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

Question

Accepted Answer

**fmt.Println** — prints to stdout, no metadata. **log.Println** — prints to stderr with timestamp, and optionally file/line info.

**fmt.Println:**
- Writes to stdout (standard output).
- No timestamp, no file info.
- Returns number of bytes written and any error.
- Adds spaces between arguments, newline at end.
- Best for user-facing output (CLI tools, server responses).

```go
fmt.Println("Processing user:", name)
// Output: Processing user Alice

// Different formatting options
fmt.Printf("Name: %s, Age: %d
", name, age)
fmt.Fprintf(w, "Response: %v", data) // write to io.Writer
```

**log.Println:**
- Writes to stderr (standard error).
- Adds timestamp: `2024/01/15 10:30:45`.
- Can add file and line number.
- Calls `os.Exit(1)` on write errors.
- Best for logging, debugging, diagnostics.

```go
log.Println("Processing user:", name)
// Output: 2024/01/15 10:30:45 Processing user Alice

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

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

**When to use fmt.Println:**
- User-facing output in CLI tools.
- Server responses (HTTP handlers).
- Testing output.

**When to use log.Println:**
- Application logging and diagnostics.
- Debugging and tracing.
- Error logging (use `log.Printf` for formatted output).

**Common mistakes:**
- Using `fmt.Println` for logging (no timestamps, goes to stdout).
- Using `log.Println` for user output (goes to stderr, has timestamps).
- Not configuring log flags (default may not include file/line).
- Using `log.Fatal` which exits immediately (deferred functions won't run).
- Not setting up structured logging for production (consider `slog` package in Go 1.21+).

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