Reading, Writing Files

Go provides robust tools for handling file operations using the os and io packages. This snippet will guide you through reading from and writing to files in Go.

Reading Files

Here's a basic way to read a file in Go:

package main

import (
	"fmt"
	"log"
	"os"
)

func main() {
	data, err := os.ReadFile("example.txt")
	if err != nil {
		log.Fatalf("failed reading file: %s", err)
	}
	fmt.Printf("File contents: %s", data)
}

Alternatively, reading a file line by line can be more efficient for large files:

package main

import (
	"bufio"
	"fmt"
	"log"
)

func main() {
	f, err := os.Open("example.txt")
	if err != nil {
		log.Fatalf("failed opening file: %s", err)
	}
	defer f.Close()

	scanner := bufio.NewScanner(f)
	for scanner.Scan() {
		fmt.Println(scanner.Text())
	}

	if err := scanner.Err(); err != nil {
		log.Fatalf("scanner error: %s", err)
	}
}

Writing Files

Writing data to a file is just as straightforward:

package main

import (
	"log"
	"os"
)

func main() {
	content := []byte("Hello, World!\n")
	err := os.WriteFile("example.txt", content, 0644)
	if err != nil {
		log.Fatalf("failed writing to file: %s", err)
	}
}

For appending data, use:

package main

import (
	"log"
	"os"
)

func main() {
	f, err := os.OpenFile("example.txt", os.O_APPEND|os.O_WRONLY, 0644)
	if err != nil {
		log.Fatalf("failed opening file: %s", err)
	}
	defer f.Close()

	if _, err := f.WriteString("Appended content\n"); err != nil {
		log.Fatalf("failed writing to file: %s", err)
	}
}

Best Practices

• Always close files with defer immediately after opening them
• Use log for error handling instead of panic in production setups
• Check for scanner errors after scanning is complete
• When writing, handle permissions with the proper file mode (e.g., 0644)

Common Pitfalls

• Forgetting to close files, leading to resource leaks
• Not checking for errors and simply assuming file operations succeed
• Using os.ReadFile for large files without considering memory constraints
• Not considering file permissions when creating or writing to files

Performance Tips

• Use bufio for reading large files line by line to reduce memory usage
• Preallocate byte slices if you know the file size for performance gains
• Consider using os.File methods directly for lower-level file operations
• Avoid using os.WriteFile for repeated small writes to reduce system calls