Read from File
Examples
Filter
package main
import (
"bufio"
"fmt"
"io"
"io/ioutil"
"os"
)
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")
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))
f.Close()
}