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 Opening 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, 0)
    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]))

    // 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, 0)
    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, 0) accomplishes this.
    _, err = f.Seek(0, 0)
    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 Opening 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