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

Question

Accepted Answer

**fmt.Println** — output to stdout without metadata. **log.Println** — output to stderr with timestamp and optional file/line info.

**fmt.Println:**
- Destination: `os.Stdout`.
- Metadata: None (no timestamp, no source info).
- Behavior: adds spaces between args, appends newline.
- Returns: `(n int, err error)`.
- Use case: user-facing output.

```go
// User output
fmt.Println("Welcome, " + name)
// Output: Welcome, Alice

// Formatted output
fmt.Printf("Score: %.2f
", score)

// To custom writer
fmt.Fprintf(w, "Response: %v
", data)
```

**log.Println:**
- Destination: `os.Stderr`.
- Metadata: Date/time, optionally file/line.
- Behavior: adds spaces between args, newline.
- Side effect: calls `os.Exit(1)` on write error.
- Use case: logging and debugging.

```go
// Application logging
log.Println("Request from", r.RemoteAddr)
// Output: 2024/01/15 10:30:45 Request from 192.168.1.1

// With source info
log.SetFlags(log.Ldate | log.Ltime | log.Lshortfile)
log.Println("Debug point")
// Output: 2024/01/15 10:30:45 main.go:42 Debug point

// Formatted log
log.Printf("User %d logged in from %s
", userID, ip)
```

**Structured logging (Go 1.21+):**
```go
import "log/slog"

slog.Info("server started", "port", 8080)
slog.Error("request failed", "error", err, "path", r.URL.Path)
```

**When to use fmt:**
- CLI tool output.
- HTTP responses.
- Test output.

**When to use log:**
- Application logging.
- Error tracking.
- Audit trails.

**Common mistakes:**
- Using `fmt.Println` for logs (no timestamps).
- Using `log.Println` for user output (stderr).
- Not configuring log output.
- Using `log.Fatal` which exits immediately.
- Not using structured logging in production.

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