Cookbooks
Swift
Chapter 6. Files and I/O
Writing to standard error

Swift:
Writing to standard error

How to:

In Swift, writing to standard error can be done using the FileHandle class for direct stderr access. Here’s a simple example:

import Foundation

// Define a message
let errorMessage = "An error occurred.\n"

// Convert the message to data
if let data = errorMessage.data(using: .utf8) {
    // Write the error message to stderr
    FileHandle.standardError.write(data)
}

Output to stderr (typically viewed in a console or terminal):

An error occurred.

For more complex logging or when working with external libraries, one might consider using a third-party library like SwiftLog. Although SwiftLog doesn’t write to stderr directly out of the box, you can implement a custom logging backend to achieve this. Here’s a simplified example of defining a custom log handler that writes to stderr:

First, add SwiftLog to your project dependencies in Package.swift:

// swift-tools-version:5.3

import PackageDescription

let package = Package(
    name: "YourPackageName",
    dependencies: [
        .package(url: "https://github.com/apple/swift-log.git", from: "1.0.0"),
    ],
    targets: [
        .target(
            name: "YourTargetName",
            dependencies: [
                .product(name: "Logging", package: "swift-log"),
            ]),
    ]
)

Then, implement a custom log handler that writes to stderr:

import Logging
import Foundation

struct StderrLogHandler: LogHandler {
    let label: String
    
    var logLevel: Logger.Level = .info
    
    func log(level: Logger.Level, message: Logger.Message, metadata: Logger.Metadata?, source: String, file: String, function: String, line: UInt) {
        let output = "\(message)\n"
        if let data = output.data(using: .utf8) {
            FileHandle.standardError.write(data)
        }
    }
    
    subscript(metadataKey metadataKey: String) -> Logger.Metadata.Value? {
        get { return nil }
        set(newValue) { }
    }
    
    var metadata: Logger.Metadata {
        get { return [:] }
        set(newMetadata) { }
    }
}

// Usage
LoggingSystem.bootstrap(StderrLogHandler.init)
let logger = Logger(label: "com.example.yourapp")

logger.error("This is an error message")

Output to stderr:

This is an error message

This custom handler allows you to route your SwiftLog error messages directly to standard error, integrating seamlessly with other log messages your application might generate.

Writing a text file