How to Read a File Line by Line in Go

If you need to read a file line by line in Go, then you can use the bufio package as follows:

package main
import (
    "bufio"
    "fmt"
    "log"
    "os"
)
func main() {
    file, err := os.Open("/path/to/file.txt")
    if err != nil {
        log.Fatal(err)
    }
    defer file.Close()
    scanner := bufio.NewScanner(file)
    for scanner.Scan() {
        fmt.Println(scanner.Text())
    }
    if err := scanner.Err(); err != nil {
        log.Fatal(err)
    }
}

Just note that the Scanner will throw an error if any line has a character count longer than 65536. This relates back to a 64K size limit. In this case, you can use the Buffer method to increase the capacity.

As illustrated with the maxCapacity variable below on line 17:

package main
import (
    "bufio"
    "fmt"
    "log"
    "os"
)
func main() {
    file, err := os.Open("/path/to/file.txt")
    if err != nil {
        log.Fatal(err)
    }
    defer file.Close()
    scanner := bufio.NewScanner(file)

    // -- swap this variable out
    const maxCapacity = longLineLen  // your required line length
    buf := make([]byte, maxCapacity)
    // --

    for scanner.Scan() {
        fmt.Println(scanner.Text())
    }
    if err := scanner.Err(); err != nil {
        log.Fatal(err)
    }
}