What causes "invalid newline in %s=... value" in golang/go?

Trim trailing newlines from command substitution: assign then strip, or use `${var%$'\n'}`.

invalid newline in %s=... value

Error message

invalid newline in %s=... value

What it means

Returned by checkEnvWrite when the value written via `go env -w` contains any vertical tab, carriage return, or newline (\v \r \n). The go env config file is one line per entry, so embedded newlines would corrupt the file structure.

Source

Thrown at src/cmd/go/internal/envcmd/env.go:700

		if err != nil {
			return fmt.Errorf("invalid %s: %v", key, err)
		}
		if len(args) == 0 {
			return fmt.Errorf("%s entry cannot contain only space", key)
		}
		if !filepath.IsAbs(args[0]) && args[0] != filepath.Base(args[0]) {
			return fmt.Errorf("%s entry is relative; must be absolute path: %q", key, args[0])
		}
	}

	if !utf8.ValidString(val) {
		return fmt.Errorf("invalid UTF-8 in %s=... value", key)
	}
	if strings.Contains(val, "\x00") {
		return fmt.Errorf("invalid NUL in %s=... value", key)
	}
	if strings.ContainsAny(val, "\v\r\n") {
		return fmt.Errorf("invalid newline in %s=... value", key)
	}
	return nil
}

func readEnvFileLines(mustExist bool) []string {
	file, _, err := cfg.EnvFile()
	if file == "" {
		if mustExist {
			base.Fatalf("go: cannot find go env config: %v", err)
		}
		return nil
	}
	data, err := os.ReadFile(file)
	if err != nil && (!os.IsNotExist(err) || mustExist) {
		base.Fatalf("go: reading go env config: %v", err)
	}
	lines := strings.SplitAfter(string(data), "\n")
	if lines[len(lines)-1] == "" {

View on GitHub (pinned to b6b368adc5)

Solutions

Trim trailing newlines from command substitution: assign then strip, or use `${var%$'\n'}`.
Reject multiline values at your config layer with strings.ContainsAny(v, "\v\r\n").
Use single-line equivalents (e.g. semicolons instead of newlines in compiler flags).

Example fix

# before
$ go env -w GOPROXY=$(printf 'a\nb')
error: invalid newline in GOPROXY=... value
# after — strip newlines first
$ go env -w GOPROXY=$(printf 'a\nb' | tr -d '\r\n')

Defensive patterns

Strategy: validation

Validate before calling

import "strings"

if strings.ContainsAny(val, "\v\r\n") {
    return fmt.Errorf("value for %s contains newlines", key)
}

Prevention

Trim trailing newlines from command substitution before persisting.
Reject multiline values at config-ingestion time.
Prefer single-line representations in env-driven configs.

When it happens

Trigger: Any `go env -w KEY=...` value containing \v, \r, or \n — typically multiline text pasted from a terminal, or values captured from `$(cmd)` whose output ends in a newline.

Common situations: Pasting multiline strings; command substitution that includes trailing newlines; copy-paste from documentation with line breaks; values containing Windows CRLF line endings.

Related errors

AI-assisted analysis of golang/go@b6b368adc5 (2026-08-12). Data as JSON: /api/errors/b24b3911a8ae016a. Report an issue: GitHub.