Can I assign the default values to array in bash? - bash

Question

* I previously asked a question but it was not the correct question. Now I made the correct question and fixed sample code. And I will up an answer which partially cites an answer to the previous question. *
I would like to default value to arrays in bash. Please see following,
function chmod_chown_func() {
local file_path="$1"
local chmod_options[2]=${2:='-R 744'} # This line has error.
local chown_options[2]=${3:='-R root:root'} # This line has error.
sudo chmod "${chmod_options[#]}" "${file_path}"
sudo chown "${chown_options[#]}" "${file_path}"
}
chmod_chown_func "test.txt"
The error message is
$2: cannot assign in this way
Thank you very much.

Answer 1

Parameter Expansions
Yes, the expansion ${a:=default} changes the value of a.
It is called "Assign Default Values" in the bash manual.
$ unset a
$ echo "<${a}> and <${a:=default}>, But <${a}>"
<> and <default>, But <default>
But that syntax could not be applied to positional parameters.
Positional parameters can be (mostly) changed with set.
$ echo "$#"
a b c
$ set -- d e f
$ echo "$#"
d e f
But you can use the expansion of "Use default value" as called in the manual:
$ unset a
$ echo "<${a}> and <${a:-default}>, But <${a}>"
<> and <default>, But <>
To assign value(s) to an array variable.
A common idiom is
$ array=( aaa bbb ccc )
$ echo "${array[1]}"
bbb
Or:
$ declare -a array=( aaa bbb ccc )
Which also will make the variable local to a function if used inside the function.
However, it comes with the detail that wildcards (*, ? and []) will be expanded (unless quoted or the option set -f is used).
Overall, it is better to use read:
$ IFS=' ' read -a array <<<"$a"
Array index
You can not assign a whole array by using one index. This:
chmod_options[2]=${2:-'-R 744'}
Will only create one array value, at index 2. A better way will be:
chmod_options=( ${2:--R 744} )
Or, as explained above:
IFS=' ' read -a chmod_options <<<"${2:--R 744}"

Answer 2

The follwings are error points and an answer code.
Error 1:
The default value by ${variable:='some value'} does not work with positional parameter.
It should be ${variable:-'some value'}
Error 2:
To assign an default value to an array, declare an array and assign a default array value to it.
An example answer code is following
function chmod_chown_func() {
local file_path="$1"
local -a chmod_options=${2:-( -R 744 )}
local -a chown_options=${3:-( -R root:root )}
sudo chmod "${chmod_options[#]}" "${file_path}"
sudo chown "${chown_options[#]}" "${file_path}"
}

Answer 3

I didn't have much luck with the existing answers, so might as well KISS and use a little logic to determine if the array is empty, and then assign the default value if necessary.
local -a chmod_options="$2"
local -a chown_options="$3"
# Assign default value if chmod_options is an empty string or array
[[ -z "$chmod_options" || ${#chmod_options[#]} -eq 0 ]] && chmod_options=('-R 744')
# Assign default value if chown_options is an empty string or array
[[ -z "$chown_options" || ${#chown_options[#]} -eq 0 ]] && chown_options=('root:root')