You have two options (see this StackOverflow answer [here][1]):
- **Preferred:** Surround the invocation in `$()`
- Surround the invocation in back ticks 

> NOTE: [back ticks are legacy][2], the former method is preferred.

```bash
output=$(curl -I http://google.com | head -n 1| cut -d $' ' -f2)
echo "$output";

output=`curl -I http://google.com | head -n 1| cut -d $' ' -f2`
echo "$output";
```

 [1]: https://stackoverflow.com/questions/8737638/assign-curl-output-to-variable-in-bash?rq=1
 [2]: https://github.com/koalaman/shellcheck/wiki/SC2006