61-reading-files.md

This Go code demonstrates reading a file using various methods from the os and io packages. Let's go through the code with inline comments and explanations:

// Importing necessary packages.
import (
	"bufio"
	"fmt"
	"io"
	"os"
)

// Function to check for errors and panic if an error is encountered.
func check(e error) {
	if e != nil {
		panic(e)
	}
}

// The main function, where the execution of the program begins.
func main() {
	// Reading the entire contents of the file using os.ReadFile.
	dat, err := os.ReadFile("/tmp/dat")
	check(err)
	fmt.Print(string(dat))

	// Opening the file for further reading.
	f, err := os.Open("/tmp/dat")
	check(err)

	// Reading the first 5 bytes from the file.
	b1 := make([]byte, 5)
	n1, err := f.Read(b1)
	check(err)
	fmt.Printf("%d bytes: %s\n", n1, string(b1[:n1]))

	// Seeking to a specific position (offset 6) in the file and reading 2 bytes.
	o2, err := f.Seek(6, 0)
	check(err)
	b2 := make([]byte, 2)
	n2, err := f.Read(b2)
	check(err)
	fmt.Printf("%d bytes @ %d: %v\n", n2, o2, string(b2[:n2]))

	// Performing another seek and reading at least 2 bytes into a buffer.
	o3, err := f.Seek(6, 0)
	check(err)
	b3 := make([]byte, 2)
	n3, err := io.ReadAtLeast(f, b3, 2)
	check(err)
	fmt.Printf("%d bytes @ %d: %s\n", n3, o3, string(b3))

	// Seeking back to the beginning of the file.
	_, err = f.Seek(0, 0)
	check(err)

	// Using a buffered reader to peek at the first 5 bytes without consuming them.
	r4 := bufio.NewReader(f)
	b4, err := r4.Peek(5)
	check(err)
	fmt.Printf("5 bytes: %s\n", string(b4))

	// Closing the file.
	f.Close()
}

output

• First make sure you have the dummy files on your working directory.
• You might need to change some code for that.
• Then run the .go file with the above code.

hello
go
5 bytes: hello
2 bytes @ 6: go
2 bytes @ 6: go
5 bytes: hello

Explanation:

1. Reading Entire File:

   • os.ReadFile("/tmp/dat") reads the entire contents of the file into a byte slice.

2. Reading Specific Bytes:

   • os.Open("/tmp/dat") opens the file for further reading.
   • f.Read(b1) reads the first 5 bytes from the file.

3. Seeking and Reading:

   • f.Seek(6, 0) seeks to the 6th byte in the file.
   • f.Read(b2) reads 2 bytes from the current position.

4. Seeking, Reading At Least, and Buffered Reading:

   • f.Seek(6, 0) seeks to the 6th byte again.
   • io.ReadAtLeast(f, b3, 2) reads at least 2 bytes into a buffer.

5. Peeking with Buffered Reader:

   • bufio.NewReader(f) creates a buffered reader.
   • r4.Peek(5) peeks at the first 5 bytes without consuming them.

6. Closing the File:

   • f.Close() closes the file.

This code demonstrates various ways to read from a file in Go using different methods from the os and io packages.