Required Argument Aware Flag Validation

SKILL: Required Argument Aware Flag Validation

Domain: validator Trigger: Apply this whenever flag validation cannot rely on shell parsers and must check --flag=value, combined short bundles, and missing arguments explicitly. Source Pattern: Distilled from reviewed tool execution, streaming, persistence, and output-budget implementations.

Core Method

Iterate over tokens after the command verb, treat -- according to whether the tool respects it, then inspect each -flag/--flag by looking it up in the declarative config. Handle inline values (-E=), attached numeric args (-A20), and bundles of non-arg flags carefully: reject unknown flags, ensure required arguments exist, and forbid bundles that include arg-taking options. Use typed helpers (validate flag argument) to keep regex/number checks centralized.

Key Rules

• Never assume an inline assignment contains a value; --flag= must be validated as empty input, not allowed to silently consume the next token.
• For launcher commands such as xargs, stop validating wrapper flags once parsing has switched to the downstream command.
• Reject combined short flags if any member expects an argument; allow only none-type flags in bundles to avoid parser differentials.
• Guard string arguments from starting with - unless the flag explicitly allows it (e.g., git --sort -version:refname).

Example Application

Implement this in a shell command policy layer by validating the tokenized command against declarative flag metadata before execution. That ensures commands containing -S, --diff-filter, or --flag=value cannot bypass read-only guardrails through parser edge cases.

Anti-Patterns (What NOT to do)

• Do not treat -FLAG=value the same as -FLAG value; the attached format still needs its own validation rules.
• Do not skip the empty-inline-value check for -FLAG=; otherwise attackers can bypass restrictions by hiding invalid input behind =.