Previous answers suggested:
ifconfig: which is deprecated.
ip -s link: which doesn't display all the information that ifconfig displays (such as IP address).
I wanted to offer another command that worked for me. So if you're looking for IP address(es) of an interface, you might consider:
For the raw interface output:
ip a | grep s1 -A3
For IPv4 only:
ip -4 a | grep s1 -A1
ip a is an abbrevation for ip address. -A1 and -A3 paremeters determine the number of lines to display after the last occurrence of your text. If you modify the parameters in ip command, you can change the -A parameter's number to fit as well.