Can We Use Arrays as Environment Variables in Bash?

1. Overview

In Bash scripting, we may experience the need to manage and manipulate environment variables. So, environment variables can help us influence the behavior of processes and scripts, since they store useful information such as file paths, the current user’s home directory, and system settings. Now, there’s the question of whether we can use arrays as environment variables in Bash.

In this tutorial, we’ll discuss this question along with examples and workarounds.

2. Basics of Environment Variables and Arrays

These environment variables are used by the shell and other programs to store configuration settings. Common environment variables include HOME, PATH, and USER. Additionally, we can access and modify these variables using the export command in Bash:

$ export HOME="/home/francis"

This example represents the HOME environment variable which defines the current user’s home directory.

Meanwhile, arrays in Bash store multiple values which can be accessed using indices. In scripts, we can use arrays to handle a list of items like filenames.

However, we cannot use arrays directly as environment variables in Bash, since environment variables are designed to store string values, not complex data structures like arrays. Additionally, introducing environment variables breaks compatibility and portability, since environment variables aren’t limited to Bash or Linux.

3. Workarounds for Using Bash Arrays as Environment Variables

We cannot directly use arrays as environment variables but we can employ techniques to mimic this behavior. We’ll use a Bash script to illustrate.

3.1. Storing Array Values Using Delimiters

We can store the array values as a single string and separate the values using a delimiter, such as a colon or a comma:

#!/bin/bash

# Define an array
my_array=(1 2 3 4)

# Convert the array to a string with a delimiter
array_string=$(IFS=:; echo "${my_array[*]}")

# Export the string as an environment variable
export MY_ARRAY=$array_string

# Display the environment variable's value
echo "MY_ARRAY: $MY_ARRAY"

# Split the string back into an array
IFS=: read -r -a new_array <<< "$MY_ARRAY"

# Access the array elements
echo
echo "new_array elements:"
echo ${new_array[0]}  # Output: 1
echo ${new_array[1]}  # Output: 2
echo ${new_array[2]}  # Output: 3
echo ${new_array[3]}  # Output: 4

Let’s analyze the Bash script:

• my_array=(1 2 3 4) – defines the array my_array with the elements 1, 2, 3 and 4
• array_string=$(IFS=:; echo “${my_array[*]}”) – this line converts the array into a single string where the elements are separated by a colon (:)
• export MY_ARRAY=$array_string – assigns the string array_string to the environment variable MY_ARRAY using the export command
• echo $MY_ARRAY – prints the value of the environment variable MY_ARRAY in the terminal
• IFS=: read -r -a new_array <<< “$MY_ARRAY” – restores the value of the environment variable into the array new_array

Under the comment Access the array elements we first print an empty line to improve readability, then print the header new_array elements: and print the elements of the new_array using their indices:

$ bash script.sh
MY_ARRAY: 1:2:3:4

new_array elements:
1
2
3
4

In summary, we transform the array elements into a single string and store it as an environment variable. Additionally, we restore this value stored string into an array.

3.2. Storing Array Values as Indexed Environment Variables

Another approach we can use is storing each element of the array as a separate environment variable and including its index in the name:

#!/bin/bash

# Define an array
my_array=(1 2 3 4)

# Export each element as an indexed environment variable
for i in "${!my_array[@]}"; do
    export MY_ARRAY_$i="${my_array[$i]}"
done

# Access the elements using the indices
echo $MY_ARRAY_0  # Output: 1
echo $MY_ARRAY_1  # Output: 2
echo $MY_ARRAY_2  # Output: 3
echo $MY_ARRAY_3  # Output: 4

Here, we utilize the for loop to store each array element as a different environment variable:

• for i in “${!my_array[@]}”; do – iterates over the indices of my_array
• export MY_ARRAY_$i=”${my_array[$i]}” – this line inside the loop captures the element at index i and exports it as an environment variable constructed with the index

Now, let’s execute the script:

$ bash script.sh
1
2
3
4

The Bash script above prints each environment variable.

3.3. Storing Array Values in a File

We can utilize this approach if the array is too large or complex to store in an environment variable. It entails writing the array to a file and then reading from the file when the need arises:

#!/bin/bash

# Define an array
my_array=(1 2 3 4 5 6)

# Write the array to a file
echo "${my_array[@]}" > /tmp/my_array.txt

# Export the file path as an environment variable
export MY_ARRAY_FILE=/tmp/my_array.txt

# Read the array from the file
new_array=($(cat $MY_ARRAY_FILE))

# Access the array elements
echo ${new_array[0]}  # Output: 1

Let’s break down the script:

• echo “${my_array[@]}” > /tmp/my_array.txt – the command echo “${my_array[@]}” prints all the elements whereas > /tmp/my_array.txt redirects this output to the file /tmp/my_array.txt
• export MY_ARRAY_FILE=/tmp/my_array.txt – sets the environment variable MY_ARRAY_FILE to the file path /tmp/my_array.txt where we stored the array
• new_array=($(cat $MY_ARRAY_FILE)) – uses cat to read the contents of the file specified in MY_ARRAY_FILE, splits the output into elements separated by whitespace, and then assigns these elements to the array new_array

Above, the command substitution (${…}) captures the output of the cat command which is (1 2 3 4 5 6).

To demonstrate, let’s execute the script:

$ bash script.sh
1

This script stores the contents of the array in a file and the file path in an environment variable. Then it reads the file contents to restore it to the array new_array and prints its first element. We can utilize this approach to persist and share array data between different scripts.

4. Conclusion

In this article, we discussed whether we can use arrays as environment variables in Bash.

So, we explored several workarounds since Bash doesn’t support using arrays directly as environment variables. The workarounds include storing the contents of the array using delimiters, as index environment variables, and files. We can select each approach based on preference or use case. Additionally, we can utilize these approaches to leverage environment variables in managing Linux processes, as well as arrays in Bash.