using double hyphen as an arg

[sudhackar]

Hello

I was writing an app that can take -- as one of the values in the args.

Something like

$ process -- test --

I can capture the second -- but not the first.
But I don't seem to be able to get it in

.arg(Arg::new("foo").action(ArgAction::Append))

I looked at trailing_var_arg(true) and allow_hyphen_values(true) but see no usecase supporting this

Here's a minimal repro

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=df089bcf97d6fd99a7894b371a27ee5b

[epage]

Could you describe your use case for treating -- as a value? That being an argument escape character (and the need for one) is fairly fundamental.

Understanding the use case helps to give an idea of both the importance within an application and how likely it is to be used across applications to weigh it, especially as we try to focus on our binary size and compile times.

If we do end up supporting something like this, I wonder if pluggable lexers from #2468 could help us handle this.

[sudhackar]

Hello @epage

The usecase could be similar to echo

/usr/bin/echo  -- 'foo'
-- foo

Also see : uutils/coreutils#5840 I was looking at

[epage]

For now, I feel like this is specialized enough that I would wait on #2468.

echos CLI is limited enough that you could workaround this by

• Implementing the parser via clap_lex (sharing a dependency with clap)
• Still implement the CLI in clap and delegate to it for the --help output

[sudhackar]

Alright. I'll take a look and wait for this to be done. Thanks a lot!

[jasonterando]

Sorry to necro this, but did come to a conclusion, one way or the other? It looks like #5055 may be related. If a fix was merged in, could somebody point me to where my syntax is wrong?

My specific use case is a utility that can optionally read from STDIN and write to STDOUT. I've used other utilities where double hyphens are used to represent these values (as opposed to file names "STDIN" and 'STDOUT". I would like to do the same. Here is what I have tried thus far.

#[derive(Parser, Debug)]
#[command(version, about, long_about = None, allow_hyphen_values = true)]
struct Args {
    /// Name of the file to process (or -- to read from STDIN)
    file: String,
    /// Global parameter file name (overriding default location, if available)
    #[arg(short, long)]
    globals: Option<String>,
    /// Print configuration information
    #[arg(short, long, default_value_t = false)]
    info: bool,
    /// Name of the output file name for test results (or -- to write to STDOUT)
    #[arg(short, long)]
    output: Option<String>,
}

Usage: my-app [OPTIONS] <FILE>

Arguments:
  <FILE>  Name of the file to process (or -- to read from STDIN)

Options:
  -g, --globals <GLOBALS>  Global parameter file name (overriding default location, if available)
  -i, --info               Print configuration information
  -o, --output <OUTPUT>    Name of the output file name for test results (or -- to write to STDOUT)
  -h, --help               Print help
  -V, --version            Print version

[epage]

In almost every utility I've worked with that has a placeholder for STDIN/STDOUT, it has been - and not --.

[jasonterando]

Yah, you're right. Recency bias on my part (I've been working lately with some proprietary print imaging tools that do the double hyphen). Single hyphen works great, I'm good. Thanks for the super-quick reply :)