Reading Files

Reading and writing files are basic tasks needed for many Go programs. First we'll look at some examples of reading files.

package main

import (
	"bufio"
	"fmt"
	"io"
	"os"
	"path/filepath"
)

Reading files requires checking most calls for errors. This helper will streamline our error checks below.

func check(e error) {
	if e != nil {
		panic(e)
	}
}

func main() {

First we write a small file to read back.

	path := filepath.Join(os.TempDir(), "dat")
	check(os.WriteFile(path, []byte("hello\ngo\n"), 0644))
	defer os.Remove(path)

Perhaps the most basic file reading task is slurping a file's entire contents into memory.

	dat, err := os.ReadFile(path)
	check(err)
	fmt.Print(string(dat))

You'll often want more control over how and what parts of a file are read. For these tasks, start by Opening a file to obtain an os.File value.

	f, err := os.Open(path)
	check(err)

Read some bytes from the beginning of the file. Allow up to 5 to be read but also note how many actually were read.

	b1 := make([]byte, 5)
	n1, err := f.Read(b1)
	check(err)
	fmt.Printf("%d bytes: %s\n", n1, string(b1[:n1]))

You can also Seek to a known location in the file and Read from there.

	o2, err := f.Seek(6, io.SeekStart)
	check(err)
	b2 := make([]byte, 2)
	n2, err := f.Read(b2)
	check(err)
	fmt.Printf("%d bytes @ %d: ", n2, o2)
	fmt.Printf("%v\n", string(b2[:n2]))

Other methods of seeking are relative to the current cursor position, and relative to the end of the file.

	_, err = f.Seek(2, io.SeekCurrent)
	check(err)

	_, err = f.Seek(-4, io.SeekEnd)
	check(err)

The io package provides some functions that may be helpful for file reading. For example, reads like the ones above can be more robustly implemented with ReadAtLeast.

	o3, err := f.Seek(6, io.SeekStart)
	check(err)
	b3 := make([]byte, 2)
	n3, err := io.ReadAtLeast(f, b3, 2)
	check(err)
	fmt.Printf("%d bytes @ %d: %s\n", n3, o3, string(b3))

There is no built-in rewind, but Seek(0, io.SeekStart) accomplishes this.

	_, err = f.Seek(0, io.SeekStart)
	check(err)

The bufio package implements a buffered reader that may be useful both for its efficiency with many small reads and because of the additional reading methods it provides.

	r4 := bufio.NewReader(f)
	b4, err := r4.Peek(5)
	check(err)
	fmt.Printf("5 bytes: %s\n", string(b4))

A bufio.Scanner reads a file line by line.

	f.Seek(0, io.SeekStart)
	scanner := bufio.NewScanner(f)
	for line := 1; scanner.Scan(); line++ {
		fmt.Println(line, scanner.Text())
	}
	check(scanner.Err())

Close the file when you're done (usually this would be scheduled immediately after Opening with defer).

	f.Close()

Opening a missing file returns an error you can test for.

	_, err = os.Open(filepath.Join(os.TempDir(), "missing"))
	fmt.Println(os.IsNotExist(err))
}

$ go run ./70_reading_files
hello
go
5 bytes: hello
2 bytes @ 6: go
2 bytes @ 6: go
5 bytes: hello
1 hello
2 go
true

View on GitHub