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"
    "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, 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 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.

by Mark McGranaghan and Eli Bendersky | source | license