How to Use Local Variables in Function in Bash

In Bash, declare local variables inside a function using the local keyword before the variable name. This limits the variable's scope to the function, preventing it from affecting or being affected by variables outside the function.

Syntax

Use the local keyword inside a function to declare a variable that only exists within that function's scope.

• local variable_name=value: Declares a local variable and optionally assigns a value.
• The variable is not accessible outside the function.

bash

function example_function() {
    local my_var="Hello"
    echo "$my_var"
}

Example

This example shows how a local variable inside a function does not affect a variable with the same name outside the function.

bash

my_var="Outside"

echo "Before function call: $my_var"

function test_local() {
    local my_var="Inside"
    echo "Inside function: $my_var"
}

test_local

echo "After function call: $my_var"

Output

Before function call: Outside
Inside function: Inside
After function call: Outside

Common Pitfalls

One common mistake is forgetting to use local, which makes the variable global and can overwrite variables outside the function.

Another issue is using local outside a function, which causes an error.

bash

my_var="Global"

function wrong_usage() {
    my_var="Changed"
    echo "Inside function without local: $my_var"
}

wrong_usage

echo "Outside after function call: $my_var"

function correct_usage() {
    local my_var="Local"
    echo "Inside function with local: $my_var"
}

correct_usage

echo "Outside after correct usage: $my_var"

Output

Inside function without local: Changed
Outside after function call: Changed
Inside function with local: Local
Outside after correct usage: Changed

Quick Reference

Command	Description
local var=value	Declare a local variable inside a function with an optional initial value
local var	Declare a local variable without assigning a value
var=value	Declare or assign a global variable (outside or inside function without local)
local outside_function	Causes error: local can only be used inside functions

Key Takeaways

Use the local keyword inside functions to declare variables scoped only to that function.

Local variables prevent accidental changes to variables outside the function.

Forgetting local makes variables global, which can cause bugs.

local cannot be used outside functions; it will cause an error.

Always declare variables local inside functions unless you want to modify global variables.