use dhcp on eth0 using command line

If your dhcp is properly configured to give you an IP address, the command:

dhclient eth0 -v 

should work. The option -v enable verbose log messages, it can be useful.

If your eth0 is already up, before asking for a new IP address, try to deconfigure eth0.

To configure the network interfaces based on interface definitions in the file /etc/network/interfaces you can use ifup and ifdown commands.