File globbing pattern, !(*example), behaves differently in bash script than it does in bash shell

Question

The following works when pasted directly into my bash terminal (I call bash explicitly, bash version: 4.4.19(1)-release (x86_64-pc-linux-gnu))

for filename in /home/dean/Downloads/!(*example).txt; do
    echo "${filename}"
done

This command echoes back all of the txt files that do not have 'example' in the filename.

But when I convert this into a script called temp.sh, chmod +x temp.sh and call it by ./temp.sh:

#!/usr/bin/env bash

for filename in /home/dean/Downloads/!(*example).txt; do
    echo "${filename}"
done

I get the following error:

dean@dean-thinkpad-p52s:~/Downloads$ ./temp.sh 
./temp.sh: line 3: syntax error near unexpected token `('
./temp.sh: line 3: `for filename in /home/dean/Downloads/!(*example).txt; do'

I fail to understand the problem here. Why is it doing exactly what I want in the shell but not in the script.

Edit (to answer panki's question):

The difference between when env is called in shell/terminal and when env is called in shell/script:

dean@dean-thinkpad-p52s:~/Downloads$ diff example_myshell.txt example_called_script.txt 
5a6
> _=/usr/bin/env
36,37d36
< TERM=xterm-256color
< SHELL=/bin/bash
38a38,39
> SHELL=/bin/bash
> TERM=xterm-256color
45c46
< PYENV_SHELL=bash
---
> SHLVL=4
47c48
< SHLVL=3
---
> PYENV_SHELL=bash
61d61
< _=/usr/bin/env

Answer 1

The !(...) Korn shell extended operator is only available in bash when you turn the extglob option on (it is off by default).

You may have extglob turned on in your interactive shell via ~/.bashrc or other initialization file, but notice that those files are not sourced when running scripts, and that option is not inherited from the calling shell (unless the BASHOPTS variable in the environment, but it would be a bad idea to have it there).

Explicitly turning it on with

shopt -s extglob

at the beginning of your script should work.

Notice that the shopt -s extglob only has effect beginning with the next line which wasn't already parsed. This means that you cannot use shopt -s extglob like set -f, to only turn the extended patterns on in a subshell:

# this won't work
(
  shopt -s extglob
  echo !(no such file)
)

You'd have to do something like:

(
  shopt -s extglob
  eval 'echo !(no such file)'
)