# Use conditional constructs in AWSTOE
<a name="toe-conditional-constructs"></a>

Conditional constructs perform different actions in your component document based on whether the specified conditional expression evaluates to `true` or `false`. You can use the `if` construct to control the flow of execution in your component document.

## if Construct
<a name="toe-conditional-if"></a>

You can use the `if` construct to evaluate whether a step should run or not. By default, when the `if` conditional expression evaluates to `true`, AWSTOE runs the step, and when the condition evaluates to `false`, AWSTOE skips the step. If a step is skipped, it's treated as a successful step when AWSTOE evaluates whether the phase and document ran successfully.

**Note**  
An `if` statement is only evaluated one time, even if the step triggers a restart. If a step restarts, it recognizes that the `if` statement has already been evaluated, and continues where it left off.

### Syntax
<a name="toe-conditional-if-syntax"></a>

```
if:
  - <conditional expression>:
      [then: <step action>]
      [else: <step action>]
```


| Key name | Required | Description | 
| --- | --- | --- | 
| conditional expression | Yes |  The conditional expression can contain exactly one of the following types of operators at the top level. [\[See the AWS documentation website for more details\]](http://docs.aws.amazon.com/imagebuilder/latest/userguide/toe-conditional-constructs.html) If your expression must satisfy multiple conditions, use a logical operator to specify your conditions.  | 
| then | No |  Defines the action to take if the conditional expression evaluates to `true`.  | 
| else | No |  Defines the action to take if the conditional expression evaluates to `false`.  | 
| step action | Conditional |  When you use `then` or `else`, you must specify one of the following step actions: [\[See the AWS documentation website for more details\]](http://docs.aws.amazon.com/imagebuilder/latest/userguide/toe-conditional-constructs.html)  | 

**Example 1: Install package**  
The following example steps from an AWSTOE component document use logical operators to test a parameter value and run the appropriate package manager commands to install an application if the package is unzipped.

```
    - name: InstallUnzipAptGet
      action: ExecuteBash
      if:
        and:
            - binaryExists: 'apt-get'
            - not:
                binaryExists: 'unzip'
      inputs:
        commands:
            - sudo apt-get update
            - sudo apt-get install -y unzip

    - name: InstallUnzipYum
      action: ExecuteBash
      if:
        and:
            - binaryExists: 'yum'
            - not:
                binaryExists: 'unzip'
      inputs:
        commands:
            - sudo yum install -y unzip

    - name: InstallUnzipZypper
      action: ExecuteBash
      if:
        and:
            - binaryExists: 'zypper'
            - not:
                binaryExists: 'unzip'
      inputs:
        commands:
            - sudo zypper refresh
            - sudo zypper install -y unzip
```

**Example 2: Skip a step**  
The following example shows two ways to skip a step. One uses a logical operator, and one uses a comparison operator with the `Skip` step action.

```
# Creates a file if it does not exist using not
- name: CreateMyConfigFile-1
  action: ExecuteBash
  if:
    not:
      fileExists: '/etc/my_config'
  inputs:
    commands:
      - echo "Hello world" > '/etc/my_config'

# Creates a file if it does not exist using then and else
- name: CreateMyConfigFile-2
  action: ExecuteBash
  if:
    fileExists: '/etc/my_config'
    then: Skip
    else: Execute
  inputs:
    commands:
      - echo "Hello world" > '/etc/my_config'
```