Morganamilo/paru · error

option expects a value

Error message

option {} expects a value

What it means

This is a command-line argument validation failure inside handle_arg (src/command_line.rs), the per-argument dispatcher invoked by parse_arg while building the Config from argv. It fires when the parser matched an option that is registered by takes_value() as TakesValue::Required (an option whose value is not optional) but no value could be attached to it: the option was the last token on the command line, or it appeared in a position where the parser explicitly passes value=None (such as the non-forced handle_arg call for bare arguments). Because there is no sane default for a mandatory option's payload, handle_arg bails out of the whole parse instead of continuing, aborting startup.

Solutions

  1. Supply the value: `--sortby name` or `--sortby=name`
  2. Check scripts for empty variables used as option values
  3. Consult `--help` to see which options require a value

Example fix

// before
args: ["--sortby"]
// after
args: ["--sortby", "name"]  // or "--sortby=name"
Defensive patterns

Strategy: validation

Validate before calling

const REQUIRES_VALUE: &[&str] = &["sortby", "searchby", "limit", "completioninterval", "sortby"];
function validate_args(args: string[]): string | null {
  for (let i = 0; i < args.length; i++) {
    const name = args[i].split('=')[0].replace(/^--?/, '');
    if (REQUIRES_VALUE.includes(name) && !args[i].includes('=') && i + 1 >= args.length)
      return `--${name} expects a value`;
  }
  return null;
}

Try / catch

match cli::parse(&args) {
    Err(e) if e.to_string().contains("expects a value") => {
        eprintln!("{}; usage: --option value or --option=value", e);
        std::process::exit(2);
    }
    Err(e) => return Err(e),
    Ok(cmd) => cmd.run(),
}

Prevention

When it happens

Trigger: Passing a value-requiring flag like `--sortby`, `--limit`, `--completioninterval`, or `--searchby` without an `=value` or a following argument, e.g. `--sortby` alone, or as the last token on the command line.

Common situations: Users typing a flag without its value or forgetting the `=`; shell scripts dropping an empty variable (`--limit "$EMPTY"`); truncated commands in CI pipelines.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

AI-assisted analysis of Morganamilo/paru@9ac3578807 (2026-09-12). Data as JSON: /api/errors/c1214af5394097d0. Report an issue: GitHub.

Appendix: source

Thrown at src/command_line.rs:134

                }
                self.handle_arg(arg, None, op_count, false)?;
            }
            Ok(false)
        } else {
            self.targets.push(arg.to_string());
            Ok(false)
        }
    }

    fn handle_arg(
        &mut self,
        arg: Arg,
        mut value: Option<&str>,
        op_count: &mut u8,
        forced: bool,
    ) -> Result<()> {
        match takes_value(arg) {
            TakesValue::Required if value.is_none() => bail!(tr!("option {} expects a value", arg)),
            _ => (),
        }

        if takes_value(arg) != TakesValue::Required && !forced {
            value = None;
        }

        if arg.is_pacman_global() {
            self.globals.args.push(crate::args::Arg {
                key: arg.arg(),
                value: value.map(|s| s.to_string()),
            });
            self.args.args.push(crate::args::Arg {
                key: arg.arg(),
                value: value.map(|s| s.to_string()),
            });
        }

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