# Use conditional constructs in AWSTOE
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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
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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.

**Note**  
Conditional expressions support a maximum nesting depth of 4 levels. If you nest logical operators (`and`, `or`, `not`) beyond 4 levels deep, validation fails.

### Syntax
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```
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.+  **Comparison operator** – For a list of comparison operators, and information about how they work in AWSTOE component documents, see [Comparison operators](toe-comparison-operators.md). <br />+  **Logical operator** – Logical operators include `and`, `or`, and `not`, and operate on one or more comparison operators. For more information about how logical operators work in AWSTOE component documents, see [Logical operators](toe-logical-operators.md). <br />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:+  **Abort** – AWSTOE marks the step as failed. <br />+  **Execute** – AWSTOE runs the step. <br />+  **Skip** – AWSTOE skips the step.  | 

**Example 1: Install package**  
The following example steps from an AWSTOE component document use logical operators to detect which package manager is available and install the unzip utility only when the unzip binary is not already present.

```
    - 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'
```