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Reading Files

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. 1.
    Reading Entire File:
    • os.ReadFile("/tmp/dat") reads the entire contents of the file into a byte slice.
  2. 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. 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. 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. 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. 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.