Chapter 6: Control Structures and Functions¶

Learning Objectives¶

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

• Understand and use conditional statements (if, case)
• Utilize looping constructs (for, while, until) in scripts
• Create and call Bash functions
• Apply control structures and functions to build readable, modular scripts
• Recognize macOS-specific quirks when using these constructs

Introduction¶

Control structures and functions are the backbone of any scripting language, enabling dynamic behavior and code reuse. Bash on macOS supports the same powerful constructs as other Unix-based systems, but scripting with security and clarity in mind is key when administering macOS at scale.

6.1 Conditional Statements¶

if Statements¶

Conditional execution allows your script to make decisions.

if [ "$1" = "macOS" ]; then
  echo "Running on macOS"
else
  echo "Not macOS"
fi

Tip for macOS¶

Use sw_vers or uname to programmatically detect the macOS version or platform.

if [[ "$(uname)" == "Darwin" ]]; then
  echo "Confirmed: macOS system"
fi

elif and else¶

if [ -f "/etc/hosts" ]; then
  echo "Hosts file found"
elif [ -f "/private/etc/hosts" ]; then
  echo "Hosts file in legacy location"
else
  echo "Hosts file not found"
fi

6.2 case Statements¶

Great for handling multiple possibilities, especially for command-line arguments or user input.

case "$1" in
  start)
    echo "Starting service..."
    ;;
  stop)
    echo "Stopping service..."
    ;;
  restart)
    echo "Restarting service..."
    ;;
  *)
    echo "Usage: $0 {start|stop|restart}"
    exit 1
    ;;
esac

6.3 Loops¶

for Loops¶

for user in $(dscl . list /Users | grep -v '^_'); do
  echo "Checking user: $user"
done

This loop iterates over user accounts (excluding system ones) on macOS.

while Loops¶

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

until Loops¶

until [ -f /tmp/signal.done ]; do
  echo "Waiting for signal..."
  sleep 2
done

6.4 Bash Functions¶

Functions allow code reuse and better structure.

greet_user() {
  local user="$1"
  echo "Welcome, $user!"
}

greet_user "Admin"

Function with Return Value¶

add_numbers() {
  # Note: In arithmetic context ($(( ))), variables don't need quotes
  local sum=$(( $1 + $2 ))
  echo "$sum"
}

result=$(add_numbers 3 4)
echo "The sum is: $result"

6.5 Real-World Example: Lock Screen Reminder¶

This script displays a lock screen reminder if a user forgets to lock their Mac:

#!/bin/bash

check_idle_time() {
  idle_ms=$(ioreg -c IOHIDSystem | awk '/HIDIdleTime/ {print $NF / 1000000; exit}')
  echo "${idle_ms%.*}"
}

alert_user() {
  osascript -e 'display notification "Lock your screen when away!" with title "Security Reminder"'
}

main() {
  idle=$(check_idle_time)
  if [ -n "$idle" ] && [ "$idle" -ge 300 ]; then
    alert_user
  fi
}

main

6.6 macOS-Specific Notes¶

• osascript allows integration of AppleScript within Bash for user notifications.
• GUI prompts or notifications via scripts require System Events permissions under System Preferences > Privacy & Security > Automation.
• File paths and user account handling differ from Linux—use dscl or id carefully.

Chapter 6 Exercise¶

1. Write a script that loops through all .log files in /var/log and prints the filename and number of lines.
2. Create a function called backup_home that accepts a username and uses rsync to back up their home directory to /Users/Backups/.
3. Modify the Lock Screen Reminder example to send a message only once every 10 minutes.

macOS Scripting Tips¶

• Use osascript for GUI integration with Bash scripts, but remember it requires appropriate permissions under System Preferences > Privacy & Security > Automation.
• Functions help organize code; use local variables within functions to avoid conflicts with global scope.
• macOS-specific commands like dscl can help iterate over user accounts more reliably than parsing /etc/passwd.
• Use sw_vers or uname to programmatically detect macOS version and platform for cross-compatibility checks.
• Consider using launchctl to schedule functions and scripts to run automatically on macOS.