linux.softwareshinobi.com/landing/docs/Bash-Scripts/010-bash-conditionals.md

# 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.