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Reading a file line by line in Go

Reading a file line by line in Go

Clive B.

—

The Problem

I don’t know how to read a file line by line in Go.

The Solution

In Go, you can use the bufio.Scanner function, which accepts an io.Reader interface, to read lines separated by a newline.

Files opened using os.Open satisfy the io.Reader interface.

The following code prints file contents line by line:

Click to Copy
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() {
        // This will print the file contents line by line.
        fmt.Println(scanner.Text())
    }

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

Advanced Usage

If you want to scan through a file and split its contents by something other than newlines, you can specify a custom split function. Go supplies a few custom split functions in the standard library, namely bufio.ScanBytes, bufio.ScanLines, bufio.ScanRunes, and bufio.ScanWords.

For example, you can split a text by spaces so that each space-separated word is printed individually:

Click to Copy
package main

import (
    "bufio"
    "bytes"
    "fmt"
    "log"
)

func main() {
    words := bytes.NewReader([]byte("This is some example text."))

    scanner := bufio.NewScanner(words)

    scanner.Split(bufio.ScanWords)

    for scanner.Scan() {
        fmt.Println(scanner.Text())
    }

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

This prints:

Click to Copy
This
is
some
example
text.

Further Reading

  • The Go bufio package
  • The Reader interface in the Go io package
  • The Open function in the Go os package
  • The Newline Wikipedia entry
  • SentryGo Error Tracking and Performance Monitoring
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