How to Read File Line by Line in Go: Simple Guide

In Go, you can read a file line by line using bufio.Scanner. Open the file with os.Open, create a scanner with bufio.NewScanner, then loop with scanner.Scan() to read each line.

Syntax

To read a file line by line in Go, you typically use these steps:

• os.Open to open the file.
• bufio.NewScanner to create a scanner for the file.
• Use scanner.Scan() in a loop to read each line.
• scanner.Text() gives the current line as a string.
• Always check for errors with scanner.Err() after the loop.

go

file, err := os.Open("filename.txt")
if err != nil {
    // handle error
}
defer file.Close()

scanner := bufio.NewScanner(file)
for scanner.Scan() {
    line := scanner.Text()
    // process line
}

if err := scanner.Err(); err != nil {
    // handle error
}

Example

This example shows how to open a file named example.txt and print each line to the console.

go

package main

import (
    "bufio"
    "fmt"
    "os"
)

func main() {
    file, err := os.Open("example.txt")
    if err != nil {
        fmt.Println("Error opening file:", err)
        return
    }
    defer file.Close()

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

    if err := scanner.Err(); err != nil {
        fmt.Println("Error reading file:", err)
    }
}

Output

Hello, world!
This is line 2.
Last line here.

Common Pitfalls

Common mistakes when reading files line by line in Go include:

• Not closing the file with defer file.Close(), which can cause resource leaks.
• Ignoring errors from scanner.Err() after the loop.
• Using ioutil.ReadAll or reading the whole file at once when only line-by-line reading is needed.
• Assuming lines end with \n only; bufio.Scanner handles line endings correctly.

go

/* Wrong way: Not closing file and ignoring errors */
file, _ := os.Open("file.txt")
scanner := bufio.NewScanner(file)
for scanner.Scan() {
    fmt.Println(scanner.Text())
}
// Missing file.Close() and error check

/* Right way: */
file, err := os.Open("file.txt")
if err != nil {
    // handle error
}
defer file.Close()
scanner := bufio.NewScanner(file)
for scanner.Scan() {
    fmt.Println(scanner.Text())
}
if err := scanner.Err(); err != nil {
    // handle error
}

Quick Reference

Remember these key points when reading files line by line in Go:

• Use os.Open to open files.
• Create a scanner with bufio.NewScanner.
• Loop with scanner.Scan() to read lines.
• Get each line with scanner.Text().
• Always close the file and check for errors.

Key Takeaways

Use bufio.Scanner with os.Open to read files line by line efficiently.

Always close the file with defer to avoid resource leaks.

Check for errors after scanning with scanner.Err().

Avoid reading the entire file if you only need line-by-line processing.

bufio.Scanner handles different line endings automatically.