Write a program that downloads itself

Question

Write a program that connects to this site, downloads the very answer in which it is posted, extracts its own source code and prints it out. The output must be identical to the source code. Shortest code (in bytes) wins.

Rules:

• No URL shorteners allowed.
• The answer must have a regular format - a heading with the language name and size, optional description, code block, optional description and explanation. No unnatural delimiters allowed.
• The output must originate from the actual code block posted on the site.
• The functionality must not depend on the position in the answer list; it should work even if there are multiple pages and the answer it not on the first one.
• New: special note for answers that are supposed to be run in a browser: it is ok to require running them on the codegolf domain (to obey the same-origin policy) but the domain and path should be included in the solution in order to make it fair.

Answer 1

Bash + coreutils + Lynx browser, 61 bytes

Thanks to @FDinoff for the tips:

lynx -dump codegolf.stackexchange.com/posts/28164/body|grep 2

Answer 2

Ruby, 155 186 195 148 138 110 97 characters

require'open-uri';puts open('http://codegolf.stackexchange.com/posts/28159/body').read[/req.+;/];

I had to make it one line, because otherwise it would output newlines as \n instead of actual newlines.

• +31 characters because I didn't notice some characters were being escaped.
• +9 characters to get rid of the annoying backslash.
• Thanks to Nathan Osman for saving 2 chars, and Ventero for saving 55 (!!!) by removing the need for most of the fixes listed above.

---

The explanation

Let's beautify this a bit first. However, I'm going to have to use a somewhat... interesting notation in this code. I can't use semicolons at all in this post, for reasons explained later, so I will use {SEMI} in place of semicolons instead.

require 'open-uri'
resp = open('http://codegolf.stackexchange.com/posts/28159/body').read
puts resp.match(/req.+{SEMI}/){SEMI}

Alright, now let's walk through this. The first two lines are fairly self-explanatory -- they fetch the HTML text of this answer.

Now, the last line is the interesting one here. You see that seemingly useless semicolon at the end of the code? It's absolutely required, and here's why.

First, resp.match extracts the code to be printed. The regexp it uses for this is the trick: /req.+{SEMI}/. It grabs the start of the code, REQuire'net/http', by searching for req (re would grab my REputation). Then, it finds the end of the code by searching for a semicolon! Since + is greedy by default, it will keep going until it finds the semicolon that signifies the end of the code. See why I can't use semicolons anymore?

After that, I don't have to unescape anything thanks to Ventero's fix of not using \ at all anymore. All I have to do is fix {AMPERSAND} changing into {AMPERSAND}amp{SEMI}, which can be achieved simply by removing the amp{SEMI} part. No need for this anymore because of new URL. After that, the original code has been retrieved! (Note: I can't use the ampersand either, because that gets HTML-encoded which causes a semicolon to be created.)

Answer 3

PowerShell - 69 62

(irm codegolf.stackexchange.com/posts/28236/body).div.pre.code

Answer 4

JavaScript - 123 122 101 95 92 91 87 86 114

with(new XMLHttpRequest)send(open(0,/\codegolf.stackexchange.com\posts\28175\body/,0)),alert(/w.*/.exec(response))

Runs in the console of your web browser on this page. Tested on the latest Chrome and Firefox.

edit: +28 bytes to add the full domain.

Firefox doesn't like my Regex URL trick anymore with this update :(

Here's the rule-breaking 86 byte solution:

with(new XMLHttpRequest)send(open(0,/posts\28175\body/,0)),alert(/w.*/.exec(response))

Answer 5

Ruby + wget + gunzip, 159 86 82 71

Using tip of @FDinoff to use http://codegolf.stackexchange.com/posts/28173/body.

puts `wget -qO- codegolf.stackexchange.com/posts/28173/body`[/pu.*\]/]

Tested. Thanks to @ace and @Bob for command line optimization.

Answer 6

CJam - 53

"codegolf.stackexchange.com/posts/28184/body"g54/1=);

I'm making this community wiki since I'm answering my own question and don't really want to compete :p
Credits to FDinoff for the URL choice.

Answer 7

Rebmu, 91 characters

Due to the Catch-22 I have to post to get this answer's URL. :-/ Okay, got it.

paTSrd http://codegolf.stackexchange.com/a/28154[th<a name="28154">th<code>cpCto</code>]prC

Rebmu is a dialect of Rebol, and you can read all 'bout it. The equivalent Rebol here would be:

parse to-string read http://codegolf.stackexchange.com/a/28154 [
    thru <a name="28154">
    thru <code>
    copy c to </code>
]
print c

Rebol's PARSE is a sort of highly-literate answer to RegEx. It starts a parser position of the input (which can be any series, including structural blocks...binary data...or string types). The rules are a language for how the parse position moves.

Tags and URLs are really just strings under the hood in the language. But they are "flavored", and as Rebol is dynamically typed you can check that type. READ for instance knows that if you give it a URL-flavored string, then it should dispatch to a scheme handler to do the reading. (In this case, the one registered for HTTP). You get back UTF-8 bytes by default, so we use to-string to decode that and get a series of codepoints in a normal Unicode string.

In the case of the parse dialect, encountering a tag type is just matched as if it were a string that looked like the tag. THRU is an instruction meaning "skip until the ensuing rule is matched, and then place the match position at the end of what you just matched." (TO is the analogue that matches, but leaves the parse position before the element).

So we zip along past the <a name="28154">. Then we zip past the next occurrence of <code>, with our parse position now located right after the >. PARSE's COPY command then lets us copy data up to another rule, in this case that rule is [TO </code>]... so we get into the variable C everything up until right before that <.

Cool, huh? :-)

Technically I could shave more off it, for instance by seeking TO "</" and that saves three characters--there's no need to match the whole </code> end tag when just </ would do. Similar arguments could me made for the start tag. But Rebmu is about literate golfing...even if you might think it looks odd at first!

UPDATE: the /body trick is out of the bag, but I'm similarly going to leave it as-is...because I think it is more educational this way.

Answer 8

Java now 634, 852, was 1004

Code has been updated; thanks for suggestions. Golfed: now replaces &gt with >

//bacchus
package golf;
import java.net.*;
import java.util.*;
public class G{
public static void main(String[] a) throws Exception {
Scanner z;
URL u;
int x=0;
String s;
u=new URL("http://codegolf.stackexchange.com/questions/28154/write-a-program-that-downloads-itself");
z=new Scanner(u.openConnection().getInputStream());
z.useDelimiter("\\s*//bacchus\\s*");
while(z.hasNext())
{
s=z.next();
s=s.replace(">", ">");
if(x>0)System.out.println("//bacchus\n"+s);
x++;
if(x>2)break;
}
System.out.println("//bacchus\n");
}
}
//bacchus

Submitting for testing, I will edit and try golfing it shortly. Needed to change x>1 to x>2 because test string is also in my code. Note: Code golf replaces > symbol to &gt.

//bacchus
package golf;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLConnection;

public class Golf {

    public static void main(String[] args) throws IOException {
        URL u;
        URLConnection c;
        InputStream i;
        InputStreamReader r;
        BufferedReader b;
        String s;
        int x=0;
        try {
            u=new URL("http://codegolf.stackexchange.com/questions/28154/write-a-program-that-downloads-itself");
            c=u.openConnection();
            i=c.getInputStream();
            r=new InputStreamReader(i);
            b=new BufferedReader(r);
            while((s=b.readLine())!=null)
            {
                if(s.contains("//bacchus")) x++;
                if(x>0)System.out.println(s);
                if(x>2) break;
            }
            i.close();
            b.close();
        } catch (MalformedURLException ex) {

        }
    }

}
//bacchus

Answer 9

Python, 175 167 bytes

This uses two external libraries; I didn't read that it was unauthorized.

import bs4,requests
print(bs4.BeautifulSoup(requests.get('http://codegolf.stackexchange.com/q/28154').text).select('#answer-28171')[0].select('pre > code')[0].string)

Longer, but nicer looking code:

import bs4, requests
request = requests.get('http://codegolf.stackexchange.com/q/28154')
soup = bs4.BeautifulSoup(request.text)
answer = soup.select('#answer-28171')[0]
code = answer.select('pre > code')[1].string
print(code)

Answer 10

w3m 45 characters

w3m codegolf.stackexchange.com/a/28336|grep ☻

Answer 11

JavaScript, 228

r=new XMLHttpRequest()
c='code'
r.open('GET','//'+c+'golf.stackexchange.com/posts/28157/body')
r.onreadystatechange=function(){this.readyState==4&&alert((a=r.responseText).substr(i=a.indexOf(c)+5,a.indexOf('/'+c)-i-1))}
r.send()

Runs on this page.

Answer 12

Haskell, 563 613 bytes

import Control.Monad
import Data.List
import Network.HTTP
m%f=join(fmap f m)
q s=(simpleHTTP(getRequest"http://codegolf.stackexchange.com/questions/28154/write-a-program-that-downloads-itself?answertab=oldest#tab-top"))%getResponseBody%(putStrLn.head.filter((==)(s++show s)).map(take 613).tails)
main=q"import Control.Monad\nimport Data.List\nimport Network.HTTP\nm%f=join(fmap f m)\nq s=(simpleHTTP(getRequest\"http://codegolf.stackexchange.com/questions/28154/write-a-program-that-downloads-itself?answertab=oldest#tab-top\"))%getResponseBody%(putStrLn.head.filter((==)(s++show s)).map(take 613).tails)\nmain=q"

Tested. Has page support via "oldest posts" feature. Uses quine-line structure to find what to print. The import Control.Monad is only because >>= generates > in HTML.

Answer 13

Javascript +jQuery, 87, 67

I'm not sure wether I'm allowed to use jQuery, but:

$('body').load('//codegolf.stackexchange.com/posts/28268/body pre')

---

Javascript + jQuery, if excecuted in this page: 27, 25

For fun, if it would be excecuted here:

$('[id$=268] pre').html()

$('[id$=28268] pre').html()

Answer 14

Dart, 164

I thought I'd try this in Dart, is pretty fun to use imo.

This can be run in the console in DartEditor, but does require the http package added in pubspec.yaml

import"package:http/http.dart"as h;h.read("http://codegolf.stackexchange.com/posts/28215/body").then((s){print(new RegExp(r"im.+(?:})").firstMatch(s).group(0));});}

Ungolfed version:

import "package:http/http.dart" as h;

void main()
{
  h.read("http://codegolf.stackexchange.com/posts/28215/body").then((s)
  {
    print(new RegExp(r"im.+(?:})").firstMatch(s).group(0));
  });
}

Answer 15

R 114 characters

library(XML);cat(xpathSApply(xmlParse("http://codegolf.stackexchange.com/posts/28216/body"),'//code',xmlValue)[1])

No real magic here: it takes the value of the field between the html tags <code></code>. Uses library XML (as one can see in the code quite obviously). Outputs the result as stdout.

Answer 16

APL (Dyalog Unicode), 108 bytes

6 3⌷⎕XML{(⎕SE.SALT.New'HttpCommand'('GET'⍵)).Run.Data}'https://codegolf.stackexchange.com/posts/211205/body'

Don't Try it online, try it locally

Correct link coming soon.

Output:

Answer 17

Java, 300 294

import java.net.*;import java.util.*;public class G{public static void main (String [] a) throws Exception{Scanner s=new Scanner(new URL("http://codegolf.stackexchange.com/posts/28189/body").openConnection().getInputStream()).useDelimiter("./?[c]ode\\W");s.next();System.out.print(s.next());}}

An improved version of bacchusbeale's answer which:

• doesn't close resources unnecessarily
• doesn't declare unnecessary variables
• uses a Scanner to avoid having to loop over the input
• uses a regexp that doesn't match itself to avoid having to skip over a middle occurrence of the start/end marker.

Updated:

• Use a direct URL to the post, so we don't need a unique comment to identify the start/end of the code; now uses <code>[...]</code> as the delimiters to search for (actually using the regular expression "./?[c]ode\W", so as to avoid having to decode < and > -- the "\W" is necessary rather than the shorter "." to avoid it matching part of the URL to the post, unfortunately, which costs 2 characters, and the square brackets around c prevent the regex matching itself).

Answer 18

w3m 55 bytes

w3m codegolf.stackexchange.com/posts/28242/body|grep x

Based on @DigitalTrauma

Answer 19

Ruby, 237 215 146 132

require'mechanize'
a=Mechanize.new
puts a.get('http://codegolf.stackexchange.com/a/28159').search('.lang-rb code:nth-child(1)').text

Answer 20

Processing, 90

print(loadStrings("http://codegolf.stackexchange.com/posts/28657/body")[2].substring(11));

Edit: Finally got it!

Answer 21

Python, 164

Works by extracting the text between the code tags. It is quite long but it will always function correctly unless the html page is edited directly or a new code block is added before the one below (having a block of code after should have no effect on the output of the program).

import urllib2
print urllib2.urlopen("http://codegolf.stackexchange.com/posts/28617/body").read().split(chr(60)+"code"+chr(62))[1].split(chr(60)+"/code"+chr(62))[0]

Answer 22

bash + awk, 71 bytes

curl -sL codegolf.stackexchange.com/q/28154 |awk -F\> '/\#/ {print $3}'

Answer 23

05AB1E, 50 bytes

žY’ƒËŠˆ.‚‹º.ŒŒ/„¤/’•4m6•’/†Î’J.w4°.W'ƒË¡1è¦6F¨}.V

Screenshot of a local run:

(CMD doesn't support the 05AB1E codepage, hence the weird output for non-ASCII characters.)

Explanation:

žY                 # Push constant "https://"
’ƒËŠˆ.‚‹º.ŒŒ/„¤/’ # Push dictionary string "codegolf.stackexchange.com/posts/"
•4m6•              # Push compressed integer 272346
’/†Î’              # Push dictionary string "/body"
J                  # Join all strings on the stack together:
                   #  "https://codegolf.stackexchange.com/posts/272346/body"
 .w                # Go to this url, and push its HTML contents
   4°.W            #†Sleep for 10 seconds (10**4 ms)
   'ƒË¡           '# Split it on dictionary word "code"
       1è          # Pop and leave the second item (0-based 1st)
         ¦         # Remove its leading ">"
          6F       # Loop 6 times:
            ¨      #  Remove its trailing character ("}.V\n/>")
           }.V     # After the loop: evaluate this string as 05AB1E code
                   # (after which the result is output implicitly)

See this 05AB1E tip of mine (sections How to use the dictionary? and How to compress large integers?) to understand why ’ƒËŠˆ.‚‹º.ŒŒ/„¤/’ is "codegolf.stackexchange.com/posts/"; ’/†Î’ is "/body"; 'ƒË is "code"; and •4m6• is 272346.

† The sleep is apparently necessary, since otherwise Stackexchange itself will give the error: You may only load 1 post body every second.

Note: I've used 6 instead of 3 to also remove the trailing .V (and optional }), otherwise it would go into an infinite loop of getting its HTML contents without ever printing anything.. If getting into an infinite loop was the intend of this challenge, the trailing 6F¨}.V could be ¨¨¨.V (-1 byte) instead.

Answer 24

Javascript, 138

a=window.open("http://codegolf.stackexchange.com/posts/28160/body");setTimeout('alert(a.document.body.innerHTML.match(/a=.*9\\)/)[0])',99)

This works assuming that the page loads in under 99 ms. It also has to be run via a console opened on a codegolf.SE page, because of the same origin policy.

Answer 25

Perl 5.10, 155 127 122 117 bytes

use XML::LibXML;say XML::LibXML->new->parse_file('http://codegolf.stackexchange.com/posts/28330/body')->find('//pre')

Using XML::LibXML.

Answer 26

Shell and xmllint, 82 bytes

xmllint --xpath 'string(//pre)' http://codegolf.stackexchange.com/posts/28333/body

Answer 27

Swift 5.10 (macOS/iPadOS), 174 176 bytes

import Foundation;print(try{String.init}()(URL(string:"http://codegolf.stackexchange.com/posts/272295/body")!).matches(of:#/im.+;/#)[0].output.replacing(#/.quot;/#,with:"\""));

This uses pretty much the same trick as this Ruby answer.

It seems that the lack of a newline after import Foundation broke Stack Exchange's syntax highlighter... (to be fair, though, Xcode acts the same way).

The Linux port of Foundation doesn't yet have networking support (attempts to use networking APIs will crash at runtime), so this only works on Darwin platforms for the time being.