With a bash script, can I read the mac address of my eth0 and print it to a 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:  Bcast:  Mask:
          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)

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

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

Redirect that into a file:

$ ifconfig eth0 | awk '/HWaddr/ {print $NF}' > filename
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    You can also use th ip command: /sbin/ip link show eth0. – Keith Apr 18 '11 at 20:39
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    @Michael On Linux anyway, it is the preferred tool. In fact you must use to configure some advanced features. But ifconfig lives on... – Keith Apr 18 '11 at 20:55
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    The international approach is LC_ALL=C ifconfig eth0 | awk '/HWaddr/ {print $NF}', because the output might be localized, not matching 'HWaddr'. LC_ALL=C uses the international standard. – user unknown Apr 19 '11 at 0:21
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    @user Ah, cool. As far as I know it'll always be the first line, so you could just get away with awk '{print $NF; exit}' too – Michael Mrozek Apr 19 '11 at 0:31
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    Yes, if you're the AWK-type. I'm the sed-type, but would here prefer an | head -n 1 then. – user unknown Apr 19 '11 at 3:53

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.

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    @michelemarcon: This does not use perl. It uses recent GNU grep and grep has an option for using perl-style regular expressions. – camh Apr 19 '11 at 13:18
  • I have busybox grep, thanks however for the clarification. +1 – michelemarcon Apr 19 '11 at 13:20
#! /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.

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    I don't have perl on my system... :( However, +1 – michelemarcon Apr 18 '11 at 15:44

Using ip -br link show eth0 will print something like this:

$ ip -br link show eth0 
eth0             UP             85:e2:62:9c:b2:02 <BROADCAST,MULTICAST,UP,LOWER_UP>

You only need the 3rd column, so:

$ ip -br link show eth0 | awk '{ print $3 }'
$ ip -br link show eth0 | awk '{ print $3 }' > file

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