How to use ‘readarray’ in bash to read lines from a file into a 2D array

This is the expected behavior. readarray will create an array where each element of the array is a line in the input.

If you want to see the whole array you need to use

echo "${myarray[@]}"

as echo "$myarray will only output myarray[0], and ${myarray[1]} is the second line of the data.

What you are looking for is a two-dimensional array. See for instance this.

If you want an array with the content of the first line, you can do like this:

$ read -a arr < demo.txt 
$ echo ${arr[0]}
1
$ echo ${arr[1]}
2
$ echo ${arr[2]}
3