shell

name	shell
description	Shell runtime rules for mvdan/sh virtual shell in internal/runtime/virtual.go. Covers positional arguments gotcha (prepend "--"), bash strict mode (set -euo pipefail), arithmetic increment pitfalls.
disable-model-invocation	false

Invowk Shell Runtime Rules

This skill covers how Invowk handles shell interpreters and script execution internally. NOT general bash scripting guidance.

Use this skill when working on:

• internal/runtime/virtual.go - Virtual shell runtime
• cmd/invowk/internal_exec_virtual.go - Virtual execution command
• Shell script execution logic

---

Virtual Shell (mvdan/sh)

The virtual runtime uses mvdan/sh, a pure Go POSIX shell interpreter. This provides cross-platform bash-like script execution without requiring an external shell binary.

Positional Arguments Gotcha

CRITICAL: Always prepend "--" when passing positional arguments to interp.Params().

The interp.Params() function configures shell parameters, but it follows POSIX shell conventions where arguments starting with - are interpreted as shell options (like -e, -u, -x).

The Problem

Without "--", positional arguments like -v or --env=staging are interpreted as shell options:

// WRONG: Will fail with "invalid option: -v"
if len(args) > 0 {
    opts = append(opts, interp.Params(args...))
}

Error messages you might see:

• failed to create interpreter: invalid option: "-v"
• failed to create interpreter: invalid option: "--"
• failed to create interpreter: invalid option: "--env=staging"

The Solution

Prepend "--" to signal the end of options:

// CORRECT: "--" signals end of options, remaining args become $1, $2, etc.
if len(args) > 0 {
    params := append([]string{"--"}, args...)
    opts = append(opts, interp.Params(params...))
}

Why This Works

In POSIX shells, "--" is the standard delimiter that terminates option parsing. Everything after "--" is treated as a positional parameter, regardless of whether it starts with -:

# Shell equivalent:
set -- -v --env=staging  # Sets $1="-v", $2="--env=staging"

Affected Locations

When working with mvdan/sh in this codebase, ensure "--" is prepended in:

1. internal/runtime/virtual.go - Execute() method
2. internal/runtime/virtual.go - ExecuteCapture() method
3. cmd/invowk/internal_exec_virtual.go - runInternalExecVirtual() function

Testing

This issue manifests on Windows CI because the virtual runtime is the only bash-compatible option there. When adding new mvdan/sh integration points:

1. Test with arguments starting with - (e.g., -v, --flag=value)
2. Run make test-cli to verify flag handling works
3. Ensure Windows CI passes

---

Bash Script Execution

Strict Mode (set -euo pipefail)

All bash scripts in this project use strict mode for safety:

set -euo pipefail

This enables:

• -e (errexit): Exit on any command failure
• -u (nounset): Error on undefined variables
• -o pipefail: Propagate errors through pipes

Arithmetic Increment Gotcha

CRITICAL: Never use ((var++)) with set -e when var might be 0.

In bash, arithmetic expressions return exit status based on the expression's value:

• ((0)) returns exit status 1 (false)
• ((1)) returns exit status 0 (true)

The post-increment ((x++)) evaluates to the original value of x:

# DANGEROUS with set -e:
COUNTER=0
((COUNTER++))  # Evaluates to 0 (the original value), exits with status 1!
               # Script terminates here due to set -e

Safe Arithmetic Patterns

Use assignment syntax instead of increment operators:

# CORRECT: Assignment always succeeds
COUNTER=$((COUNTER + 1))

# CORRECT: Alternative with let and || true guard
let COUNTER++ || true

# CORRECT: Using arithmetic expansion in assignment
: $((COUNTER++))  # The : (colon) command always succeeds

Anti-Patterns to Avoid

# WRONG: Will fail if COUNTER is 0
((COUNTER++))

# WRONG: Will fail if FAILED is 0
((FAILED++))

# WRONG: Will fail if SKIPPED is 0
((SKIPPED++))

Real-World Example

The VHS test scripts use counters for PASSED, FAILED, and SKIPPED tests:

# WRONG - Script exits on first skip when SKIPPED is 0:
SKIPPED=0
if [[ ! -f "$golden_file" ]]; then
    ((SKIPPED++))  # Exit status 1, script terminates!
    continue
fi

# CORRECT - Assignment syntax always works:
SKIPPED=0
if [[ ! -f "$golden_file" ]]; then
    SKIPPED=$((SKIPPED + 1))  # Always succeeds
    continue
fi

---

Common Pitfalls

• Missing "--" delimiter - Always use append([]string{"--"}, args...) when calling interp.Params() with user-provided positional arguments.
• Arithmetic with set -e - Always use VAR=$((VAR + 1)) instead of ((VAR++)) when VAR might be 0.