Here we won't be surprised. As in any programming language, a loop in bash is a block of code that iterates as long as the control conditional is true.

There are four types of loops in Bash: `for`, `while`, `until` and `select`.

### `for` loop

The `for` loop is very similar to its sibling in C. It looks like this:

```bash
for arg in elem1 elem2 ... elemN
do
  # statements
done
```

During each pass through the loop, `arg` takes on the value from `elem1` to `elemN`. Values may also be wildcards or [brace expansion](#brace-expansion).

Also, we can write a `for` loop in one line, but in this case there needs to be a semicolon before `do`, like below:

```bash
for i in {1..5}; do echo $i; done
```

By the way, if `for..in..do` seems a little bit weird to you, you can also write `for` in C-like style such as:

```bash
for (( i = 0; i < 10; i++ )); do
  echo $i
done
```

`for` is handy when we want to do the same operation over each file in a directory. For example, if we need to move all `.bash` files into the `script` folder and then give them execute permissions, our script would look like this:

```bash
#!/bin/bash

for FILE in $HOME/*.bash; do
  mv "$FILE" "${HOME}/scripts"
  chmod +x "${HOME}/scripts/${FILE}"
done
```

### `while` loop

The `while` loop tests a condition and loops a sequence of commands so long as that condition is `true`. A condition is nothing more than the primary used in `if..then` conditions. So a working example with `while` loop might look like this:

```bash
#!/bin/bash

# Squares of numbers from 0 through 9
x=0
while [[ $x -lt 10 ]]; do # value of x is less than 10
  echo $(($x * $x))
  x=`expr $x + 1` # increase x
done
```

Just like in the case of the `for` loop, if we want to write `do` and the condition in the same line, then we must use a semicolon before `do`.

### `until` loop

The `until` loop is the exact opposite of the `while` loop. Like a `while` loop it checks a test condition, but it keeps looping as long as this condition is `false`:

```bash
until [[ condition ]]; do
  #statements
done
```

### Loop control

There are situations when we need to stop a loop before its normal ending or step over an iteration. In these cases, we can use the shell built-in `break` and `continue` statements. Both of these work with every kind of loop.

The `break` statement is used to exit the current loop before its ending.

The `continue` statement steps over one iteration. We can use it as such:

```bash
for (( i = 0; i < 10; i++ )); do
  if [[ $(($i % 2)) == 0 ]]; then continue; fi
  echo $i
done
```

If we run the example above, it will print all odd numbers from 0 through 9.

## THE CHALLENGE

Create a file named `loops.bash`.

The first two positional parameters which will be passed into your script are the limits. You should write all even numbers in the range between the first and second positional parameters.

Example:

    ./loops.bash 10 17

Output:

    10
    12
    14
    16

Use `for`, `while` or `until` loops to solve this problem.

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