Bash – Unexpected output when accessing array element

Here's a slight modification to what you were already doing:

set -f # to prevent filename expansion
for i in ${!NIC_index[@]}
do
  printf "%-20s" ${NIC_details[i]}
  printf "\n"
done

I added a for loop to go over all of the array indexes -- that's the ${!NIC... syntax. Since it's a numeric array, you could alternatively loop directly over the indexes.

Note that I intentionally left ${NIC_details[i]} unquoted so that the shell would split the value on the spaces that you've used to separate the values. printf thus sees the 3 values and so repeats the %-20s formatting for each of them. I then add a newline after each NIC to make a nice table.

The set -f at the top is important if we leave variables unquoted; otherwise the shell will step in and attempt to glob any wildcards it sees in unquoted variables.

Sample output:

wlp2s0              192.168.1.221       xx:xx:xx:xx:xx:xx
wwan0               none                xx:xx:xx:xx:xx:xx
virbr0              192.168.122.1       00:00:00:00:00:00

You're flattening the input into a single value.

You should do

list=("${@}")

to maintain the array and the potential of whitespace in arguments.

If you miss out the " then something like ./script.sh "a b" 2 3 4 will return a length of 5 because the first argument will be split up. With " we get

$ cat x
#!/bin/bash
list=("${@}")

echo array = ${list[*]} and length = ${#list[*]}

$ ./x "a b" 2 3 4  
array = a b 2 3 4 and length = 4