Produce a download file from CGI in Lua - lua

Question

I am writing a simple CGI program in lua. What I want to achieve is produce a response from CGI which enables a file to be downloaded from the browser. But I just can't print the data. I have no idea what's going on here. Here is the code below:
print("Content-Type: text/html; charset=UTF-8")
print("Content-Length:" .. sys.getenv("CONTENT_LENGTH"))
print("Content-Disposition:",'attachment;filename="backup.tar.gz"\n')
print("Content-Type:application/x-tar-gz\n\n")
file=io.popen("some command")
output = file:read('*a')
print(output)
--file:close()
The problem is I just can't print the output whose content is binary. I can see the type of output is string.
What is the problem? Please give some hints about it. Thank you.
ADD : I have no idea about this and Where is the issue. Let me put more info about the command I want run. But I don't think that matters.
Actually, I work in openwrt, the web server uhttpd. (No LuCI here)
The command is:sysupgrade -b - 2>/dev/null. This command is used to backup the config file. I want to write a CGI to download the backup file from the web.
But I can not print the output to the server. Even in the terminal(in lua IDE) I cannot print out the output except one or two messy code. But I can write the output to a file in terminal. Maybe it has some relationship with the content of out.
When I print the content line by line, it prints some, but not all of it. After I download the file. I can't open it.

Answer 1

print("Content-Disposition:",'attachment;filename="backup.tar.gz"\n')
print("Content-Type:application/x-tar-gz\n\n")
I think you have way too many new lines. First of all, you can newline in Content-Disposition and this is in addition to the newline that print adds, which ends the headers and makes Content-Type a part of the payload (which breaks the content). You also have two newlines in Content-Type where you only need one (as one is added by the print command).
I think something like this should work:
local file = io.popen("some command")
local output = file:read('*a')
file:close()
print("Content-Type: text/html; charset=UTF-8")
print("Content-Length: " .. #output)
print("Content-Disposition: " .. 'attachment;filename="backup.tar.gz"')
print("Content-Type: application/x-tar-gz\r\n")
print(output)