Go Package os

External

• https://pkg.go.dev/os

Internal

• File Operations in Go
• Standard library

Overview

Package os provides a platform-independent interface to operating system functionality. The design is Unix-like, although the error handling is Go-like; failing calls return values of type error rather than error numbers. Often, more information is available within the error.

Accessing Environment Variables in Go

To pull the value of an environment variable and also get an indication whether the environment variable is set to "" but is still defined, use:

value, ok := os.LookupEnv("SOME_ENV_VAR")

ok is set to false if the environment variable has not been defined.

Note that:

export SOME_ENV_VAR

does not define a variable, unless a value, including empty string is provided. In this case os.LookupEnv() will return false.

ok is set to true if the environment variable has been defined, even with an empty string value.

export SOME_ENV_VAR=""

makes os.LookupEnv() return "", true.

os.File

https://pkg.go.dev/os#File

File represents an open file descriptor. An existing file can be opened with os.Open(). A new file can be created with os.Create().

os.File.Stat()

To introspect the state of the file, File.Stat() is available. The function the FileInfo structure describing the file. If there is an error, it will be of type *PathError.

f := os.Stdout
fi, err := f.Stat()
if err != nil {
   ...
}
fmt.Printf("Name: %s\n", fi.Name())
fmt.Printf("Size: %d\n", fi.Size())
fmt.Printf("Mode: %v\n", fi.Mode())
fmt.Printf("ModTime: %v\n", fi.ModTime())
fmt.Printf("IsDir: %v\n", fi.IsDir())
fmt.Printf("Sys: %v\n", fi.Sys())

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

os.Open()

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

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

If the file does not exist, it returns a fs.PathError.

Works with files and directories.

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?

Local Filesystem Interaction

os.Getwd()

Return a rooted path name corresponding to the current directory. If the current directory can be reached via multiple paths due to symbolic links, Getwd() may return any one of them.

Checking whether a File Exists

_, err := os.Stat("/path/to/whatever")
if os.IsNotExist(err) {
  // file does not exist
}

Removing (Deleting) a File

https://pkg.go.dev/os#Remove

os.Remove()

TO DEPLETE

Functions

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