Efficiently Returning Arrays from Bash Functions

Bash doesn’t directly support returning arrays from functions. However, we can achieve this using clever techniques. This article explores two effective methods: using command substitution and manipulating the Internal Field Separator (IFS).

Table of Contents

• Command Substitution
• Returning Arrays with Command Substitution
• Returning Arrays with IFS
• Choosing the Best Method

Command Substitution

Command substitution captures a command’s output and assigns it to a variable. This forms the basis of our first method. The key is to format the function’s output for easy parsing back into an array.

Returning Arrays with Command Substitution

This method involves joining array elements into a string using a delimiter, then splitting the string back into an array. Let’s look at an example:

my_array_function() {
  local array=("apple" "banana" "cherry")
  local delimiter="|"
  echo "${array[*]}" | tr ' ' 'n' | paste -sd"$delimiter" -
}

returned_string=$(my_array_function)
returned_array=(${returned_string//|$delimiter/ })

echo "Returned array: ${returned_array[@]}"

Here’s how it works:

1. my_array_function creates a local array.
2. It uses echo, tr to replace spaces with newlines and paste to join the elements with the delimiter.
3. Command substitution captures the output in returned_string.
4. Parameter expansion replaces the delimiter with spaces to create the array returned_array.

This approach is simple and effective, but the delimiter must not appear in the array elements. If it does, consider using a less common character or a more robust delimiter handling method.

Returning Arrays with IFS

The Internal Field Separator (IFS) controls how Bash splits strings into words. We can leverage this to return arrays by setting IFS within the function and printing array elements. The calling script then uses the modified IFS to parse the output.

my_array_function() {
  local array=("apple" "banana" cherry")
  local old_ifs="$IFS"
  IFS=$'n'
  printf "%sn" "${array[@]}"
  IFS="$old_ifs"
}

returned_array=($(my_array_function))

echo "Returned array: ${returned_array[@]}"

Explanation:

1. my_array_function saves the current IFS.
2. It sets IFS to a newline character.
3. It prints each array element on a new line.
4. It restores IFS.
5. The calling script uses the newline as the separator to create the array.

This method is cleaner if your array elements don’t contain newlines. For elements containing newlines, you need to choose a different, less likely IFS character.

Choosing the Best Method

Both methods provide viable solutions. The best choice depends on your specific needs and array contents. The command substitution approach offers more flexibility with delimiter selection, while using IFS can be cleaner if your data allows it. Always prioritize clean, robust, and well-commented code.