/sbin/ifconfig
and read off the "HWaddr" value.
For example:
$ /sbin/ifconfig
eth0 Link encap:Ethernet HWaddr 00:0C:76:3B:53:C1
inet addr:128.100.5.88 Bcast:128.100.5.255 Mask:255.255.255.0
UP BROADCAST NOTRAILERS RUNNING MULTICAST MTU:1500 Metric:1
RX packets:19040 errors:0 dropped:0 overruns:0 frame:0
TX packets:4643 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
The MAC Address here is: 00:0C:76:3B:53:C1
However, this command will only work on some Linux distributions if the interface was already able to lease an IP. We are then facing a chicken and egg problem. To get around this type:
/sbin/ifconfig eth0 up
and then run the command:
/sbin/ifconfig