Can I assign the default values to array in 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.

user8017719 · Accepted Answer · 2016-11-03 13:08:48Z

2

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}"

answered Nov 3, 2016 at 13:08

user8017719

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1 Comment

mora Over a year ago

thank you for telling detailed and better answer about initialization of array with default values.

mora · Accepted Answer · 2016-11-03 10:08:40Z

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}"
}

answered Nov 3, 2016 at 10:08

mora

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Comments

Dan Bray · Accepted Answer · 2021-02-28 11:40:04Z

0

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')

answered Feb 28, 2021 at 11:40

Dan Bray

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Can I assign the default values to array in bash?

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Parameter Expansions

To assign value(s) to an array variable.

Array index

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Array index

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