cat /proc/net/dev to get the list of available interfaces (most likely you'll have eth0 and wlan0). Then ifup the interface via sudo ifconfig eth0 up. You may need to run sudo dhclient eth0 once the interface is up to get an IP address.

For wireless interface, after ifup, you'll need Network Manager to configure it to connect to a given AP.

Also if static IP is needed, then I normally used these commands to setup the interface:

sudo ifconfig eth0 192.168.0.118

sudo gedit /etc/resolv.conf # then add a line nameserver 192.168.0.1 in that file

sudo route add default gw 192.168.0.1

Are you looking for sudo service networking start ?

I believe network-manager handles the networking and interface on Ubuntu 12.04 so it should be like:

sudo service network-manager start

[EDIT]
Just an update in support of my comment below, for automating SUDO when piping the password to it from external source use:

echo "password" | sudo -S service network-manager start

Which will not ask for the password interactively.

-S Switch here make sudo to read the password from standard input and not from terminal.

You (and all of us) do not have an eth0 for years now. Ubuntu uses predictive names. ifconfig -a shows you the name you need to feed ifconfig.

Example from my machines:

$ ifconfig -a
...
wlo1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500    
        inet 192.168.178.19  netmask 255.255.255.0  broadcast 192.168.178.255
        inet6 fe80::b971:f72f:c416:2a96  prefixlen 64  scopeid 0x20<link>
        ether fc:77:74:bc:87:43  txqueuelen 1000  (Ethernet)
        RX packets 2733275  bytes 3448410896 (3.4 GB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 1070857  bytes 381490320 (381.4 MB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

So for me ifconfig wlo1 will show what you are asking the system with eth0.