picocli does not seem to support - as a positional parameter

[helpermethod]

Hi,

I'm preparing some demo project for picocli and stumbled over some interesting, currently unsupported case.

Some shell commands, like cat accept - as a positional parameter

cat -

which means cat will read it's input from STDIN instead of a file.

But it seems picocli currently doesn't support such a case?

When I call

@Command(name = "mst-cli", mixinStandardHelpOptions = true)
public class Cli implements Runnable {
    @Parameters(index = "0")
    private String json;
    @Parameters(index = "1")
    private String template;
    // more code
}

with the following parameters

- ~/hello.mustache

picocli complains that the second parameter isn't set :/.

But

~/data.json ~/hello.mustache

works.

[remkop]

Is there a stack trace or more detailed error message?

[remkop]

I agree that the single dash character - is a common argument like you describe and it should be possible to have a positional parameter in picocli take this value. I’ll look into it when I get to my PC.

[remkop]

I was able to reproduce the problem with this test:

@Test
public void testSingleDashPositionalParam() {
    @Command(name = "dashtest", mixinStandardHelpOptions = true)
    class App {
        @Parameters(index = "0")
        private String json;
        @Parameters(index = "1")
        private String template;
    }
    System.setProperty("picocli.trace", "DEBUG");
    App app = new App();
    CommandLine commandLine = new CommandLine(app);
    
    commandLine.parseArgs("-", "~/hello.mustache");
    assertEquals("-", app.json);
    assertEquals("~/hello.mustache", app.template);
}

I see the following DEBUG output:

[picocli DEBUG] Processing argument '-'. Remainder=[~/hello.mustache]
[picocli DEBUG] '-' cannot be separated into <option>=<option-parameter>
[picocli DEBUG] Could not find option '-', deciding whether to treat as unmatched option or positional parameter...
[picocli DEBUG] - resembles an option: 4 matching prefix chars out of 4 option names
[picocli DEBUG] Processing argument '~/hello.mustache'. Remainder=[]
[picocli DEBUG] '~/hello.mustache' cannot be separated into <option>=<option-parameter>
[picocli DEBUG] Could not find option '~/hello.mustache', deciding whether to treat as unmatched option or positional parameter...
[picocli DEBUG] ~/hello.mustache doesn't resemble an option: 0 matching prefix chars out of 4 option names
[picocli DEBUG] No option named '~/hello.mustache' found. Processing remainder as positional parameters
[picocli DEBUG] Processing next arg as a positional parameter at index=0. Remainder=[~/hello.mustache]
[picocli DEBUG] Position 0 is in index range 0. Trying to assign args to field String picocli.CommandLineTest$95App.json, arity=1
[picocli INFO] Setting field String picocli.CommandLineTest$95App.json to '~/hello.mustache' (was 'null') for args[0] at position 0
[picocli DEBUG] Consumed 1 arguments and 0 interactive values, moving position to index 1.

picocli.CommandLine$MissingParameterException: Missing required parameter: <template>

So the problem is that the single dash resembles an option in picocli's heuristics for unmatched arguments.

The workaround is to set

    commandLine.setUnmatchedOptionsArePositionalParams(true);

This will make parse the input as expected. Maybe the heuristic should be improved.

[remkop]

Fixed in master.
Improved the heuristics for unmatched options: single-character arguments that don't exactly match options (like -) are now always considered positional parameters.

Please verify.

[remkop]

Picocli 3.9.3 has been released. This includes a fix for this issue.

Enjoy!

[helpermethod]

Thank you very much. Was very low on time the last few weeks so couldn't test the change :/.