How to check if internet connection is available via Terminal

Question

I'm trying to write a Bash script that is aware of internet connectivity.

How can I achieve that?

For example:

if (something that checks internet connection);then
curl -o new someurl
else 
exit 1

Solution 1

Try using the curl command and checking the exit status in an if statement.

curl -D- -o /dev/null -s http://www.google.com
if [[ $? == 0 ]]; then
  foocode
else
  exit 1
fi

Solution 2

I was using snmp when I had a router that support it to get the ip assigned to the Wan interface (.253 was the router addr):

IP=$(/usr/bin/snmpwalk -v2c -Ov -cpublic 192.168.1.253 \
          IP-MIB::ipAdEntAddr 2>/dev/null \
          | /bin/grep -v "0.0.0.0\|127.0.0.1\|192.168.1.253")

Or maybe you could webscarp it from the router's stat pages with curl.

Currently instead I use to check/get the ip from the service I use for the dynamic dns:

IP=$(curl -s 'http://checkip.dyndns.org' | sed 's/.*Current IP Address: \([0-9\.]*\).*/\1/g')

and then:

if [ "$IP" != "" ] && [ "$IP" != "0.0.0.0" ]; then
   # connected