list_test.sh

#!/bin/bash
set -u

THELIST=( 'one' 'two' 'three' 'four' 'five' 'six' 'seven' 'eight' 'nine' 'ten' )
THEITEM='twelve'
declare -a EXTLIST=()

EXTLIST=( 'eleven' 'twelve' )
if [[ ${#EXTLIST[@]} -gt 0 ]]; then
    THELIST+=( "${EXTLIST[@]}" )
fi

contains() {
    echo "The list: $1"
    [[ "$1" =~ (^|[[:space:]])"$2"($|[[:space:]]) ]] && return 0 || return 1
}

if contains "${THELIST[*]}" "$THEITEM"; then
    echo "\$THELIST contains ${THEITEM}"
else
    echo "\$THELIST doesn't contain ${THEITEM}"
fi