Go by Example: 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"
    "io/ioutil"
    "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 := ioutil.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")

Read some bytes from the beginning of the file. Allow up to 5 to be read but also not 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))

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: %s\n", n2, o2, string(b2))

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 it’s 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

Next we’ll look at writing files.

Next example: Writing Files.

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