\'" adds a new line, only, if doesn't exists
            echo "$2"==== | fold -w 4 | sed '$ d' | tr -d '\n' | tr '_-' '/+' | base64 -d | sed '$a\' && exit 0
        fi
    fi
fi

print_help