Use conditional constructs in EC2 TOE - EC2 Image Builder
if Construct
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Use conditional constructs in EC2 TOE

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

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, EC2 TOE runs the step, and when the condition evaluates to false, EC2 TOE skips the step. If a step is skipped, it's treated as a successful step when EC2 TOE 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

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 EC2 TOE component documents, see Comparison operators.

  • 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 EC2 TOE component documents, see Logical operators.

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 – EC2 TOE marks the step as failed.

  • Execute – EC2 TOE runs the step.

  • Skip – EC2 TOE skips the step.
Example 1: Install package

The following example steps from an EC2 TOE 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'