Environment variables vs local variables in Bash Scripting - Hands-On Comparison

Environment variables vs local variables in Bash scripting

📖 Scenario: You are writing a simple Bash script to understand how environment variables and local variables work differently. This is important when you want to control which variables are visible to other programs or scripts.

🎯 Goal: Learn how to create local variables and environment variables in Bash, and see how they behave differently when running a child script.

📋 What You'll Learn

Create a local variable in a Bash script

Create an environment variable in a Bash script

Write a child script that prints both variables

Observe which variables are accessible in the child script

💡 Why This Matters

🌍 Real World

Understanding environment vs local variables helps when writing scripts that call other scripts or programs, ensuring the right data is shared or kept private.

💼 Career

Many automation and deployment tasks require managing environment variables correctly to configure software and services.

1

Create a Bash script with a local variable

Create a Bash script called parent.sh and inside it, create a local variable called local_var with the value local_value.

Bash Scripting

# Create a local variable called local_var with value local_value
# Your code here

Hint

Use the syntax variable_name="value" to create a local variable in Bash.

2

Add an environment variable to the script

In parent.sh, add an environment variable called env_var with the value env_value by exporting it.

Bash Scripting

local_var="local_value"
# Export an environment variable called env_var with value env_value
# Your code here

Hint

Use export variable_name="value" to create an environment variable in Bash.

3

Create a child script to print both variables

Create a new Bash script called child.sh that prints the values of local_var and env_var using echo.

Bash Scripting

local_var="local_value"
export env_var="env_value"

# Create child.sh script below
# Your code here

Hint

Use echo "variable_name=$variable_name" to print variable values in Bash.

4

Run child script from parent and observe output

In parent.sh, add a line to run child.sh after setting the variables. Then run bash parent.sh in the terminal and observe the output.

Bash Scripting

local_var="local_value"
export env_var="env_value"

# child.sh script
#!/bin/bash
echo "local_var=$local_var"
echo "env_var=$env_var"

# Run child.sh from parent.sh
# Your code here

Hint

Use bash ./child.sh to run the child script from the parent script.

Notice that local_var is empty in the child script output because it is local, but env_var is visible because it is exported.

Practice

1. What is the main difference between an environment variable and a local variable in a Bash script?

easy

A. Local variables are available to child processes, environment variables are not.

B. Environment variables are available to child processes, local variables are not.

C. Environment variables can only be numbers, local variables can be strings.

D. Local variables are global, environment variables are only inside functions.

Solution

1. Step 1: Understand environment variables

   Environment variables are set with export and are inherited by child processes.

2. Step 2: Understand local variables

   Local variables exist only in the current shell or function and are not passed to child processes.

3. Final Answer:

   Environment variables are available to child processes, local variables are not. -> Option B

4. Quick Check:

   Environment = exported, local = not exported [OK]

Hint: Remember: export makes variables visible to child processes [OK]

Common Mistakes:

• Thinking local variables are inherited by child processes
• Confusing export with local keyword
• Assuming environment variables are only inside functions

2. Which of the following is the correct way to create a local variable inside a Bash function?

easy

A. VAR=value

B. export VAR=value

C. local VAR=value

D. setlocal VAR=value

Solution

1. Step 1: Identify local variable syntax in Bash functions

   Inside a function, the keyword local declares a variable limited to that function.

2. Step 2: Compare options

   export makes variables environment variables, setlocal is not a Bash command, and VAR=value without local is global or inherited.

3. Final Answer:

   local VAR=value -> Option C

4. Quick Check:

   Use local inside functions for local variables [OK]

Hint: Use 'local' keyword inside functions for local variables [OK]

Common Mistakes:

• Using export instead of local inside functions
• Assuming VAR=value inside function is local
• Using non-existent commands like setlocal

3. What will be the output of this script?

VAR=hello
function test_func() {
  local VAR=world
  echo $VAR
}
test_func
echo $VAR

medium

A. world
hello

B. hello
hello

C. world
world

D. hello
world

Solution

1. Step 1: Analyze variable inside function

   The function declares a local variable VAR with value 'world', so echo $VAR inside prints 'world'.

2. Step 2: Analyze variable outside function

   The global VAR remains 'hello', so echo $VAR after function call prints 'hello'.

3. Final Answer:

   world hello -> Option A

4. Quick Check:

   Local inside function, global outside [OK]

Hint: Local variables override global only inside functions [OK]

Common Mistakes:

• Assuming local variable changes global variable
• Confusing output order
• Ignoring local keyword effect

4. Identify the error in this script snippet:

export VAR=hello
function test() {
  VAR=world
  local VAR
  echo $VAR
}
test
echo $VAR

medium

A. local must be declared before assigning VAR inside function

B. export cannot be used outside functions

C. echo $VAR inside function will print empty

D. Function syntax is incorrect

Solution

1. Step 1: Understand local variable declaration order

   In Bash, local VAR must appear before assigning a value to VAR inside the function to make it local.

2. Step 2: Analyze the script behavior

   Here, VAR=world assigns to global VAR before local VAR is declared, so local VAR is empty and echo $VAR prints empty.

3. Final Answer:

   local must be declared before assigning VAR inside function -> Option A

4. Quick Check:

   Declare local before assignment inside functions [OK]

Hint: Declare local variables before assigning values inside functions [OK]

Common Mistakes:

• Assigning before declaring local causes unexpected behavior
• Thinking export is invalid outside functions
• Assuming function syntax is wrong

5. You want to write a Bash script that sets a variable PATH_VAR inside a function and have it available outside the function and in any child processes. Which is the correct approach?

hard

A. Inside the function, use export PATH_VAR=value after declaring local PATH_VAR.

B. Declare local PATH_VAR=value inside the function and then export PATH_VAR outside the function.

C. Use local PATH_VAR=value inside the function and do nothing else.

D. Assign PATH_VAR=value inside the function and then export PATH_VAR inside the function without local.

Solution

1. Step 1: Understand local vs exported variables

   Declaring a variable as local limits it to the function scope, so it won't be available outside or to child processes.

2. Step 2: Correct way to export variable from function

   Assign the variable without local inside the function, then use export PATH_VAR inside the function to make it environment variable accessible outside and to child processes.

3. Final Answer:

   Assign PATH_VAR=value inside the function and then export PATH_VAR inside the function without local. -> Option D

4. Quick Check:

   Export without local to share variable outside function [OK]

Hint: Don't use local if you want variable exported outside function [OK]

Common Mistakes:

• Using local and export together inside function
• Exporting variable only outside function after local declaration
• Assuming local variables can be exported