faster code for my code

[kucingkembar]

hi, sorry for my bad English,
I have code to remove line(s) that do not start with '[' and end with ']':

    with open('input.raw') as f:
        lines = ""
        for line in f:
            lina = line.strip()
            #print(line)
            if lina.startswith("["):
                if lina.endswith("]"):
                    lines = lines + lina + "\n"
    writing = open('output.raw', 'w')
    writing.write(lines)
    writing.close()

but somehow my file is over 200Mb, and it take hour(s) to get it done,
som how make my code faster?