Go: What is the difference between fmt.Println and log.Println?

fmt.Println — stdout, no metadata. log.Println — stderr, with timestamp.

fmt.Println:

• Writes to os.Stdout.
• No timestamp.
• Adds spaces between args.

1fmt.Println("Hello, World!")
2// Output: Hello, World!
3
4fmt.Printf("User: %s\n", name)

log.Println:

• Writes to os.Stderr.
• Adds timestamp.
• Can add file/line.

1log.Println("Server started")
2// Output: 2024/01/15 10:30:45 Server started
3
4log.SetFlags(log.Ldate | log.Ltime | log.Lshortfile)
5log.Println("Debug")
6// Output: 2024/01/15 10:30:45 main.go:42 Debug

When to use fmt:

• CLI output.
• HTTP responses.

When to use log:

• Application logging.
• Debugging.

Common mistakes:

• Using fmt.Println for logs.
• Using log.Println for user output.
• Not configuring log output.
• Using log.Fatal (exits immediately).
• Not using structured logging in production.