What is expr doing when processing arrays?

Question

I have a BASH script problem that I've narrowed down but I can't understand what I'm seeing. I want to get a substring of an array in a succinct way. Here's what I'm seeing in bash, with 'set -x' so that commands are printed as they are executed:

$: read -a colours
+ read -a colours
blue green red yellow pink purple
$: echo ${colours[@]}
+ echo blue green red yellow pink purple
blue green red yellow pink purple
$: expr substr "${colours[@]}" 16 6
+ expr substr blue green red yellow pink purple 16 6
expr: syntax error: unexpected argument ‘yellow’
$: expr substr \"${colours[@]}\" 16 6
+ expr substr '"blue' green red yellow pink 'purple"' 16 6
expr: syntax error: unexpected argument ‘yellow’

The last attempt at the expr substr command returned odd quoting for the array elements, so where did that come from? Help me understand what expr substr is doing here or is this a bug?

EDIT: I've listed below some desired behaviour on a flat quoted string, but why can't I quote an array in the same way?

$: all_colours=${colours[@]}
+ all_colours='blue green red yellow pink purple'
$: expr substr $all_colours 16 6
+ expr substr blue green red yellow pink purple 16 6
expr: syntax error: unexpected argument ‘yellow’
$: expr substr "$all_colours" 16 6
+ expr substr 'blue green red yellow pink purple' 16 6
yellow

Answer 1

Expr is a command line tool, it is independent from the bash. It does not know anything about arrays. It gets command line arguments, exactly those what we refer as $1, $2, $3 etc in our scripts.

Important thing is: bash knows arrays, but as you call a binary, it can only get a list of arguments as its parameters.

If you call expr with an array expanded on your syntax, you will get what you said. expr substr will expect numbers, you give to it many text words, so it will complain about too much arguments and their bad type.

I am not sure, what are you trying to do with this substring - an array might have a range, or a "sub-array", or an interval but I am not sure, how you define the substring of an array.