If you don’t know , Shellshock (CVE-2014-6271, CVE-2014-7169, CVE-2014-7186, CVE-2014-7187, CVE-2014-6277, CVE-2014-6278 ) is is a vulnerability in GNU’s bash shell that gives attackers access to run remote commands on a vulnerable system .
Okay lets see how you can perform shellshock vulnerability check .

You can use https://shellshocker.net/ to check your website and local system .

curl https://shellshocker.net/shellshock_test.sh | bash

You will get sample output like this

You can check any website with shellshocket.net , you just have to provide address to cgi script like http://somesite/cgi-bin/something.cgi .

https://shellshocker.net/#systemtest
shellshocker github