How can I check Internet access using a Bash script on Linux?

Question

In my school, the Internet is not available (every night after 23:00 the school will kill the Internet connection, to put us to bed >..<), Then the ping will never stop, though I have used the parameter ping -w1 ....

That is, when I use: ping -q -w1 -c1 8.8.8.8 to check if the Internet connection is up/down, it will be there without any output and doesn't exit, just like I am using a single cat.

I don't know why it's like this, But I think the problem is related to the school-internet-service. Any suggestion? (I think wget may be a good alternative, but how can I use it?)

Answer 1

Using wget:

#!/bin/bash
    
wget -q --tries=10 --timeout=20 --spider http://google.com
if [[ $? -eq 0 ]]; then
    echo "Online"
else
    echo "Offline"
fi

Answer 2

If the school actually turns off their router instead of redirecting all traffic to a "why aren't you in bed" page, then there's no need to download an entire web page or send HTTP headers. All you have to do is just make a connection and check if someone's listening.

nc -z 8.8.8.8 53

This will output "Connection to 8.8.8.8 port 53 [tcp/domain] succeeded!" and return a value of 0 if someone's listening.

If you want to use it in a shell script:

nc -z 8.8.8.8 53  >/dev/null 2>&1
online=$?
if [ $online -eq 0 ]; then
    echo "Online"
else
    echo "Offline"
fi

Answer 3

Use:

#!/bin/bash

INTERNET_STATUS="UNKNOWN"
TIMESTAMP=`date +%s`
while [ 1 ]
 do
    ping -c 1 -W 0.7 8.8.4.4 > /dev/null 2>&1
    if [ $? -eq 0 ] ; then
        if [ "$INTERNET_STATUS" != "UP" ]; then
            echo "UP   `date +%Y-%m-%dT%H:%M:%S%Z` $((`date +%s`-$TIMESTAMP))";
            INTERNET_STATUS="UP"
        fi
    else
        if [ "$INTERNET_STATUS" = "UP" ]; then
            echo "DOWN `date +%Y-%m-%dT%H:%M:%S%Z` $((`date +%s`-$TIMESTAMP))";
            INTERNET_STATUS="DOWN"
        fi
    fi
    sleep 1
 done;

The output will produce something like:

./internet_check.sh

UP   2016-05-10T23:23:06BST 4
DOWN 2016-05-10T23:23:25BST 19
UP   2016-05-10T23:23:32BST 7

The number in the end of a line shows duration of previous state, i.e. 19 up, 7 seconds down.

Answer 4

Without wget

#!/bin/bash

echo -e "GET http://google.com HTTP/1.0\n\n" | nc google.com 80 > /dev/null 2>&1

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

Enjoy ;)

Answer 5

Use the timeout option -t:

ping -q -t 5 -w1 -c1 8.8.8.8 t

Answer 6

Install fping: > fewer problems than ping.

fping google.com | grep alive

To use, for example, like:

#!/bin/bash

itest=$(fping google.com | grep alive)

while [ "$itest" == "" ]
        do
        sleep 5
        itest=$(fping google.com | grep alive)
done
echo now online

Answer 7

Using the example above. I wrote this script to log the state of your connection: https://gist.github.com/cganterh/ffc2fffa8263857cbece

First, save the following code into a name.sh file.

#!/bin/bash

while true
do
    wget -q --tries=10 --timeout=20 -O - http://google.com > /dev/null
    if [[ $? -eq 0 ]]; then
        echo $(date) "1" | tee -a log.csv
    else
        echo $(date) "0" | tee -a log.csv
    fi
    sleep 5
done

Then, execute name.sh file in terminal, and then check the log state information in log.csv of the same folder.

Answer 8

Reliable old ping in a separate Bash script:

#!/bin/bash
ipaddr='8.8.8.8' # Google's public DNS server
[[ -z `ping -c1 $ipaddr |& grep -o 'Network is unreachable'` ]] || exit 1
[[ -z `ping -c3 $ipaddr |& grep -o '100% packet loss'` ]] && exit 0 || exit 1

---

Put this in a separate script. It will handle different network situations as (1) not being connected to a network, (2) connected to the network, but cannot access the Internet (or at least Google), and (3) connected to the Internet.

---

You may later use the exit code of the script to check connectivity, e.g.

~$ script-name && echo online || echo offline

Answer 9

I decided to combine a few of the previous answers, so I could later create a plot showing ups, downs, and their durations:

#!/bin/bash
#
# pinger is a bash shell script that monitors the network
# status every 15 seconds and records if it is up '1' or down '0'
# into the file log.csv from whence it may be plotted.
#
# author: J. W. Wooten, Ph.D.
# since: 11/12/2019
# version: 1.0
#
TIMESTAMP=`date +%s`
while [ 1 ]
  do
    nc -z -w 5 8.8.8.8 53  >/dev/null 2>&1
online=$?
    TIME=`date +%s`
    if [ $online -eq 0 ]; then
      echo "`date +%Y-%m-%d_%H:%M:%S_%Z` 1 $(($TIME-$TIMESTAMP))" | tee -a log.csv
    else
      echo "`date +%Y-%m-%d_%H:%M:%S_%Z` 0 $(($TIME-$TIMESTAMP))" | tee -a log.csv
    fi
    TIMESTAMP=$TIME
    sleep 15
  done;

This outputs to a CSV file every 15 seconds. Using Excel or Numbers, you can read the file and create a plot which will show when the Internet connection was not available and also the duration. If it changes from your sleep interval, then it is spending time trying to connect. I hope to add the ability to send me a text when it detects network is down next.

Answer 10

A variation on Majal's solution is just to test the return code from ping, which returns 0 if the site responds, 1 if there is no reply and 2 if the network is unreachable.

    ping -c 1 -t 5 8.8.8.8 2&>1
    rc=$?
    [[ $rc -eq 0 ]] && { echo "Connected to the Internet" ; exit 0 ; } \
 || [[ $rc -eq 1 ]] && { echo "No reply from Google DNS" ; exit 1 ; } \
 || [[ $rc -eq 2 ]] && { echo "Network unreachable" ; exit 2 ; }

Using ping has the advantage of not needing to download anything, improving the speed of the test.