File Handling Programs

Learn file handling programs through an executable example, then connect the small demonstration to production code.

File exercises introduce reading, writing, error checking and cleanup. Prefer temporary directories for runnable demonstrations so programs do not overwrite real user files.

This example creates a temporary text file, reads it back and removes the directory at the end.

Follow this diagram before executing the example. Check where a value or decision changes what runs next.

This example is a complete program. Save it as main.go or paste it into the Go Playground.

Go code

package main

import (
	"fmt"
	"os"
	"path/filepath"
)

func main() {
	dir, err := os.MkdirTemp("", "go-file-example-")
	if err != nil {
		panic(err)
	}
	defer os.RemoveAll(dir)
	path := filepath.Join(dir, "note.txt")
	if err := os.WriteFile(path, []byte("Learn Go"), 0600); err != nil {
		panic(err)
	}
	data, err := os.ReadFile(path)
	if err != nil {
		panic(err)
	}
	fmt.Println("Content:", string(data))
}

Output

Content: Learn Go

The program gets an isolated temporary directory, writes Learn Go with restrictive file permissions, then reads and prints the same bytes.

• Check filesystem errors rather than assuming paths and permissions work.
• Do not use hard-coded shared temporary filenames that another process might overwrite.

Append a second line and print the resulting file contents.

Tip: Edit the example above, predict the output, and run the modified code to test your reasoning.

You have practiced File Handling Programs in a small, self-contained program. The important habit is to predict what runs, inspect the actual output and distinguish the language rule from the particular example.

Before continuing, make sure you can explain the program to someone who has not seen it, including the line that changes its result.