Generates a randomized locally-administered unicast MAC address
Interesting idea, and pretty good first Bash script.

Bash arithmetics

Instead of this:


let "byte=$RANDOM%255"



I find this more readable:

((byte = RANDOM % 255))


You can use math like this within ((...)), without $ of variables, and with spaces around operators to keep the expressions nicely readable.

You can use this in conditions too,
in fact, instead of this:


if [ $i -eq 0 ];then



It's better to write like this:

if (( i == 0 )); then


And here, this is really awkward, as you are running a sub-shell, and storing the output of echo commands:


prependZero=$([ $byte -lt 16 ] &amp;&amp; echo 1 || echo 0)



When it could be simply:

((prependZero = byte &lt; 16))


Padding with 0

Since you already use printf to convert from integer to hexadecimal,
you can actually use it to 0-pad too:

byte=$(printf "%02x" "$byte")


This will pad with zeros on the left to make it 2-digits wide    

Suggested implementation

With the above tips, the script can be simpler and cleaner.

#!/bin/bash

macArr=( 0 0 0 0 0 0 )
for ((i = 0; i &lt; ${#macArr[@]}; ++i))
do
    ((byte = RANDOM % 255))

    # the first byte must have the least significant bits set to "10" 
    # in order to identify as a locally administered unicast MAC
    if (( i == 0 )); then
        ((byte = ((byte &gt;&gt; 1) | 1) &lt;&lt; 1))
    fi

    # convert byte from decimal int to hex string
    byte=$(printf "%02x" "$byte")

    # uppercase any letters in the string
    macArr[$i]="${byte^^}"
done
printf "%s" "${macArr[@]}"