Print mac address to file

ifconfig will output information about your interfaces, including the MAC address:

$ ifconfig eth0
eth0      Link encap:Ethernet  HWaddr 00:11:22:33:44:55  
          inet addr:10.0.0.1  Bcast:10.0.0.255  Mask:255.0.0.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:289748093 errors:0 dropped:0 overruns:0 frame:0
          TX packets:232688719 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:3264330708 (3.0 GiB)  TX bytes:4137701627 (3.8 GiB)
          Interrupt:17 

The HWaddr is what you want, so you can use awk to filter it:

$ ifconfig eth0 | awk '/HWaddr/ {print $NF}'
00:11:22:33:44:55

Redirect that into a file:

$ ifconfig eth0 | awk '/HWaddr/ {print $NF}' > filename

Here's a modern Linux method:

ip -o link show dev eth0 | grep -Po 'ether \K[^ ]*'

It's modern in that ifconfig has long been deprecated in favour of ip from the iproute2 package, and that grep has the -P option for perl regular expressions for the zero-width positive look-behind assertion.

grep -o is nice for text extraction. sed is traditionally used for that but I find the perl-style zero-width assertions clearer than a sed substitution command.

You don't actually need the -o (oneline) option to ip, but I prefer to use it when extracting network information since I find it cleaner having one record per line. If you're doing more complicated matches or extractions (usually with awk), -o is essential for a clean script, so for the sake of consistency and a common pattern, I always use it.

#! /bin/sh

/sbin/ifconfig eth0 | perl -ne 'print "$1\n" if /HWaddr\s+(\S+)/' >file

There are other tools that could cut the MAC address out of ifconfig's output, of course. I just like Perl.