85 lines
2.2 KiB
Go
85 lines
2.2 KiB
Go
|
// 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 `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))
|
||
|
|
|
||
|
|
// 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 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()
|
||
|
|
|
||
|
|
}
|