Go by Example

Reading Files

Read manageable files with os.ReadFile or scan lines from os.Open; check scanner errors and account for its token limit.

Use os.ReadFile when a file has a known, manageable size. For line-oriented processing, bufio.Scanner reads incrementally without loading the whole file.

Open a file, close it after use, and check Scanner.Err after the scan loop because read errors also stop scanning.

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

In production

os.ReadFile uses memory proportional to the file size. Scanner's default token buffer is 64 KiB, including any line delimiter; increase it with Scanner.Buffer before scanning or use bufio.Reader for longer lines. Always check the final read error, even when the loop ended normally.

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