I want to use a bash script on my raspberry pi to check whether the internet connection is still present.

I found this script which I extended:

#!/bin/bash

# I do not work properly
if [[ "$(ping -c 1 8.8.8.8 | grep 'Network is unreachable' )" != "" ]]; then
 echo "Network isn't present"
 sudo shutdown -r 0
 exit 1
fi

# I work properly
if [[ "$(ping -c 1 8.8.8.8 | grep '100% packet loss' )" != "" ]]; then
 echo "Internet isn't present"
 sudo shutdown -r 0
 exit 1
else
 echo "Internet is present"
 exit 0
fi

The second part works nice, the first one, however, does not.

I unplugged my Ethernet cable and get an Error message like connect: Network is unreachable. So as expected. But I am not understanding why my script is not picking it up? I think it has something to do with it being an error message and not an output. But I do not know how I would have to adjust my script to grep from error messages as well, assuming this is correct.

I found a way how to get the output to be registered by grep by using this line ping -c 1 8.8.8.8 2> >(grep 'unreachable';). However, this still makes my script fail because it is no longer recognized by the rest of that line then. And I also do not entirely understand what 2> >() does.

You don't really need a string comparison here:

ping command gives an appropriate return code after it completes execution.

So, you could use something like:

 function check_connectivity() 

 local test_ip
 local test_count

 test_ip="8.8.8.8"
 test_count=1

 if ping -c $test_count $test_ip > /dev/null; then
 echo "Have internet connectivity"
 else
 echo "Do not have connectivity"
 fi
 

 check_connectivity

Try this:

x=`ping -c1 google.com 2>&1 | grep failure`
if [ ! "$x" = "" ]; then
echo "Network isn't present"
sudo shutdown -r 0
exit 1
fi

Or for your script I would do:

if [[ "$(ping -c 1 8.8.8.8 | grep '100% packet loss' )" != "" ]]; then
 echo "Internet isn't present"
 sudo shutdown -r 0
 exit 1
elif [[ "$(ping -c 1 8.8.8.8 | grep 'packet loss' | grep -o "1[0-9][0-9]+%")" != "" ]]; then
 echo "Network isn't present"
 sudo shutdown -r 0
 exit 1
else
 echo "Internet is present"
 exit 0
fi

But i recommending using /sys/class/net instead

# Test for network conection
for interface in $(ls /sys/class/net/ | grep -v lo);do
if [[ $(cat /sys/class/net/$interface/carrier) = 1 ]]; then OnLine=1;fi;done
if ! [ $OnLine ]; then echo "Internet isn't present" > /dev/stderr;sudo shutdown -r 0; exit; fi