# os

type FileMode uint32

const (
    // The single letters are the abbreviations
    // used by the String method's formatting.
    ModeDir        FileMode = 1 << (32 - 1 - iota) // d: is a directory
    ModeAppend                                     // a: append-only
    ModeExclusive                                  // l: exclusive use
    ModeTemporary                                  // T: temporary file (not backed up)
    ModeSymlink                                    // L: symbolic link
    ModeDevice                                     // D: device file
    ModeNamedPipe                                  // p: named pipe (FIFO)
    ModeSocket                                     // S: Unix domain socket
    ModeSetuid                                     // u: setuid
    ModeSetgid                                     // g: setgid
    ModeCharDevice                                 // c: Unix character device, when ModeDevice is set
    ModeSticky                                     // t: sticky

    // Mask for the type bits. For regular files, none will be set.
    ModeType = ModeDir | ModeSymlink | ModeNamedPipe | ModeSocket | ModeDevice

    ModePerm FileMode = 0777 // Unix permission bits
)

func (m FileMode) IsDir() bool {
    return m&ModeDir != 0
}

func (m FileMode) IsRegular() bool {
    return m&ModeType == 0
}

func (m FileMode) Perm() FileMode {
    return m & ModePerm
}

fmt.Println(os.ModeDir.String())         // d---------
fmt.Println(os.ModePerm.String())        // -rwxrwxrwx
fmt.Println(os.ModeDir | os.ModeSymlink) // dL---------

key := "hello_go"
os.Setenv(key, "hello-go")
fmt.Println(os.Getenv(key))                           // hello-go
fmt.Println(os.LookupEnv(key))                        // hello-go true
fmt.Println(os.ExpandEnv("the value is ${hello_go}")) // the value is hello-go
fmt.Println(os.ExpandEnv("the value is $hello_go"))   // the value is hello-go

func ExpandEnv(s string) string {
    return Expand(s, Getenv)
}

func Expand(s string, mapping func(string) string) string

func mapping(s string) string {
    if s == "a" {
        return "b"
    } else {
        return "a"
    }
}

func main() {
    fmt.Println(os.Expand("${a} ${b}", mapping)) // b a
}

var (
    ErrInvalid    = errors.New("invalid argument") // methods on File will return this error when the receiver is nil
    ErrPermission = errors.New("permission denied")
    ErrExist      = errors.New("file already exists")
    ErrNotExist   = errors.New("file does not exist")
)

type PathError struct {
    Op   string
    Path string
    Err  error
}

type LinkError struct {
    Op  string
    Old string
    New string
    Err error
}

type SyscallError struct {
    Syscall string
    Err     error
}

fmt.Println(os.IsExist(os.ErrExist))           // true
fmt.Println(os.IsNotExist(os.ErrNotExist))     // true
fmt.Println(os.IsPermission(os.ErrPermission)) // true

// Flags to OpenFile wrapping those of the underlying system. Not all
// flags may be implemented on a given system.
const (
    O_RDONLY int = syscall.O_RDONLY // open the file read-only.
    O_WRONLY int = syscall.O_WRONLY // open the file write-only.
    O_RDWR   int = syscall.O_RDWR   // open the file read-write.
    O_APPEND int = syscall.O_APPEND // append data to the file when writing.
    O_CREATE int = syscall.O_CREAT  // create a new file if none exists.
    O_EXCL   int = syscall.O_EXCL   // used with O_CREATE, file must not exist
    O_SYNC   int = syscall.O_SYNC   // open for synchronous I/O.
    O_TRUNC  int = syscall.O_TRUNC  // if possible, truncate file when opened.
)

// 以读写模式来创建新文件
// 如果文件已经存在，则会清空原有文件
func Create(name string) (*File, error) {
    return OpenFile(name, O_RDWR|O_CREATE|O_TRUNC, 0666)
}

// 以只读模式打开文件
// 如果文件不存在，会报错
func Open(name string) (*File, error) {
    return OpenFile(name, O_RDONLY, 0)
}