Really ugly python script to convert nd [a md like format of mine] to an html blog post for my website

nd2html.py

import sys
from datetime import date

def parse_line(line):
    if line[0] == "#":
        if line[1] == "#":
            return f"</div><div><h2>{line[2:]}</h2>"
        return f"</div><div><h1>{line[2:]}</h1>"
    if line[0] == "!":
        text_to_use = line[line.index('[') + 1:line.index(']')]
        link_to_use = line[line.index('(') + 1:line.index(')')]
        return f"<p style=\"font-size: 70%\"><img src=\"{link_to_use}\" alt=\"{text_to_use}\"><br>{text_to_use}</p>"
    if line[0] == "$":
        text_to_use = line[line.index('[') + 1:line.index(']')]
        link_to_use = line[line.index('(') + 1:line.index(')')]
        return f"<a href=\"{link_to_use}\">{text_to_use}</a>"
    return f"<p>{line}</p>"

def file_to_html(file):
    html = ""
    for line in file:
        html += parse_line(line)
    return html


def main():
    if len(sys.argv) < 2:
        print("[!] Pass in a file to convert!")
        sys.exit(-1)

    prologue = open("base/prologue.html", "r")
    epilogue = open("base/epilogue.html", "r")
    file_name = sys.argv[1]
    today = date.today()
    today_but_string = today.strftime("%d-%m-%Y")
    f = open(file_name, "r")
    html = file_to_html(f)
    f.close()
    output = open(f"{today_but_string}.html", "w")
    output.write(prologue.read())
    output.write(f"<h1>{file_name}</h1>")
    output.write(f"<h4>Posted on {today_but_string}</h4>")
    output.write(html)
    output.write(epilogue.read())

if __name__ == "__main__":
    main()