Skip to content

Exception Handling

Clikt uses exceptions internally to signal that processing has ended early for any reason. This includes incorrect command line usage, or printing a help page.

Where are Exceptions Handled? {#handling}

When you call CliktCommand.main, it will parse the command line and catch any CliktError and Abort exceptions. If it catches one, it will then print out the appropriate information and exit the process. If the caught exception is a PrintMessage or PrintHelpMessage, the process exit status will be 0 and the message will be printed to stdout. Otherwise it will exit with status 1 and print the message to stderr.

Any other types of exceptions indicate a programming error, and are not caught by main. However, convert and the other parameter transformations will wrap exceptions thrown inside them in a UsageError, so if you define a custom transformation, you don’t have to worry about an exception escaping to the user.

Handling Exceptions Manually

CliktCommand.main is just a try/catch block surrounding CliktCommand.parse, so if don’t want exceptions to be caught, you can call parse wherever you would normally call main.

fun main(args: Array<String>) = Cli().parse(args)

Which Exceptions Exist?

Clikt will throw Abort if it needs to halt execution immediately without a specific message. All other exceptions are subclasses of UsageError.

The following subclasses exist:

  • PrintMessage : The exception’s message should be printed.
  • PrintHelpMessage : The help page for the exception’s command should be printed.
  • UsageError : The command line was incorrect in some way. All other exceptions subclass from this. These exceptions are automatically augmented with extra information about the current parameter, if possible.
  • BadParameterValue : A parameter was given the correct number of values, but of invalid format or type.
  • MissingParameter : A required parameter was not provided.
  • NoSuchOption : An option was provided that does not exist.
  • IncorrectOptionValueCount : An option was supplied but the number of values supplied to the option was incorrect.
  • IncorrectArgumentValueCount : An argument was supplied but the number of values supplied was incorrect.
  • MutuallyExclusiveGroupException : Multiple options in a mutually exclusive group were supplied when the group is restricted to a single value.