Overview

This guide revisits essential shell scripting fundamentals—commands, functions, script flow, shebangs—and demonstrates how to weave them into reliable, maintainable scripts. We assume you’re already comfortable with looping (for, while) and branching (if, case) constructs; hands-on labs will reinforce these concepts.

Shell Script Execution Lifecycle

Every shell script follows a predictable lifecycle. Understanding these phases will help you write clearer, more robust Bash scripts.

Top-to-Bottom Imperative Execution

By default, Bash scripts execute commands in order from top to bottom. This imperative style is simple but can become hard to manage as scripts grow:

#!/bin/bash

echo "Hello World"
echo "Hello World one more time"
echo "Hello World one last time"

Interactive commands work the same way:

$ ls
documents  download  music  pict

We combine external binaries, built-ins, conditional logic, and special syntax to automate workflows and repetitive tasks.

Organizing Code with Functions

Functions let you group logic into reusable blocks. Declaring a function doesn’t execute it—you must explicitly call it:

#!/bin/bash

echo_function() {
    echo "This function runs only when called, even if declared above."
}

echo "This is the first line of the script."

# Invoke the function twice
echo_function
echo_function

Using functions improves readability, maintainability, and testability of your scripts.

Error Handling and Exit Codes

Reliable scripts must report success or failure at each step. Imagine dropping off two kids at school: if one is absent due to illness, you must report that accurately to avoid confusion. The same principle applies in scripting—if a directory creation fails, your script should exit immediately rather than allowing downstream errors.

Here are two common techniques:

• Use set -e at the top of your script to exit on any error.

• Check the exit status of critical commands explicitly:

  mkdir /important/dir
  if [[ $? -ne 0 ]]; then
      echo "Failed to create /important/dir" >&2
      exit 1
  fi
  

With these principles—shebang usage, script lifecycle, imperative and functional structures, and robust error handling—you’re ready to write and maintain high-quality Bash scripts. See you in the next lesson!