I was wondering about single parentheses in bash. I know that they are used for executing commands in subshells and that they are used for creating arrays, but are they used for anything else?
One thing that caught my attention is that when you use the in variable assignment, like
var=(hello)
echo $var # hello
bash doesn't generate an error or anything, and the output is the same as if
var=hello
Are these two variable definitions the same or is there a difference?
In your case parenthesis () are used as array definition, for example
a=(one two three) # array definition
echo "${a}" # print first element of array a
echo "${a[0]}" # print first element of array a
echo "${a[1]}" # print *second* element of array a
echo "${#a[@]}" # print number of elements in array a
If you put single variable in array then you just have an array with single element.
To answer your other question whether parenthesis are also used for anything else: there are many situations in bash that in combination with other characters they can be used as:
$()<() and >()(command)(())fun () { echo x; }?(), *(), +(), @(), !() (only if extglob is enable) 
var=(1 2 3); echo $varis the same asvar=(1 2 3); echo ${var[0]}? – Mattias Jul 23 '15 at 13:36bashis the same. – jimmij Jul 23 '15 at 13:38""inecho "${a}"? – nn0p Feb 10 '19 at 14:42a. Consider for examplea=*, then tryecho $aandecho "$a". – jimmij Feb 10 '19 at 14:52