Yrurtj

I need to create a script that outputs the internal IP address, that is configured as the default Interface.

Here's another slightly terser method using procfs (assumes you're using Linux):

default_iface=$(awk '$2 == 00000000 { print $1 }' /proc/net/route)

ip addr show dev "$default_iface" | awk '$1 ~ /^inet/ { sub("/.*", "", $2); print $2 }'

This returns both the IPv4 and (if available) the IPv6 address of the interface. You can change the test if you only want one or the other (look for inet for IPv4, and inet6 for IPv6).

$ default_iface=$(awk '$2 == 00000000 { print $1 }' /proc/net/route)

$ ip addr show dev "$default_iface" | awk '$1 ~ /^inet/ { sub("/.*", "", $2); print $2 }'

10.0.2.15

fe80::a00:27ff:fe45:b085

$ ip addr show dev "$default_iface" | awk '$1 == "inet" { sub("/.*", "", $2); print $2 }'

10.0.2.15

$ ip addr show dev "$default_iface" | awk '$1 == "inet6" { sub("/.*", "", $2); print $2 }'

fe80::a00:27ff:fe45:b085

Here's what I wrote:

1. Get the default interface from the "route" command.

It will print out which interface is the "default". For my host, I need to get the last column of the default line.

[root@pppdc9prd3ga mdesales]# route

Kernel IP routing table

Destination     Gateway         Genmask         Flags Metric Ref    Use Iface

192.168.4.0     *               255.255.252.0   U     0      0        0 bridge0

10.132.60.0     *               255.255.252.0   U     0      0        0 eth4

link-local      *               255.255.0.0     U     1002   0        0 eth4

link-local      *               255.255.0.0     U     1003   0        0 bridge0

default         10.132.60.1     0.0.0.0         UG    0      0        0 eth4

2. Use "ifconfig" to retrieve the IP address of that interface.

Just getting the addr: value.

[root@pppdc9prd3ga mdesales]# ifconfig eth4

eth4      Link encap:Ethernet  HWaddr 00:50:56:01:42:91  

          inet addr:10.132.63.191  Bcast:10.132.63.255  Mask:255.255.252.0

          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1

          RX packets:1346288 errors:0 dropped:0 overruns:0 frame:0

          TX packets:438844 errors:0 dropped:0 overruns:0 carrier:0

          collisions:0 txqueuelen:1000 

          RX bytes:276243478 (263.4 MiB)  TX bytes:116188062 (110.8 MiB)

So here's the script I came up with.

/app/myPublicIp.sh 

defaultInterface=$(route | grep default | awk '{print $(NF)}')

ifconfig $defaultInterface | grep 'inet addr:' | cut -d: -f2 |  awk '{ print $1}'

Here's it executing:

/app/ipFor.sh 

10.132.63.191

Hope it helps!

Lots of good answers here, but wanted to throw in my usual approach:

The simplest solution is to get the route for a public internet address:

$ ip route get 1.1.1.1 | grep -oP 'src KS+'

192.168.0.20

Another solution is to get the default gateway, and then get the IP addr used to communicate with that gateway:

$ ip route get $(ip route show 0.0.0.0/0 | grep -oP 'via KS+') | grep -oP 'src KS+'

192.168.0.20

If what you want is the IP address assigned to the default interface (which is what I understood from the comments under the question), using the Swiss army knife of network setup (ip) should be enough:

$ ip route | grep '^default'

default via 10.176.143.1 dev eth1  metric 203 

$ ip addr show eth1

4: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000

    link/ether c0:de:f1:72:30:48 brd ff:ff:ff:ff:ff:ff

    inet 10.176.143.71/24 brd 10.176.143.255 scope global eth1

       valid_lft forever preferred_lft forever

This shortens to

$ ip addr show 

    $( /sbin/ip route 

        | grep '^default'

        | sed 's/^.*dev ([^ t]*) .*$/1/' )

which is ugly, because it is parsing something that probably wasn't really meant to be parsed (output of ip route), but should work.

My favorite one is following.

Get the default interface:

$ ip r | grep -oP 'default .* K.+'

eth0

Get the ip of an interface:

$ ip a show eth0 | grep -oP 'inet K[d.]+'

10.33.44.135

Combined:

$ ip a show $(ip r | grep -oP 'default .* K.+') | grep -oP 'inet K[d.]+'

10.33.44.135

Simple Command with default interface.

ip route | grep src | awk '{print $NF; exit}'

Tested on All Unix OS