Like in other languages, Bash conditionals let us decide whether to perform an action or not.  The result is determined by evaluating an expression, which should be enclosed in `[[ ]]`.

Conditional expression may contain `&&` and `||` operators, which are _AND_ and _OR_ respectively. Beside this, there are many [other handy expressions](https://github.com/denysdovhan/bash-handbook#primary-and-combining-expressions).

Before we start, let's see what primaries are.

### Primary and combining expressions

Expressions enclosed inside `[[ ]]` (or `[ ]` for `sh`) are called **test commands** or **primaries**. These expressions help us to indicate results of a conditional. In the tables below, we are using `[ ]`, because it works for `sh` too. Here is an answer about [the difference between double and single square brackets in bash](http://serverfault.com/a/52050).

**Working with the file system:**

| Primary       | Meaning                                                      |
| :-----------: | :----------------------------------------------------------- |
| `[ -e FILE ]` | True if `FILE` **e**xists.                                   |
| `[ -d FILE ]` | True if `FILE` exists and is a **d**irectory.                |
| `[ -r FILE ]` | True if `FILE` exists and is **r**eadable.                   |
| `[ -w FILE ]` | True if `FILE` exists and is **w**ritable.                   |
| `[ -x FILE ]` | True if `FILE` exists and is e**x**ecutable.                 |
| `[ FILE1 -nt FILE2 ]` | FILE1 is **n**ewer **t**han FILE2.                   |
| `[ FILE1 -ot FILE2 ]` | FILE1 is **o**lder **t**han FILE2.                   |

**Working with strings:**

| Primary        | Meaning                                                     |
| :------------: | :---------------------------------------------------------- |
| `[ -z STR ]`   | `STR` is empty (the length is **z**ero).                    |
| `[ -n STR ]`   |`STR` is not empty (the length is **n**on-zero).             |
| `[ STR1 == STR2 ]` | `STR1` and `STR2` are equal.                            |
| `[ STR1 != STR2 ]` | `STR1` and `STR2` are not equal.                        |

**Arithmetic binary operators:**

| Primary             | Meaning                                                |
| :-----------------: | :----------------------------------------------------- |
| `[ ARG1 -eq ARG2 ]` | `ARG1` is **eq**ual to `ARG2`.                         |
| `[ ARG1 -ne ARG2 ]` | `ARG1` is **n**ot **e**qual to `ARG2`.                 |
| `[ ARG1 -lt ARG2 ]` | `ARG1` is **l**ess **t**han `ARG2`.                    |
| `[ ARG1 -le ARG2 ]` | `ARG1` is **l**ess than or **e**qual to `ARG2`.        |
| `[ ARG1 -gt ARG2 ]` | `ARG1` is **g**reater **t**han `ARG2`.                 |
| `[ ARG1 -ge ARG2 ]` | `ARG1` is **g**reater or **e**qual to `ARG2`.          |

Conditions may be combined using these **combining expressions:**

| Operation      | Effect                                                      |
| :------------: | :---------------------------------------------------------- |
| `[ ! EXPR ]`   | True if `EXPR` is false.                                    |
| `[ (EXPR) ]`   | Returns the value of `EXPR`.                                |
| `[ EXPR1 -a EXPR2 ]` | Logical _AND_. True if `EXPR1` **a**nd `EXPR2` are true. |
| `[ EXPR1 -o EXPR2 ]` | Logical _OR_. True if `EXPR1` **o**r `EXPR2` are true.|

Of course, there are more useful primaries and you can easily find them in the [Bash man pages](http://www.gnu.org/software/bash/manual/html_node/Bash-Conditional-Expressions.html)

### Using an `if` statement

`if` statements work the same as in other programming languages. If the expression within the braces is true, the code between `then` and `fi` is executed. `fi` indicates the end of the conditionally executed code.

```bash
# Single-line
if [[ 1 -eq 1 ]]; then echo "true"; fi

# Multi-line
if [[ 1 -eq 1 ]]; then
  echo "true"
fi
```

Likewise, we could use an `if..else` statement such as:

```bash
# Single-line
if [[ 2 -ne 1 ]]; then echo "true"; else echo "false"; fi

# Multi-line
if [[ 2 -ne 1 ]]; then
  echo "true"
else
  echo "false"
fi
```

Sometimes `if..else` statements are not enough to do what we want to do. In this case we shouldn't forget about the existence of `if..elif..else` statements, which always come in handy.

Look at the example below:

```bash
if [[ `uname` == "Adam" ]]; then
  echo "Do not eat an apple!"
elif [[ `uname` == "Eva" ]]; then
  echo "Do not take an apple!"
else
  echo "Apples are delicious!"
fi
```

## THE CHALLENGE

Create a file named `if.bash`.

Using `if` statements and primaries, output `Good morning!` if the first positional parameter is less than 12. Output `Good afternoon!` if it is equal to/greater than 12 but less than 18. Otherwise, output `Good evening!` if it is equal to/greater than 18. Take care about cases when the positional argument is less than 0 and greater than 24 (print `Error!` in these cases).

For example:

    ./if.bash -5
    ./if.bash 12
    ./if.bash 21

Output:

    Error!
    Good afternoon!
    Good evening!

---