[Help-bash] What is the different between `unset a[n-1]` in interactive mode and non-interactive mode?

[Peng Yu]

Hi,

I can remove an array element in the following script.

$ cat main.sh
#!/usr/bin/env bash
# vim: set noexpandtab tabstop=2:

set -v
array=(a b c)
n=3
unset array[n-1]
declare -p array
$ ./main.sh
array=(a b c)
n=3
unset array[n-1]
declare -p array
declare -a array=([0]="a" [1]="b")


But when I run the same commands in the command line, it does not
delete the element.

$ array=(a b c)
$ n=3
$ unset array[n-1]
$ declare -p array
declare -a array=([0]="a" [1]="b" [2]="c")

However, if I quote it, then the element will be deleted.
$ unset 'array[n-1]'
$ declare -p array
declare -a array=([0]="a" [1]="b")

Why is there such a difference between the interactive mode and the
non-interactive mode? Thanks.

-- 
Regards,
Peng