Using bufio.Scanner is the most common and efficient method in Go to read files line by line, and is suitable for handling scenarios such as large files, log parsing or configuration files. 1. Open the file using os.Open and make sure to close the file via defer file.Close(). 2. Create a scanner instance through bufio.NewScanner. 3. Call scanner.Scan() in the for loop to read line by line until false is returned to indicate that the end of the file is reached or an error occurs. 4. Use scanner.Text() to get the current line content (excluding newlines). 5. Check scanner.Err() after the loop is over to catch possible read errors. This method is memory efficient and avoids loading the entire file at once. Therefore, in most cases, bufio.Scanner instead of os.ReadFile combined with strings.Split is recommended, which will load the entire file into memory and is not suitable for large file processing.
Reading a file line by line in Go is straightforward using the bufio.Scanner . This is the most common and efficient way for text files where you want to process one line at a time.
Here's how you do it:
Use os.Open with bufio.NewScanner
package main
import (
"bufio"
"fmt"
"log"
"os"
)
func main() {
file, err := os.Open("example.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)
}
}Key Points
-
os.Open: Opens the file in read-only mode. -
defer file.Close(): Ensures the file is closed after reading. -
bufio.NewScanner: Creates a scanner that reads the file in chunks, splitting by lines. -
scanner.Scan(): Advanceds to the next line (returnsfalseat EOF or on error). -
scanner.Text(): Returns the current line as a string (without the newline character). - Check
scanner.Err(): Always check for errors after the loop, because some errors might be caught only after scanning finishes.
When to Use This Method
- Processing large files (avoids loading everything into memory).
- Log parsing, config files, or any line-based input.
- You don't need to store all lines — just process them one by one.
Alternative: Read All Lines at Once
If you need all lines in a slice, you could do:
data, err := os.ReadFile("example.txt")
if err != nil {
log.Fatal(err)
}
lines := strings.Split(string(data), "\n")
for _, line := range lines {
fmt.Println(line)
}But this loads the entire file into memory — not ideal for large files.
So, for most cases, use bufio.Scanner — it's simple, memory-efficient, and idiomatic in Go.
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