Functions

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This session teaches how to structure large scripts effectively by creating and utilizing reusable functions, improving readability, maintainability, and reducing code duplication.

Learning Objectives

  • Define and call simple Bash functions.
  • Understand and manage variable scope within functions (local vs. global).
  • Pass arguments to functions and handle return values.
  • Learn to modularize scripts by sourcing function definitions from external files.

Topics Covered

1. Defining Functions

  • Syntax: Defining functions using function name { ... } or the more portable name () { ... }.
  • Execution: Calling a function simply by its name within the script.

2. Function Arguments and Return Values

  • Arguments: Functions treat arguments just like scripts, using $1, $2, etc.
  • Passing Arrays (Advanced):

bash my_func() { local -a arr=("${@}") echo "First element: ${arr[0]}" } list=("apple" "banana") my_func "${list[@]}"

  • Return Values:
  • Functions return an exit code using return (0-255).
  • Data can be "returned" via echo and command substitution.
  • Namerefs (Very Advanced): Using declare -n to modify a variable passed by name.

3. Variable Scope and Best Practices

  • Global vs Local: Always use local for variables inside functions to avoid "polluting" the global namespace.
  • The main Pattern: For larger scripts, wrap your logic in a main() function.

bash main() { setup execute_task } main "$@"

4. Script Modularity

  • Sourcing Files: Using the source command or the dot operator (.) to execute another script file in the current shell environment, making its functions available. This is key for creating script libraries.

Lab/Assessment Focus

Goal: Create a modular calculator tool.

  1. lib_math.sh: Create a library containing functions for add, multiply, and a recursive factorial function.
  2. app.sh:
    • Sources lib_math.sh.
    • Uses a main() function to parse arguments.
    • Checks if the first argument is "fact" or "calc".
    • Calls the appropriate function and handles errors if the input is not a number.

Advanced Topic References

Loops SQL Intro