env variable mac address script problem

schris403 · 08-01-2007, 07:22 AM

I have been trying for days to try and get the following script to load the mac variable upon boot in the rc.local file and just can't seem to get it to work. It does work when I manually run the line but when I run it as a normal sh script or in rc.local, it doesn't work.

mac=`ifconfig -a eth0 | grep "HWaddr" | cut -d " " -f 11 | tr -s : .`

Maybe there is a different way of accomplishing this, what I want is a variable with my eth0 mac address with . instead of : seperating them. I will be using this on OSX mostly. Any help would be greatly appreciated.

Thanks so much!!

Chris

Centinul · 08-01-2007, 08:00 AM

I'm assuming it doesn't work because the scope of the variable is limited to the script you are trying to run. Effectively once the script ends any variables that are created in it are destroyed.

I'm not familiar with OS X but is there anyway you can just put that assignment in your .bash_profile?

This tutorial essentially outlines what I have said above. Configuring your OS X Unix Environment

HTH,

Centinul

bigearsbilly · 08-01-2007, 08:11 AM

some guesses,
maybe the PATH ain't set up right yet?

try using the full path name for ifconfig etc
(all though it sounds unlikely now I write it)

is it DHCP? maybe eth0 isn't up and running yet when it's called?
isn't rc.local where you set up interfaces with wep keys etc.

I would suggest making a script, set -x and call that in rc.local with all output logged to a file (exec > logfile 2>&1)

split it up see what you are getting

theNbomr · 08-01-2007, 08:12 AM

You haven't shown what actually does result, but I'll offer that the script is not finding ifconfig. Try using an absolute path to it, probably '/sbin/ifconfig'. The rest of the logic looks fine to me, although it is very sensitive to the formatting of ifconfig's output.
--- rod.

schris403 · 08-01-2007, 08:55 AM

It does all indeed work when run manually from a command line, what it outputs is the following example on one line:
00.00.00.00.00.00

It also works with most Linux flavors I have tried it on but the problem goes back to getting it to run as a script or as a startup script to set the variable.

Thanks!
Chris

theNbomr · 08-01-2007, 09:55 AM

Okay, I think I understand what's happening, and it goes back to understanding the scope of an environment. When you run the command as a script, typically it will look something like:

Code:

#! /bin/bash
mac=`ifconfig -a eth0 | grep "HWaddr" | cut -d " " -f 11 | tr -s : .`

When executed, a child shell is launched, and the variable $mac is created correctly in that shell (only). When the shell exits, its environment vanishes with it, and all traces of $mac are lost. The variable is not magically transferred up to the parent shell. To get around this, bash and most other shells allow you to 'source' a file; that is it is executed in the context of the current shell process, just as if you were typing it at your keyboard. So, if you put the command string into a file 'getmac', and then source it:

Code:

source getmac

you should see the $mac variable created in the current shell.

Code:

echo $mac

This same principle is at play when the script is executed in the rc.local script. Once that script has run to completion, the $mac variable no longer exists. Moreover, it only ever did exist for the owner of the script, which is root. To cause the variable to be available to all users, you must have rc.local create a second script which can be sourced by either /etc/bashrc or by user-specific ~/.bashrc scripts. This might be done something like this:
(in rc.local, Redhat style)

Code:

mac=`/sbin/ifconfig -a eth0 | grep "HWaddr" | cut -d " " -f 11 | tr -s : .`
echo "export MAC=$mac" > /etc/profile.d/eth0_mac.sh

Now, when /etc/bashrc executes on each user login, it will source your small script, and create an environment variable '$MAC' containing the MAC address of eth0.
One last point: in bash, the '.' character is shorthand for the command word 'source'. THis notation is more commonly used than the longhand word 'source'.
Hope this is on track.
--- rod.

schris403 · 08-01-2007, 02:05 PM

Thanks! This does help temendously and everything seems to work except for when I want to install it as a system wide variable using export. I've tried using bashrc to run:
source /etc/eth0_mac.sh

it just doesn't seem to run when it is in bashrc. I also tried running it as a cronjob with no luck. It does work though when run manually at a command line. Any thoughts?

Chris

theNbomr · 08-01-2007, 03:04 PM

Check the permissions on the script. Users would need read-execute access. Also, if you used my example, the script would be in /etc/profile.d/, and my /etc/bashrc runs every readable Bourne shell script that it finds there:

Code:

if ! shopt -q login_shell ; then # We're not a login shell
        for i in /etc/profile.d/*.sh; do
            if [ -r "$i" ]; then
                . $i
            fi
        done
        unset i
fi

In the installation I'm presently using, all scripts in /etc/profile.d are permission level 755; all owner & group-onwer root.

--- rod.