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landing/docs/Bash-Scripts/010-bash-conditionals.md

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+# Conditionals
+
+You've mastered conditional expressions. Now, let's put them to work using `if`, `if-else`, `elif`, and `case` statements to control your script's flow.
+
+---
+
+### If Statements
+
+The `if` statement executes commands only if a condition is true.
+
+```bash
+if [[ some_test ]]
+then
+    <commands>
+fi
+```
+
+Example: Prompt for a name, then check if it's empty.
+
+```bash
+touch if_example.sh
+```
+
+Open `if_example.sh` and add:
+
+```bash
+#!/usr/bin/env bash
+
+read -p "What is your name? " name
+
+if [[ -z "${name}" ]]; then # Check if name is empty
+    echo "Please enter your name!"
+fi
+```
+
+Save and exit. Make it executable and run:
+
+```bash
+chmod +x if_example.sh
+./if_example.sh
+```
+
+### If-Else Statements
+
+Use `else` to define actions when the `if` condition is false.
+
+Create `if_else_example.sh`:
+
+```bash
+touch if_else_example.sh
+```
+
+Open `if_else_example.sh` and add:
+
+```bash
+#!/usr/bin/env bash
+
+read -p "What is your name? " name
+
+if [[ -z "${name}" ]]; then
+    echo "Please enter your name!"
+else
+    echo "Hi there ${name}!"
+fi
+```
+
+Save, make executable, and run.
+
+### Admin/Root Checks
+
+Check user roles. For admin verification:
+
+Create `admin_check.sh`:
+
+```bash
+touch admin_check.sh
+```
+
+Open `admin_check.sh` and add:
+
+```bash
+#!/usr/bin/env bash
+
+admin="Dev Team Six"
+
+read -p "Enter your username: " username
+
+if [[ "${username}" == "${admin}" ]]; then
+    echo "You are the admin user!"
+else
+    echo "You are NOT the admin user!"
+fi
+```
+
+Save, make executable, and run.
+
+To prevent a script from running as `root` (`EUID` is 0 for root):
+
+Create `root_check.sh`:
+
+```bash
+touch root_check.sh
+```
+
+Open `root_check.sh` and add:
+
+```bash
+#!/usr/bin/env bash
+
+if [[ $EUID -eq 0 ]]; then # Check if current user ID is 0 (root)
+    echo "This script should not be run as root. Exiting."
+    exit 1 # Exit with error code
+fi
+echo "Script running as non-root user."
+```
+
+Save, make executable, and run (test as root and non-root).
+
+Combine conditions for stricter control. This example ensures the user is neither `admin` nor `root`.
+
+Create `combined_check.sh`:
+
+```bash
+touch combined_check.sh
+```
+
+Open `combined_check.sh` and add:
+
+```bash
+#!/usr/bin/env bash
+
+admin_user="Dev Team Six"
+
+read -p "Enter your username: " username
+
+if [[ "${username}" != "${admin_user}" || $EUID -ne 0 ]]; then # OR operator
+    echo "You are not the admin or root user. Proceeding safely."
+else
+    echo "You are an admin or root user. Exercise caution."
+fi
+```
+
+Save, make executable, and run.
+
+### Elif Statements
+
+For multiple, mutually exclusive conditions, use `elif`.
+
+Create `elif_example.sh`:
+
+```bash
+touch elif_example.sh
+```
+
+Open `elif_example.sh` and add:
+
+```bash
+#!/usr/bin/env bash
+
+read -p "Enter a number: " num
+
+if [[ $num -gt 0 ]]; then
+    echo "The number is positive."
+elif [[ $num -lt 0 ]]; then
+    echo "The number is negative."
+else
+    echo "The number is 0."
+fi
+```
+
+Save, make executable, and run.
+
+### Case Statements
+
+Simplify complex conditionals with `case` when you have multiple choices.
+
+The `case` syntax:
+
+```bash
+case $variable_to_test in
+  pattern_1)
+    commands_for_pattern_1
+    ;;
+  pattern_2|pattern_3) # Multiple patterns with |
+    commands_for_pattern_2_or_3
+    ;;
+  *) # Default case (catch-all)
+    default_commands
+    ;;
+esac
+```
+
+Key points: `case` starts the statement, `in` follows the variable. Patterns end with `)`. Commands are terminated by `;;`. `*` is the default. `esac` closes the statement.
+
+Example: Check car brand and factory location.
+
+Create `car_brand_case.sh`:
+
+```bash
+touch car_brand_case.sh
+```
+
+Open `car_brand_case.sh` and add:
+
+```bash
+#!/usr/bin/env bash
+
+read -p "Enter the name of your car brand: " car
+
+case $car in
+
+  Tesla)
+    echo "${car}'s factory is in the USA."
+    ;;
+
+  BMW | Mercedes | Audi | Porsche)
+    echo "${car}'s factory is in Germany."
+    ;;
+
+  Toyota | Mazda | Mitsubishi | Subaru)
+    echo "${car}'s factory is in Japan."
+    ;;
+
+  *)
+    echo "${car} is an unknown car brand."
+    ;;
+
+esac
+```
+
+Save, make executable, and run.
+
+---
+
+### Conclusion
+
+Practice these conditional structures. Modify the examples. Mastering flow control is critical for effective scripting.