What causes "stdin is not supported" in hashicorp/nomad?

Set the Stdin field on KVBuilder before calling Add, e.g. b.Stdin = os.Stdin (or a bytes.Buffer with your JSON payload)

stdin is not supported

Error message

stdin is not supported

What it means

When a KVBuilder argument is exactly '-', the builder reads the whole value from stdin. This error is thrown when the KVBuilder's Stdin field was never set, so there is no reader to consume. The library refuses to guess or fall back to os.Stdin implicitly.

Source

Thrown at command/var.go:210

	// Regardless of validity, make sure we make our result
	if b.result == nil {
		b.result = make(map[string]any)
	}

	// Empty strings are fine, just ignored
	if raw == "" {
		return nil
	}

	// Split into key/value
	parts := strings.SplitN(raw, "=", 2)

	// If the arg is exactly "-", then we need to read from stdin
	// and merge the results into the resulting structure.
	if len(parts) == 1 {
		if raw == "-" {
			if b.Stdin == nil {
				return fmt.Errorf("stdin is not supported")
			}
			if b.stdin {
				return fmt.Errorf("stdin already consumed")
			}

			b.stdin = true
			return b.addReader(b.Stdin)
		}

		// If the arg begins with "@" then we need to read a file directly
		if raw[0] == '@' {
			f, err := os.Open(raw[1:])
			if err != nil {
				return err
			}
			defer f.Close()

			return b.addReader(f)

View on GitHub (pinned to 482b49bf1a)

Solutions

Set the Stdin field on KVBuilder before calling Add, e.g. b.Stdin = os.Stdin (or a bytes.Buffer with your JSON payload)
If no stdin input is intended, pass an explicit key=value pair instead of '-'
In tests, assign a strings.Reader or bytes.Buffer to Stdin to simulate piped input

Example fix

// before
b := &KVBuilder{Result: map[string]interface{}{}}
b.Add("-")
// after
b := &KVBuilder{Result: map[string]interface{}{}, Stdin: strings.NewReader(`{"key":"value"}`)}
b.Add("-")

Defensive patterns

Strategy: validation

Validate before calling

if b.Stdin == nil && containsDashArg(args) {
	b.Stdin = os.Stdin // or strings.NewReader(jsonPayload)
}

Prevention

Always populate KVBuilder.Stdin when constructing it programmatically
Avoid '-' arguments outside real CLI contexts; pass explicit key=value instead
Use bytes.Buffer/strings.Reader in tests to simulate stdin

When it happens

Trigger: Calling KVBuilder.Add("-") (or var CLI with a lone '-') on a builder constructed without assigning b.Stdin — typically when the builder is used programmatically outside the normal CLI wiring.

Common situations: Embedding the var parser in tests or other tools where Stdin is left nil; refactors that construct KVBuilder directly instead of through the command plumbing that wires os.Stdin in.

Related errors

AI-assisted analysis of hashicorp/nomad@482b49bf1a (2026-09-04). Data as JSON: /api/errors/af00b70318086087. Report an issue: GitHub.