Which parameter of the exec resource allows execution only if a specific condition is true?

The parameter of the exec resource that allows for execution only if a specific condition is true is the onlyif parameter. This parameter is used to specify a command that checks for a certain condition; if the command returns a successful exit status (typically a zero exit code), then the exec resource will be executed.

This functionality is very useful for ensuring that actions are only taken when certain preconditions are met, which can help avoid unnecessary executions and maintain idempotency in Puppet. For instance, if you are deploying software but want to ensure that it only runs if a specific file does not exist, you would use the onlyif parameter to check for that file before proceeding with the execution.

While other parameters like creates and unless also govern when the exec resource will execute, they function differently. Creates specifies a file or directory that, if already present, prevents the exec from running; unless will run the exec unless a specified condition is true. In contrast, onlyif actively checks and allows execution based on a positive condition.