9 commands to check if connected to internet with shell script examples

In this article I will share multiple commands and examples to check if connected to internet for your Linux machine. At the end of the article I will also share some example to test internet connection from the Linux to external network using shell script.

But before we do that there are certain things you must take care of, such as

• You have IP Address assigned to your network interface (static or dhcp)
• Your gateway is reachable from the Linux system
• If you are on virtual machine then make sure your VM is configured to be able to connect external network
• Your DNS is properly configured

Now there are multiple commands which are available in Linux which can be used to check if connected to internet, we will try to discuss some of them here in brief.

1. Ping Test

The very first tool I use to check if connected to internet is via ping utility. But do you know how to check ping or perform ping check to check if connected to internet? Ping is part of iputils rpm so make sure iputils is installed on your setup

To perform ping check, try to ping any page on internet such as google.com

# ping -c 1 google.com
PING google.com (172.217.166.110) 56(84) bytes of data.
64 bytes from maa05s09-in-f14.1e100.net (172.217.166.110): icmp_seq=1 ttl=49 time=46.9 ms

--- google.com ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 46.923/46.923/46.923/0.000 ms

Here we use -c 1 with ping tool to send only one packet. Now check the statistics section, we see 1 packet was transmitted and received and there was 0% packet loss so it means we are connected to internet.

If you plan to check if connected to internet via shell script then you can use below, anything in the output other than "0" means not connected to internet.

# ping -c 1 -q google.com >&/dev/null; echo $?
0

2. Check port availability using cat, echo..

There are various tools which can be used to check port availability which I will share in this article. But many of them are not installed by default so you may have to install them manually. Alternatively you can use below command to check if connected to internet without installing any additional rpm

# cat < /dev/null > /dev/tcp/8.8.8.8/53; echo $?
0

If the output is 0 then it means you are connected to internet, but if the output is something like below

# cat < /dev/null > /dev/tcp/8.8.8.8/53; echo $?
-bash: connect: Network is unreachable
-bash: /dev/tcp/8.8.8.8/53: Network is unreachable
1

Then your linux node is not connected to internet.

Alternatively you can also use

# echo -n >/dev/tcp/8.8.8.8/53; echo $?
0

3. DNS lookup using nslookup, host etc..

You can perform a DNS lookup any web page address to check if connected to internet. With a successful DNS lookup you should get a response something like below.

# nslookup google.com
Server:         192.168.0.10
Address:        192.168.0.10#53

Non-authoritative answer:
Name:   google.com
Address: 172.217.166.110
Name:   google.com
Address: 2404:6800:4007:811::200e

For a failed DNS lookup you should get something like

# nslookup google.com1
Server:         8.8.8.8
Address:        8.8.8.8#53

** server can't find google.com1: NXDOMAIN

There are many more commands to perform DNS lookup such as host, dig etc

# host www.google.com
www.google.com has address 216.58.196.164
www.google.com has IPv6 address 2404:6800:4007:812::2004

4. Curl

curl is a tool to transfer data from or to a server, using one of the many supported protocols such as HTTP, FTP etc. We can also use this tool to query a webpage and test internet connection in Linux.

# curl -Is  http://www.google.com | head -n 1
HTTP/1.1 200 OK

Here if you receive anything other than 200 OK then it means the server failed to connect to the provided page. So unless you provide an invalid webpage, your node is connected to internet

5. Telnet

Telnet is another tool to check port connectivity so you can check port availability of any webpage from the Linux node to check if connected to internet. We can try connecting port 53 of Google DNS to check internet connection.

# telnet 8.8.8.8 53
Trying 8.8.8.8...
Connected to 8.8.8.8.
Escape character is '^]'.
Connection closed by foreign host.

As you see the session was "connected". If you get an output like below

# telnet 8.8.8.8 53
Trying 8.8.8.8...
telnet: connect to address 8.8.8.8: Network is unreachable

then it means there is a problem with internet connectivity.

6. Nmap

nmap is normally a port scanner to check the list of open ports on a system. We will use this to connect to external network to scan the port. If it is able to connect to the external network for port scanning then we can check if connected to internet.

# nmap -p 443 google.com
Starting Nmap 7.70 ( https://nmap.org ) at 2019-11-19 07:40 IST
Nmap scan report for google.com (172.217.166.110)
Host is up (0.054s latency).
Other addresses for google.com (not scanned): 2404:6800:4007:811::200e
DNS record for 172.217.166.110: maa05s09-in-f14.1e100.net

PORT    STATE SERVICE
443/tcp open  https

Nmap done: 1 IP address (1 host up) scanned in 0.77 seconds

Here I am scanning google.com on port 443. As you see highlighted section, nmap was able to establish connection with google.com

7. netcat or nc

In some variant of Linux you will find netcat while in others nc, you can use either of these tools for port scanning. nc or ncat is part of nmap-ncat rpm. Here we use nc command to check connection to google.com on port 443

# nc -vz google.com 443
Ncat: Version 7.70 ( https://nmap.org/ncat )
Ncat: Connected to 172.217.166.110:443.
Ncat: 0 bytes sent, 0 bytes received in 0.07 seconds.

8. wget

GNU Wget is a free utility for non-interactive download of files from the Web. But we can also use this to check if connected to internet.

# wget -q --spider http://google.com ; echo $?
0

Here with --spider Wget will behave as a Web spider, which means that it will not download the pages, just check that they are there
With echo $? we check the exit status, anything other than 0 in the output means your system is not connected to internet

9. Traceroute

The traceroute command shows every step taken on the route from your machine to a specified host. Assuming you are having problems connecting to www.google.com so you can use traceroute to check all the hubs taken to reach the destination server. This will also test internet connection.

# traceroute www.google.com
traceroute to www.google.com (216.58.196.164), 30 hops max, 60 byte packets
 1  _gateway (192.168.0.1)  3.046 ms  2.873 ms  3.291 ms
 2  10.249.0.1 (10.249.0.1)  4.148 ms  4.676 ms  4.678 ms
 3  broadband.actcorp.in (202.83.20.173)  5.153 ms  5.445 ms  6.305 ms
 4  broadband.actcorp.in (202.83.20.181)  4.952 ms  6.212 ms  6.195 ms
 5  * * *
 6  72.14.194.18 (72.14.194.18)  48.202 ms  49.669 ms  49.504 ms
 7  108.170.253.113 (108.170.253.113)  50.061 ms 108.170.253.97 (108.170.253.97)  48.480 ms  48.512 ms
 8  216.239.43.239 (216.239.43.239)  49.911 ms  56.915 ms 216.239.43.235 (216.239.43.235)  57.610 ms
 9  * * *
10  * * *
11  * * *
12  * * *

Here we were able to trace the route to google.com upto a certain point.

What do those * * * mean?

Each one indicates a five-second timeout at that hop. Sometimes that could indicate that the machine simply doesn’t understand how to cope with that traceroute packet due to a bug, but a consistent set of * indicates that there’s a problem somewhere with the router to which 216.239.43.239 hands off packets.

On a Linux system with no internet connection, you would face problem with DNS resolution itself

# traceroute www.google.com
www.google.com: Name or service not known
Cannot handle "host" cmdline arg `www.google.com' on position 1 (argc 1)

How To check if connected to internet using shell script?

You can easily use any or all the above commands/methods to check if connected to internet using bash/shell script.

For example by using wget you can test internet connection in the below shell script:

#!/bin/bash

wget -q --spider https://google.com

if [ $? -eq 0 ]; then
    echo "Online"
else
    echo "Offline"
fi

Here this script will print the status of the internet, similarly for other scripts you can check the exit status and perform tasks accordingly.

Lastly I hope the steps from the article to check if connected to internet (test internet connection) on Linux was helpful. So, let me know your suggestions and feedback using the comment section.

References:
Linux: Phrasebook