How to Read the Contents of a Text File in Go?

How to Read a Text File in Go

When working with text files in Go, it's essential to understand how to read their contents. However, the question you posed, "How to read a text file? [duplicate]," suggests that this task may be more complex than it seems.

The code you provided:

package main

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

func main() {
    file, err := os.Open("file.txt")
    if err != nil {
        log.Fatal(err)
    }

    fmt.Print(file)
}

successfully reads the file, but the output is merely the pointer value of the file descriptor (*os.File). To actually obtain the file's contents, you need to employ one of several techniques:

Reading File Content into Memory

For small files, the simplest approach is to use io/ioutil.ReadAll to load the entire file into memory.

func main() {
    file, err := os.Open("file.txt")
    if err != nil {
        log.Fatal(err)
    }
    defer file.Close()

    b, err := io.ReadAll(file)
    fmt.Print(b)
}

Reading in Chunks

For larger files, reading in chunks can be more memory-efficient.

func main() {
    file, err := os.Open("file.txt")
    if err != nil {
        log.Fatal(err)
    }
    defer file.Close()

    buf := make([]byte, 32*1024)

    for {
        n, err := file.Read(buf)

        if n > 0 {
            fmt.Print(buf[:n])
        }

        if err == io.EOF {
            break
        }
        if err != nil {
            log.Printf("read %d bytes: %v", n, err)
            break
        }
    }
}

Using a Scanner

Finally, you can use the bufio package to create a Scanner that reads the file in tokens, advancing based on a separator (by default, newline).

func main() {
    file, err := os.Open("file.txt")
    if err != nil {
        log.Fatal(err)
    }
    defer file.Close()

    scanner := bufio.NewScanner(file)

    for scanner.Scan() {
        fmt.Println(scanner.Text())
        fmt.Println(scanner.Bytes())
    }
}

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