Discerning the IP

On a modern box, this is done with ip, which we'll go into greater depth with later:

$ ip address show
You can shorten ip commands to make them quicker to type (as we did in the previous chapter,) but in the preceding code I've used the full expression as it better expresses what we're doing.

This command will give you information about all the interfaces on your system. In the case of our VMs, it looks like this:

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host 
       valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether 52:54:00:c9:c7:04 brd ff:ff:ff:ff:ff:ff
    inet 10.0.2.15/24 brd 10.0.2.255 scope global noprefixroute dynamic eth0
       valid_lft 85733sec preferred_lft 85733sec
    inet6 fe80::5054:ff:fec9:c704/64 scope link 
       valid_lft forever preferred_lft forever
3: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether 08:00:27:4b:03:de brd ff:ff:ff:ff:ff:ff
    inet 192.168.33.10/24 brd 192.168.33.255 scope global noprefixroute eth1
       valid_lft forever preferred_lft forever
    inet6 fe80::a00:27ff:fe4b:3de/64 scope link 
       valid_lft forever preferred_lft forever
4: eth2: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether 08:00:27:50:a5:cb brd ff:ff:ff:ff:ff:ff

We know lo is the loopback address, which will always have the 127.0.0.1/8 setup, or some other address in that range.

If you know the device you want explicitly, you can also specify it. In the following code, we've done this with eth1:

$ ip address show dev eth1

Within the printed block, we're looking for the IPv4 address, which is listed as being in the inet family:

3: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether 08:00:27:0d:d9:0c brd ff:ff:ff:ff:ff:ff
    inet 192.168.33.10/24 brd 192.168.33.255 scope global noprefixroute eth1
       valid_lft forever preferred_lft forever
    inet6 fe80::a00:27ff:fe0d:d90c/64 scope link 
       valid_lft forever preferred_lft forever

We have the IP, 192.168.33.10, and we know the subnet it's in, /24 (255.255.255.0).

If you're a bit lazier and want to save on eye movement, you could use something like the following in a scrip to get this information:

$ ip address show dev eth1 | grep "inet " | awk '{ print $2 }'
192.168.33.10/24

The preceding code is one of many, many ways in which you could write a one-liner to generate the output you desire. Linux is flexible, and you might choose to reach the same output in a completely different way. It doesn't overly matter how you do it, but if it's something you plan on sharing, doing it in a succinct and readable style is often best.
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