Chapter 2: Bash Syntax and Command Structure¶

Learning Objectives¶

By the end of this chapter, you will be able to:

Understand how Bash parses and executes commands
Use variables, arguments, and quoting effectively
Structure your scripts with control flow: if, case, for, while
Use comments, functions, and exit codes properly
Begin writing scripts that interact with user input

Introduction: The Building Blocks of Bash¶

Before you can write powerful scripts, you need to understand how Bash interprets your input. This chapter will give you a clear, practical foundation in Bash syntax and structure—knowledge that applies equally whether you're writing a 3-line script or a full automation tool.

2.1 Commands and Arguments¶

Bash commands follow a simple structure:

command [options] [arguments]

For example:

ls -la /Users/sammy/Desktop

Note: The path /Users/sammy/Desktop is an example. In real scripts, use $HOME or ~ for user-specific paths.

ls is the command
-la are the options (flags)
/Users/sammy/Desktop is the argument

You can chain commands using &&, ||, and ;:

mkdir newfolder && cd newfolder

2.2 Variables¶

Bash variables are declared without a let or var keyword:

name="Sammy"
echo "Hello, $name!"

Remember:

No spaces around =
Use "$var" to preserve whitespace
Enclose in braces for safety: ${name}

You can also use environment variables:

echo "$HOME"

2.3 Quoting: Single, Double, and Escaping¶

'single quotes' prevent expansion.
"double quotes" allow variable and command substitution.
\ escapes the next character.

Examples:

echo 'My name is $USER'      # Prints literally
echo "My name is $USER"      # Expands $USER

2.4 Conditionals: if, elif, else¶

Basic structure:

if [ condition ]; then
  commands
elif [ other_condition ]; then
  other_commands
else
  fallback_commands
fi

Example:

if [ -f /etc/hosts ]; then
  echo "Hosts file exists."
else
  echo "Missing hosts file."
fi

Use [[ ... ]] for advanced conditions (e.g., regex).

2.5 Loops: for, while, and until¶

for loop¶

for file in *.txt; do
  echo "Processing $file"
done

while loop¶

count=1
while [ "$count" -le 5 ]; do
  echo "Count: $count"
  ((count++))
done

until loop¶

until [ "$ready" = "yes" ]; do
  echo "Waiting..."
  sleep 1
  ready="yes"
done

2.6 Functions and Return Values¶

Functions allow code reuse and structure:

say_hello() {
  echo "Hello, $1!"
}

say_hello "Sammy"

Use return to provide exit codes (0 for success):

check_dir() {
  [ -d "$1" ] && return 0 || return 1
}

if check_dir "/tmp"; then
  echo "Directory exists."
fi

2.7 Script Exit Codes¶

Every Bash command returns an exit code ($?):

0 = success
1+ = failure or different exit state

You can use exit to set a code:

exit 0   # success
exit 1   # general error

2.8 Reading User Input¶

echo -n "Enter your name: "
read username
echo "Hi, $username."

Flags:

-p prints prompt
-s hides input (e.g., passwords)
-n 1 reads a single character

Example:

read -p "Proceed (y/n)? " answer
if [[ "$answer" =~ ^[Yy]$ ]]; then
  echo "Continuing..."
else
  echo "Aborted."
fi

Chapter 2 Exercise¶

Write a script called backup.sh that:

Asks for a folder to back up
Checks if it exists
Creates a timestamped .tar.gz archive
Outputs success or failure

Hint:

#!/bin/bash
read -p "Folder to back up: " folder
if [ -d "$folder" ]; then
  tar -czf "backup_$(basename "$folder")_$(date +%F).tar.gz" "$folder"
  echo "Backup complete."
else
  echo "Folder not found."
  exit 1
fi

macOS Scripting Tips¶

Use open . or open "$folder" to open Finder to a directory.
You can pipe input to GUI dialogs with osascript.
Use read -s to securely accept passwords from users.
Use touch, say, and afplay for playful script output on macOS.