Go Lang Notes

File Operations

Create a File

f, err := os.Create("/tmp/dat2")

Opening a File

uses Package: os

f, err := os.Open("/tmp/dat")

Closing a File

You should always close the file when you are done with it.

f.close()

Reading a File

Reading a whole File (simple method)

uses Package: ioutil.ReadFile

Reading an entire file at once (not efficient for large files).

  dat, err := ioutil.ReadFile("/tmp/dat")
  if err != nil { // check for error
      panic(err)
  }
  fmt.Print(string(dat))

Reading parts (bytes) of a file

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))

Moving to a location in a file.

_, err = f.Seek(0, 0) // goes to beginning

Seek to a known location and start reading 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 above is more robust with the use of the ‘io’ package and ReadAtLeast function.

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))

Using the “bufio” package allows for better efficiency for many small reads and provides additional reading methods.

uses package: bufio r4 := bufio.NewReader(f) b4, err := r4.Peek(5) check(err) fmt.Printf("5 bytes: %s\n", string(b4))

Writing a File

Dumping a string or bytes into a file

uses package: ioutil myData := []byte("hello\ngo\n") err := ioutil.WriteFile("/tmp/data1", myData, 0644) check(err)

using bufio writer (NewWriter()) to write like we used bufio to read above. * uses package: bufio

w := bufio.NewWriter(f)
n4, err := w.WriteString("buffered\n")
fmt.Printf("wrote %d bytes\n", n4)
w.Flush() // flush ensures that all buffered operations have completed.