LinuxQuestions.org - [SOLVED] Using script variable in awk subtraction

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Using script variable in awk subtraction

I have the following script:

Code:

#!/bin/bash



##################################################################

#

# Purpose:

# When RHEL6 VMs are moved between compute pools or VLANs their

# MAC addresses change breaking the interfaces. This script 

# updates the ifcfg-eth* and network interface udev rules files

# to make the configuration match the new addresses.

#

# Changelog:

# 1.0 - Initial script - 05/26/2012 - msnyder

# 1.1 - Switched $udevpath to $udevfile incorporating the actual

#      file in the variable since it is always the same; added

#      documentation - 05/27/2012 - msnyder

# 2.0 - Rewrite to account for variable number of improper

#      udev file entries - 05/29/2012 - msnyder

#

##################################################################





#Set paths to file locations

udevfile=/etc/udev/rules.d/70-persistent-net.rules

netpath=/etc/sysconfig/network-scripts

ifaces=$(grep eth $udevfile |wc -l)

        

shft=$(($ifaces - 1))

adj=$(($ifaces - 3))



for (($i=0; $i<=$shft; $i++))

do

        #Extract MAC addresses of old and new udev entries

        #These will be used to update the ifcfg-eth* files

        MAC[$i]=$(grep eth$i $udevfile |awk -F ',' '{print $4}' |awk -F "==" '{print $2}')



        if [ $i -lt 3 ]

        then

                #If $i is less than 3 comment out the eth$i entry in the udev rules file

                sed -i "/eth$i/ s/^/#/" $udevfile

        elif [ $i -ge 3 ] && [ $i -lt $adj ]

        then

                #Delete the lines associated with the old "new" interfaces

                sed -i "/eth$i/d"

        else

                #Adjust the eth field to the correct value

                int=$(echo $i |awk -v val=$adj '{print ($1 - $val)}')

                sed -i "s/eth$i/eth$int/" $udevfile



                #Change the MAC address in the ifcfg-eth* files to match

                #the entry in the udev rules file for the new interfaces

                sed -i "s/HWADDR.*/HWADDR=${MAC[$i]}/" $netpath/ifcfg-eth$int

        fi

done



reboot

The issue is specifically with the line that assigns the output of the awk subtraction to the int variable:

Code:

int=$(echo $i |awk -v val=$adj '{print ($1 - $val)}')

I know that $val is being assigned the value of $adj as is evidenced by the screenshot.* However, the subtraction does not execute and the value of $int is not set properly (for an $adj of 5 I expect an $int of 2).

What am I doing wrong with this line that it isn't subtracting the value properly?

*I had to use a screenshot because the VMware console window doesn't support copying/pasting text.

I think I solved this. Instead of using the '-v val=$adj' assignment I used

Code:

int=$(echo $i |awk '{print ($1 - '$adj')}')

I'm now seeing the results I expect.

Firstly, your issue was the you used $adj in awk where you call variables with their name, ie. just 'adj'.

Secondly, you previously use bash math:

Code:

adj=$(($ifaces - 3))

Is there a reason to not use it here?

Also you can simplify your current usage further:

Code:

(( adj = ifaces - 3 ))

the problem with that awk is that you do not need $ inside the script:
int=$(echo $i |awk -v val=$adj '{print ($1 - $val)}')
you need to remove it - but of course, (( int = i - adj )) is much better

in line: MAC[$i]=$(grep eth$i $udevfile |awk -F ',' '{print $4}' |awk -F "==" '{print $2}')
only one awk would be enough instead of grep|awk|awk

What would the one awk in the MAC line look like?

awk -F, " /eth$i/ "' { split ($4, a, "=="); print a[2] }'
I have not tested because I have no input file, but probably works, after testing:
MAC[$i]=$( this is the awk )
if you give a sample file we can try to test it also

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I would also point out that your for loop is not going to work either as you are trying to assign the value 0 to the value of 'i' by using $i.

Also, you asked previously how to get the MAC values in your question here, which showed you a number of better methods.

Code:

ifaces=$(grep eth $udevfile |wc -l)

wc is an over kill here, try looking up the -c option for grep.

As with the math examples above you could also change your tests from the older [] form to also use (()):

Code:

if [ $i -lt 3 ]



if (( i < 3 ))

Thanks for the additional input. I discovered the assignment of 0 to $i when I started putting this into full test mode.

Thanks for the heads up with the -c option for grep. That should provide some more elegant lines in the future.