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

Question

Accepted Answer

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

**fmt.Println details:**
- Writes to `os.Stdout`.
- Adds space between arguments.
- Appends newline.
- No timestamp or source info.
- Returns `(n int, err error)`.

```go
// Basic usage
fmt.Println("User:", name, "Age:", age)
// Output: User: Alice Age: 30

// Formatting
fmt.Printf("User: %s (age %d)
", name, age)
fmt.Sprintf("User: %s", name) // returns string

// Write to custom writer
fmt.Fprintf(os.Stderr, "Warning: %s
", msg)
```

**log.Println details:**
- Writes to `os.Stderr`.
- Adds date and time prefix.
- Can add file name and line number.
- Adds space between arguments.
- Calls `os.Exit(1)` on write failure.

```go
// Default behavior
log.Println("Server starting on port 8080")
// Output: 2024/01/15 10:30:45 Server starting on port 8080

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

// Log to file
f, _ := os.OpenFile("app.log", os.O_CREATE|os.O_WRONLY|os.O_APPEND, 0644)
defer f.Close()
log.SetOutput(f)
log.Println("This goes to file")
```

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

slog.Info("User logged in", "user_id", 123, "ip", "192.168.1.1")
slog.Error("Database connection failed", "error", err)
```

**When to use fmt:**
- CLI output, user-facing messages.
- HTTP response bodies.
- Testing assertions.

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

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

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