golang/go · error

error: cannot use -w with standard input

Error message

error: cannot use -w with standard input

What it means

Thrown by gofmtMain when no file/directory arguments are supplied (len(args)==0) and the -w (write) flag is set. With no arguments gofmt reads from standard input, and writing formatted output back over stdin is meaningless/impossible, so the combination is rejected up front.

Source

Thrown at src/cmd/gofmt/gofmt.go:410

		if err != nil {
			s.AddReport(fmt.Errorf("creating cpu profile: %s", err))
			return
		}
		defer func() {
			f.Close()
			<-fdSem
		}()
		pprof.StartCPUProfile(f)
		defer pprof.StopCPUProfile()
	}

	initParserMode()
	initRewrite()

	args := flag.Args()
	if len(args) == 0 {
		if *write {
			s.AddReport(fmt.Errorf("error: cannot use -w with standard input"))
			return
		}
		s.Add(0, func(r *reporter) error {
			return processFile("<standard input>", nil, os.Stdin, r)
		})
		return
	}

	for _, arg := range args {
		// Walk each given argument as a directory tree.
		// If the argument is not a directory, it's always formatted as a Go file.
		// If the argument is a directory, we walk it, ignoring non-Go files.
		if err := filepath.WalkDir(arg, func(path string, d fs.DirEntry, err error) error {
			switch {
			case err != nil:
				return err
			case d.IsDir():
				return nil // simply recurse into directories

View on GitHub (pinned to b6b368adc5)

Solutions

1. Pass the target files explicitly: `gofmt -w <files...>` or `gofmt -w .`.
2. Drop -w when reading from stdin: `gofmt < foo.go` writes formatted output to stdout.
3. If scripting, ensure the argument list is non-empty before adding -w.

Example fix

// before
cat main.go | gofmt -w

// after
gofmt -w main.go

if *write && len(flag.Args()) == 0 {
    return errors.New("-w requires file/dir arguments; reading stdin is incompatible with -w")
}

When it happens

Trigger: Invoking `gofmt -w` with no positional arguments, or piping a file into gofmt while requesting in-place write, e.g. `cat foo.go | gofmt -w` or just `gofmt -w`.

Common situations: Forgetting to pass the file path after -w, copy-pasting a pipeline from a snippet that used -w, or scripts that conditionally drop the filename argument.

Related errors

• -C flag must be first flag on command line
• cannot combine -all and -short
• parse error
• tool %q is ambiguous; choose one of:
• no trace file supplied

AI-assisted analysis of golang/go@b6b368adc5 (2026-08-12). Data as JSON: /api/errors/60441395910b79df. Report an issue: GitHub.