Check if computer is connected to the internet

Question

Write a program which, according to whether the script has access to the internet, produces an output which is "Truthy/Falsey". You may try and connect to any existing site, at your own discretion (don't use a shady site which only has 10% uptime - try to keep to above 80% annual uptime). If the site is down, your program does not have to work.

It must be a standalone program or a function. You may use libraries outside of the standard library to achieve this. Standard loopholes are forbidden. This is code golf, so the code with the shortest byte-count wins.

Example pseudocode:

function a:
    try:
        connect to internet 
        return 1
    catch error:
        return 0

This is my first post on code golf, so if this violates any rules in any way or is a dupe, please alert me.

EDIT: Due to numerous suggestions, I have removed the UTF-8 byte count restriction

Answer 1

Bash (with dnsutils), 3 bytes

Sends a DNS request for "." (DNS root), exit code is 0 for success and >0 otherwise.

Golfed

dig

Test

% dig >/dev/null; echo $?;        
0

% nmcli nm wifi off
% dig >/dev/null; echo $?;
9

Disclaimer

This will obviously only work if your DNS server is sitting in the provider's network, i.e. in the "Internet" (as your provider network is normally a part of it), or if your system is using a public DNS server (like 8.8.8.8 from Google, which Android based systems use), as otherwise, you can get a cached copy from a local LAN server (or localhost).

But I assume this is not against the code-golf rules, as there are obviously more than one system where this does work as intended.

Pure-HTTP methods can also give false positives, due to an intermediate caching proxy, and are not guaranteed to work everywhere, so that is not something unique to this method.

A slightly more reliable version, 8 bytes

dig +tra

(a little tribute to @Digital Trauma !)

Enables the "trace mode", which will force dig to do the recursive search by itself (see https://serverfault.com/a/778830), avoiding any cache issues.

Answer 2

Bash + GNU utils, 8

• 5 bytes saved thanks to @Muzer.

wget to.

The other shell answers check the return code and echo some status output accordingly. This is unnecessary. The shell return code is already a usable Truthy/Falsey code and accessible in the $? parameter which is idiomatic for bash. Return code 0 means True. Return code >0 means False.

In use:

ubuntu@ubuntu:~$ wget to.
--2017-01-13 09:10:51--  http://to./
Resolving to. (to.)... 216.74.32.107, 216.74.32.107
Connecting to to. (to.)|216.74.32.107|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 11510 (11K) [text/html]
Saving to: ‘index.html.6’

index.html.6        100%[===================>]  11.24K  --.-KB/s    in 0.04s   

2017-01-13 09:10:51 (285 KB/s) - ‘index.html.6’ saved [11510/11510]

ubuntu@ubuntu:~$ echo $?
0
ubuntu@ubuntu:~$ sudo ifconfig ens33 down
ubuntu@ubuntu:~$ wget to.
--2017-01-13 09:11:00--  http://to./
Resolving to. (to.)... failed: Temporary failure in name resolution.
wget: unable to resolve host address ‘to.’
ubuntu@ubuntu:~$ echo $?
4
ubuntu@ubuntu:~$ sudo ifconfig ens33 up
ubuntu@ubuntu:~$ # Local network up, upstream link down
ubuntu@ubuntu:~$ wget to.
--2017-01-13 09:11:34--  http://to./
Resolving to. (to.)... failed: Name or service not known.
wget: unable to resolve host address ‘to.’
ubuntu@ubuntu:~$ echo $?
4
ubuntu@ubuntu:~$ 

Answer 3

Batch, 8 bytes

ping to.

ping will set ERRORLEVEL to 1 if the address cannot be resolved or reached.

Answer 4

05AB1E, 11 9 bytes

Saved 2 bytes on "to." courtesy of ev3commander

…to..wgX›

Checks if the length of the content at http://to. is greater than 1.
.w returns 0 on error.

Answer 5

MATL, 15 14 bytes

One byte saved thanks to Kevin Cruijssen's suggestion

'http://to.'Xi

Output is through STDOUT. This displays a non-empty string containing non-zero chars (which is truthy) if there is an Internet connection; and displays nothing (which is falsy) if there's no connection.

This can't be tested online because the Xi is not allowed in the online interpreters.

Explanation

'http://to.'  % Push this string
Xi            % Return contents of that URL as a string. If there is no Internet
              % connection this gives an error, with no output on STDOUT

Answer 6

Bash 66 62 21 bytes

ping -c1 g.gl echo $?

Thanks @Alex L. for the URL shortening tip.

Ungolfed version:

r=$(ping -c1 g.gl)
if [ $? -ne 0 ];
 then echo "0"
else echo "1"
fi

This is my first answer in Bash , i'm not sure i have shortened the script enough.

Answer 7

Java, 72 bytes

a->new java.net.InetSocketAddress("to.",80).getAddress().isReachable(9);

Answer 8

R, 20 bytes

curl::has_internet()

There's a function for exactly this task in the curl package.

Answer 9

C#, 87 bytes

_=>{try{new System.Net.WebClient().OpenRead("http://g.gl");return 1;}catch{return 0;}};

If an exception is considered falsey, which I don't think it is, then this is 65 bytes:

_=>new System.Net.WebClient().OpenRead("http://g.gl").ReadByte();

I also tried using the link http://to. as stated by @KevinCruijssen but it didn't seem to work.

Answer 10

8th, 23 21 bytes

Two bytes saved thanks to Kevin Cruijssen's suggestion and to my discovery: http://to seems to work as well as http://to. (saving another byte)

"http://to" net:get .

If site http://to can be reached, it then prints true. Otherwise it prints false. It leaves retrieved data on the stack.

Answer 11

Perl, 15 bytes

print`curl to.`

Run with:

perl -e 'print`curl to.`' 2> /dev/null

curl outputs stuffs on STDERR, don't mind them. If the computer has access to internet, it will print a few lines of html (truthy), otherwise, it will print nothing (falsy).

Saved 1 bytes by using to. (instead of my previous b.io) thanks to @Kevin Cruijssen.

Answer 12

MATLAB, 32 22 bytes

urlread('http://g.gl')

Explanation:

If the internet connection is up, this will result in ans (the default variable) being a string with the entire html-code in plain text (which is true in MATLAB).

If the internet connection is down, this will write an error message to STDERR and leave the workspace empty (which is false in MATLAB).

Unfortunately, urlread requires a full url-address, so g.gl is not enough. 11 of the 22 bytes are therefore just the url-address.

---

Alternative approach:

A solution that catches the error and leave a 0 (also false) in the workspace if the connection is down:

0;try urlread('http://g.gl'),end

0; initializes the default variable ans to 0, which is false in MATLAB. Then we try to read the url. This will give an error if the internet connection is down, or a character array if not (which is true in MATLAB).

We don't need to catch anything, so we just end. If the urlread call was successful, then ans will be a long string with the content of the website, otherwise ans=0.

Answer 13

Bash, 39 bytes

exec 4<>/dev/tcp/to./80&&echo 1||echo 0

Answer 14

JavaScript ES6, 71 43 bytes

fetch``.then(a=>alert(1)).catch(a=>alert``)

Alerts 1 if online, alerts an empty string if offline. Thanks to Patrick Roberts for helping me shave off some bytes

Old version

_=>fetch('http://enable-cors.org').then(a=>alert(a)).catch(a=>alert(0))

Alerts [object Reponse] if online, alerts 0 if offline

Removed the code snippet, it doesn't work because it loads from a different domain without CORS, but it works in the browser console

Answer 15

PHP, 23 PHP + Curl, 14

Using PHP's backtick operator:

<?=`curl to.`;

---

Orignal answer:

I will try to make a start:

<?=file('http://x.gl');

This outputs nothing if x.gl can't be reached and Array if it is.

Another version where I'm not quite sure if they fit:

<?=getmxrr('x.gl',$a);  // 22 chars

Answer 16

Powershell, 64 26 23 bytes

Saved 38 bytes, thanks to Shawn Esterman

Saved 3 bytes, and repaired script, thanks to briantist

Test-Connection -q g.gl

Answer 17

JavaScript ES6, 90 81 Bytes

f=a=>{i=new Image();i.src="//placehold.it/1x1";i.onload=b=>a(1);i.onerror=c=>a()}

JavaScript ES6, 22 21 bytes (Invalid)

Some browsers don't fully support, or produce the expected result when using navigator.onLine.

f=a=>navigator.onLine

Answer 18

Scala, 54 bytes

x=>(Runtime.getRuntime exec "ping -c 1 ai."waitFor)<1

Pretty simple; executes a ping command to http://ai./, and returns true if it exits with 0, or false otherwise.

Answer 19

Brainfuck (non-competing) 21 bytes

++++++[>++++++++<-]>.

Brainfuck can't connect to the internet (as far as I'm aware), so since the program is unable to connect, the answer is always 0

Non-competing because it seems to fall under the hard-coded output standard loophole, even though this program technically is correct for the challenge.

Answer 20

Python 3 + requests, 59 55 53 bytes

There has to be a requests answer, right?

from requests import*
try:get("http://to.")
except:Z

Exit status is 0 for internet, 1 for no internet. Example:

$ python inet.py 
$ echo $?
0
$ # Remove ethernet cable
$ python inet.py 
$ echo $?
1

Changelog:

• -4 bytes (thanks Mego)

Answer 21

Python 2.7, 70 77 Bytes

from urllib import*
a=1
try:urlopen('http://to.')
except:a=0
print a

import urllib as l
try: 
 l.urlopen('http://a.uk')
 print 1
except:
 print 0

Uses 1 for truthy, 0 for falsy. a.uk redirects to a motorbike clothing company. Saved 3 bytes by assigning to a variable and printing that. And another one for the "to." trick (confirmed to work with urllib), two for getting rid of the pesky indents.

Answer 22

Elixir, 33 bytes

{:ok,_}=:inet.getaddr('to',:inet)

0 if connected, 1 otherwise.

Answer 23

Julia + Bash (with dnsutils), 10 bytes

run(`dig`)

`command` in julia creates a cmd object that can be run with run.

Answer 24

PowerShell, 12 bytes

!!(irm g.gl)

Answer 25

Mathematica 10 Bytes

Assuming you have a valid copy of Mathematica, and login credentials on user.wolfram.com

CloudPut@1

will write the value 1 to the cloud. Truthy: CloudObject[""] Falsey: $Failed

CloudGet@%

Will return the value 1 that was uploaded to the cloud.

Answer 26

Clojure, 49 bytes

#(try(slurp"http://to.")1(catch Exception _ nil))

Returns 1 if it can connect, and nil otherwise.

Just attempts to slurp the page; throwing a NoRouteToHostException exception on failure, which is caught.

Unfortunately, the protocol and dot seem to be mandatory.

Answer 27

Javascript (Nashorn), 61 bytes

print(new java.net.InetSocketAddress("to.",80).getAddress())

Based on Huntro's Java answer.