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"
)

// 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() {

	// Perhaps the most basic file reading task is
	// slurping a file's entire contents into memory.
	dat, err := os.ReadFile("/tmp/dat")
	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 `Open`ing a file to obtain an `os.File` value.
	f, err := os.Open("/tmp/dat")
	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,
	_, err = f.Seek(4, io.SeekCurrent)
	check(err)

	// and relative to the end of the file.
	_, err = f.Seek(-10, 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))

	// Close the file when you're done (usually this would
	// be scheduled immediately after `Open`ing with
	// `defer`).
	f.Close()
}

$ echo "hello" > /tmp/dat
$ echo "go" >>   /tmp/dat
$ go run reading-files.go
hello
go
5 bytes: hello
2 bytes @ 6: go
2 bytes @ 6: go
5 bytes: hello

# Next we'll look at writing files.