Environment variables vs local variables in Bash Scripting - Visual Side-by-Side Comparison

Concept Flow - Environment variables vs local variables

Start Script

↓

Set local variable

↓

Set environment variable (export)

↓

Call child process

↓

Child sees environment variable

↓

Child does NOT see local variable

↓

End Script

This flow shows how local variables exist only in the current script, while environment variables (exported) are passed to child processes.

Execution Sample

Bash Scripting

name="local"
export city="NewYork"
echo "Local: $name"
echo "Env: $city"
bash -c 'echo "Child sees city: $city"; echo "Child sees name: $name"'

This script sets a local variable and an environment variable, then runs a child bash to show which variables are visible.

Execution Table

Step	Action	Variable 'name'	Variable 'city'	Output
1	Set local variable name="local"	local	unset
2	Export environment variable city="NewYork"	local	NewYork
3	Print local variable	local	NewYork	Local: local
4	Print environment variable	local	NewYork	Env: NewYork
5	Run child bash to print variables	local (not passed)	NewYork (passed)	Child sees city: NewYork Child sees name:

💡 Script ends after child process runs; local variable not passed, environment variable passed.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 5 (child)
name	unset	local	local	unset (not visible)
city	unset	unset	NewYork (exported)	NewYork (visible)

Key Moments - 2 Insights

Why does the child bash see the environment variable 'city' but not the local variable 'name'?

Because only variables exported with 'export' become environment variables visible to child processes, as shown in execution_table step 5 where 'city' is visible but 'name' is empty.

Does setting a variable without 'export' make it available to child processes?

No, local variables are only available in the current shell or script. The execution_table step 5 shows the child process cannot see 'name' because it was not exported.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what is the output?

A Local: NewYork

B Local: local

C Local:

D Local: name

💡 Hint

Check the 'Output' column at step 3 in the execution_table.

At which step does the variable 'city' become visible to the child process?

A Step 1

B Step 3

C Step 5

D Step 2

💡 Hint

Look at the 'Variable city' column and output at step 5 in the execution_table.

If we remove 'export' from the city variable, what happens in the child process output?

A Child sees city:

B Child sees name: local

C Child sees city: NewYork

D Child sees both variables

💡 Hint

Refer to variable_tracker and execution_table step 5 showing visibility depends on export.

Concept Snapshot

Local variables exist only in the current shell or script.
Environment variables are local variables marked with 'export'.
Only environment variables are passed to child processes.
Use 'export VAR=value' to make a variable visible to child processes.
Without export, variables remain local and invisible outside.
Child processes inherit environment variables but not local variables.

Full Transcript

This lesson shows the difference between local and environment variables in bash scripting. A local variable is set normally and exists only in the current shell. An environment variable is created by exporting a local variable with 'export'. When a child process runs, it can see environment variables but not local variables. The example script sets a local variable 'name' and exports 'city'. The child bash process prints both variables. It sees 'city' but not 'name'. This shows that only exported variables are passed down. Understanding this helps control variable visibility in scripts and subprocesses.

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