What causes "command output contains newlines: %s" in jesseduffield/lazygit?

Reduce the command to one line of output: add '| head -n1', 'awk NR==1', or make the query exact ('git rev-parse --short HEAD').

jesseduffield/lazygit · error

command output contains newlines: %s

Error message

command output contains newlines: %s

What it means

TemplateFunctionRunCommand executes a shell command from a custom-command template ({{runCommand ...}}) and requires single-line output: trailing newlines are trimmed, then any remaining '\r\n' makes it fail, because the result is interpolated back into a git command line where embedded newlines would break or inject lines.

Source

Thrown at pkg/commands/git_commands/custom.go:36

}

// Only to be used for the sake of running custom commands specified by the user.
// If you want to run a new command, try finding a place for it in one of the neighbouring
// files, or creating a new BlahCommands struct to hold it.
func (self *CustomCommands) RunWithOutput(cmdStr string) (string, error) {
	return self.cmd.New(str.ToArgv(cmdStr)).RunWithOutput()
}

// A function that can be used as a "runCommand" entry in the template.FuncMap of templates.
func (self *CustomCommands) TemplateFunctionRunCommand(cmdStr string) (string, error) {
	output, err := self.RunWithOutput(cmdStr)
	if err != nil {
		return "", err
	}
	output = strings.TrimRight(output, "\r\n")

	if strings.Contains(output, "\r\n") {
		return "", fmt.Errorf("command output contains newlines: %s", output)
	}

	return output, nil
}

View on GitHub (pinned to c477a2959b)

Solutions

Reduce the command to one line of output: add '| head -n1', 'awk NR==1', or make the query exact ('git rev-parse --short HEAD').
Strip CR if the tool emits CRLF: pipe through 'tr -d "\r"' — the guard specifically looks for '\r\n' pairs.
For list-like data, run the command in the custom command itself, not via runCommand.

Example fix

# before
command: 'echo {{runCommand "git branch --list main"}}'
# after
command: 'echo {{runCommand "git branch --list main | head -n1 | tr -d "\r""}}'

Defensive patterns

Strategy: validation

Validate before calling

// In your own runCommand-style helpers, enforce single-line output yourself:
out, err := runWithOutput(cmd)
if err != nil {
    return "", err
}
out = strings.TrimRight(out, "\r\n")
if strings.ContainsAny(out, "\r\n") {
    return "", fmt.Errorf("expected single-line output from %q", cmd)
}

Prevention

Use runCommand only for single-value lookups (rev-parse, config get, head -n1).
Pipe multi-line sources through 'head -n1' / 'awk NR==1' and 'tr -d "\r"' at the call site.
Never interpolate raw command output into shell strings without newline/quote sanitization.

When it happens

Trigger: A custom command using runCommand whose command prints multiple lines (e.g. 'git branch --list', 'ls', any table output) — after trimming the final newline, interior CRLF pairs still remain and trigger the error.

Common situations: Piping a multi-line git query into a template placeholder; forgetting that the helper is designed for single-value lookups (version strings, hashes, names), not lists.

Related errors

AI-assisted analysis of jesseduffield/lazygit@c477a2959b (2026-08-15). Data as JSON: /api/errors/075db546af072edf. Report an issue: GitHub.