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Asked: 2026-06-04T13:47:05+00:00

I am trying to write a script that uses a user-created MAC address when

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I am trying to write a script that uses a user-created MAC address when using the wlan0 interface.

To test, I made a file called testFile.txt, which is a copy of /etc/network/interfaces. If there are interfaces after wlan0, I cannot use echo "$var" >> testFile.txt because that simply adds the text to the end.

I am able to find the end of the wlan0 interface text, but I am not sure how to insert there. Below is what I currently have:

#!/bin/bash

echo "Enter MAC Address"
read var
log=$(cat testFile.txt | grep -o "wlan0.*" | grep -o dhcp)
echo $log

echo $log prints dhcp.

I tried adding | echo "hwaddress ether $var" >> testFile.txt to $log but that still appends to the end of the file.
How do I insert directly after $log?

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1 Answer

  1. Editorial Team

    The Problem

    1. Useless use of cat. Don’t do that.
    2. You’re not using the right tool for the job. Use GNU sed to make life easier on yourself.

    The Solution

    # Insert hwaddress line into /etc/network/interfaces.
read -p "Enter MAC Address: "
sudo sed -i.bak "/iface wlan0/a\    hwaddress $REPLY" /etc/network/interfaces

# Replace existing hwaddress line in /etc/network/interfaces.
read -p "Enter MAC Address: "
sudo sed -ri.bak "s/(hwaddress).*/\1 $REPLY/"
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