Go Package os

External

• https://pkg.go.dev/os

Internal

• Standard library
• File Operations in Go

Reading and Writing an Entire File in/from Memory

Read with ReadFile()

To read the content of an entire file into a byte slice:

bs, err := os.ReadFile("/Users/ovidiu/tmp/test.txt")

os.ReadFile() closes the file after use.

Write with WriteFile()

To write in-memory data into a file:

s := "This\nis multi-line\nfile\ncontent\n"
err := os.WriteFile("/Users/ovidiu/tmp/test2.txt", []byte(s), 0777)

os.WriteFile() closes the file after use.

Sequentially Reading and Writing Byte Arrays

Reading

Open the file with os.Open() and then use the File Read() method to read into a byte slice.

bs := make([]byte, 10)
f, err := os.Open("/Users/ovidiu/tmp/test.txt")
if err != nil {
  // ...
  return
}
defer f.Close()
for {
  cnt, err := f.Read(bs)
  if err != nil {
    fmt.Println(err)
    f.Close()
    return
  }
  fmt.Println("read ", cnt, " bytes")
}

The file must be explicitly closed after use with Close().

Writing

os.Create()

https://golang.org/pkg/os/#Create

Open the file with os.Create(). It will return a os.File that implements io.Reader, io.Writer and io.Closer interfaces.

Then use the File Write() method write the content of a byte slice, or WriteString() to write strings. The file must be explicitly closed after use with Close():

var err error
f, err := os.Create("/Users/ovidiu/tmp/test3.txt")
if err != nil {
 // ...
}
_, err = f.WriteString("This is the first line\n")
if err != nil {
  // ...
}
_, err = f.WriteString("This is the second line\n")
if err != nil {
  // ...
}
err = f.Close()
if err != nil {
  // ...
}

The file content will be:

This is the first line
This is the second line

What happens if open the file with os.Open() and then I write?

TO DEPLETE

Functions

os.Open()

• https://golang.org/pkg/os/#Open

Works with files and directories.

os.File

• https://golang.org/pkg/os/#File

os.File Methods

• File.Stat()
• File.Read()
• File.Close()
• File.Readdir()

FileInfo

• https://golang.org/pkg/os/#FileInfo

FileInfo Methods

• Size()

os.Args

os.Args is a string slice that holds the command-line arguments, starting with the program name. For more details on how to use see:

Command Line Parsing