local

What is local?

The local command is used to declare variables that are only accessible within a specific function or scope. Without local, variables declared within a function would become global, potentially leading to unintended side effects and making your scripts harder to maintain and debug.

Declaring Local Variables

The basic syntax is straightforward:

local variable_name=value

Let’s illustrate this with a simple example:

my_function() {
  local my_local_variable="This is a local variable"
  echo "Inside function: $my_local_variable"
}

my_global_variable="This is a global variable"

my_function
echo "Outside function: $my_local_variable"  # This will result in an error or an empty string
echo "Outside function: $my_global_variable"

In this script, my_local_variable is declared as local within my_function. The output demonstrates that it’s only accessible within the function. Attempting to access it outside results in it not being defined. my_global_variable, however, remains accessible globally.

Using local with Arrays

local also works seamlessly with arrays:

my_array_function() {
  local my_local_array=("element1" "element2" "element3")
  echo "Inside function: ${my_local_array[0]}"
  echo "Inside function: ${my_local_array[@]}"
}

my_array_function
#echo "Outside function: ${my_local_array[0]}" # This will result in an error or an empty string

This example showcases declaring and accessing a local array within a function. Again, accessing the array outside the function will fail.

local and Arithmetic Operations

You can also use local with variables involved in arithmetic operations:

arithmetic_function() {
  local x=10
  local y=5
  local sum=$((x + y))
  echo "Sum inside function: $sum"
}

arithmetic_function

Here, x, y, and sum are all local to the function.

local and complex scenarios: nested functions

The power of local truly shines when dealing with nested functions:

outer_function() {
  local outer_var="Outer Variable"

  inner_function() {
    local inner_var="Inner Variable"
    echo "Inner function: Outer var = $outer_var, Inner var = $inner_var"
  }

  inner_function
  #echo "Outer function: Inner var = $inner_var" # this will result in an error because inner_var is not accessible here
}


outer_function

Even though inner_function is nested, outer_var remains accessible because it’s in the parent scope. However, inner_var remains confined to inner_function.

Error Handling and local

If you try to use a variable that hasn’t been declared with local within a function, the shell might behave differently depending on your shell configuration (e.g., throwing an error, using a global variable with the same name, etc.) Using local consistently enhances the predictability and robustness of your shell scripts.

Best Practices

Always use local when declaring variables within functions to prevent unintended modification of global variables and improve code clarity and maintainability. This is a cornerstone of writing well-structured shell scripts.