Scope

Chapter 16: Scope and Variable Visibility

Overview

Understanding variable scope is crucial for writing maintainable bash scripts. This chapter covers global variables, local variables, function scope, environment variables, and best practices for managing variable visibility in your scripts.

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Global vs Local Variables

Global Variables

global_var.sh

#!/usr/bin/env bash
# Global variable - accessible everywhere
GLOBAL_VAR="I'm global"

show_global() {
    echo "Inside function: $GLOBAL_VAR"
}

echo "Before function: $GLOBAL_VAR"
show_global
echo "After function: $GLOBAL_VAR"

Local Variables

local_var.sh

#!/usr/bin/env bash
GLOBAL="Global"

test_local() {
    local LOCAL="I'm local"
    echo "Inside: GLOBAL=$GLOBAL, LOCAL=$LOCAL"
}

test_local
# echo "$LOCAL"  # Error: LOCAL is not defined here

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Function Scope

Modifying Global Variables

modify_global.sh

#!/usr/bin/env bash
counter=0

increment() {
    counter=$((counter + 1))
    echo "Inside: counter=$counter"
}

echo "Before: counter=$counter"
increment
increment
increment
echo "After: counter=$counter"

Passing Variables

pass_var.sh

#!/usr/bin/env bash
# Pass by value
double() {
    local num=$1
    echo $((num * 2))
}

result=$(double 5)
echo "Result: $result"

# Pass by reference (bash 4.3+)
modify() {
    local -n ref="$1"
    ref="modified"
}

value="original"
modify value
echo "Value: $value"

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Environment Scope

Exporting Variables

export_var.sh

#!/usr/bin/env bash
# Export makes variable available to child processes
export MY_VAR="exported"

# Child process can see it
bash -c 'echo "Child sees: $MY_VAR"'

# Non-exported variable
NOT_EXPORTED="hidden"
bash -c 'echo "Child sees: $NOT_EXPORTED"'  # Empty

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Practical Examples

Example 1: Configuration Management

config_scope.sh

#!/usr/bin/env bash
# Global config
CONFIG_FILE="/etc/app/config"

load_config() {
    # Local variables for function
    local key="$1"
    local default="${2:-}"

    # Use global config
    echo "Loading $key from $CONFIG_FILE (default: $default)"
}

get_config() {
    local key="$1"
    local default="${2:-default_value}"

    # Return config value
    echo "$default"
}

# Usage
load_config "database"
load_config "port"

Example 2: Private Variables in Scripts

private_vars.sh

#!/usr/bin/env bash
# Script-private helper function
_helper() {
    local secret="This is private"
    echo "Helper: $secret"
}

public_function() {
    local public_var="This is public"
    echo "Public: $public_var"
}

# Call functions
_helper
public_function

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Best Practices

Always Use local

best_local.sh

#!/usr/bin/env bash
# WRONG: Using global variables in functions
process() {
    result="processed"  # Pollutes global scope
}

# CORRECT: Use local
process() {
    local result="processed"
    echo "$result"
}

Minimize Global State

minimize_global.sh

#!/usr/bin/env bash
# WRONG: Multiple globals
var1="value1"
var2="value2"
var3="value3"

# CORRECT: Group related variables
declare -A CONFIG
CONFIG[key1]="value1"
CONFIG[key2]="value2"
CONFIG[key3]="value3"

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Summary

In this chapter, you learned about:

• ✅ Global variables
• ✅ Local variables
• ✅ Function scope
• ✅ Modifying global variables
• ✅ Passing variables (by value and reference)
• ✅ Environment scope
• ✅ Exporting variables
• ✅ Best practices

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Exercises

Level 1: Basics

1. Create a script with global and local variables
2. Modify a global variable from a function

Level 2: Intermediate

3. Create a configuration system using scope
4. Implement private/public function pattern

Level 3: Advanced

5. Create a state management system
6. Implement pass-by-reference

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Next Steps

Continue to the next chapter to learn about command line arguments.

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Previous Chapter: Functions Next Chapter: Command Line Arguments