📄 Go – Read File Line by Line with bufio.Scanner (2025 Guide)

🧲 Introduction – Why Read a File Line by Line?

Reading a file line by line is one of the most common file-processing tasks in Go. It’s ideal for log processing, configuration files, data ingestion, and streaming large text files without consuming too much memory.

🎯 In this guide, you’ll learn:

• How to read a file line by line in Go
• Use bufio.Scanner with ScanLines
• Handle errors and large files efficiently
• Best practices for scalable file parsing

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✅ Example – Read File Line by Line

package main

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

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

    scanner := bufio.NewScanner(file)

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

    if err := scanner.Err(); err != nil {
        log.Fatal(err)
    }
}

📤 Output Example (for a file containing):

Line 1
Line 2
Line 3

Line 1  
Line 2  
Line 3

✅ This method uses bufio.Scanner with the default splitter (ScanLines) to read each line one by one.

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⚙️ How It Works

• scanner.Scan() reads the next line of input
• scanner.Text() returns the current line as a string
• Loop continues until EOF or error occurs

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🔄 Customize Line Parsing Logic

lineNumber := 1
for scanner.Scan() {
    fmt.Printf("Line %d: %s\n", lineNumber, scanner.Text())
    lineNumber++
}

✅ Add line numbers, conditional parsing, or filtering inside the loop.

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🧠 Best Practices

Practice	Why It Helps
✅ Use defer file.Close()	Ensures proper cleanup
✅ Always check scanner.Err()	Detects read errors
✅ Suitable for large files	Reads line-by-line without memory overhead
❌ Avoid using ioutil.ReadAll()	Loads entire file—inefficient for big files

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🧰 Alternative – Use bufio.Reader.ReadString('\n')

reader := bufio.NewReader(file)
for {
    line, err := reader.ReadString('\n')
    if err != nil {
        break
    }
    fmt.Print(line)
}

✅ More control, but requires manual handling of end-of-file and partial lines.

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📌 Summary – Recap & Next Steps

Go’s bufio.Scanner makes it easy to read files line by line, especially for log processors, large datasets, or configuration readers. It’s lightweight, safe, and idiomatic Go.

🔍 Key Takeaways:

• Use bufio.Scanner to stream files line-by-line
• Efficient for large files (no need to load into memory)
• Default split function is ScanLines
• Always check for scan errors and defer file close

⚙️ Next: Learn how to Read Word by Word, Write Lines to Files, or Count Lines in a File.

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❓ FAQs – Go Read File Line by Line

❓ What package should I use to read a file line by line?
✅ Use bufio with os.Open() and bufio.NewScanner().

❓ How do I read a file efficiently in Go?
✅ Use streaming techniques like bufio.Scanner or bufio.Reader.

❓ What happens if the file has millions of lines?
✅ bufio.Scanner is designed to handle large files efficiently, reading line by line.

❓ Can I count lines while reading?
✅ Yes, increment a counter inside the scan loop.

❓ Is ioutil.ReadAll() better for small files?
✅ Yes, but avoid it for large files—prefer streaming with bufio.

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