How to read a text file? [duplicate]

Question

I'm trying to read "file.txt" and put the contents into a variable using Golang. Here is what I've tried...

package main  import (     "fmt"     "os"     "log" )  func main() {     file, err := os.Open("file.txt")     if err != nil {         log.Fatal(err)     }      fmt.Print(file) } 

The file gets read successfully and the return from os.Open returns a type of *os.File

Answer 1

It depends on what you are trying to do.

file, err := os.Open("file.txt") fmt.print(file) 

The reason it outputs &{0xc082016240}, is because you are printing the pointer value of a file-descriptor (*os.File), not file-content. To obtain file-content, you may READ from a file-descriptor.

---

To read all file content(in bytes) to memory, ioutil.ReadAll

package main  import (     "fmt"     "io/ioutil"     "os"     "log" )  func main() {     file, err := os.Open("file.txt")     if err != nil {         log.Fatal(err)     }     defer func() {         if err = file.Close(); err != nil {             log.Fatal(err)         }     }()     b, err := ioutil.ReadAll(file)   fmt.Print(b) } 

But sometimes, if the file size is big, it might be more memory-efficient to just read in chunks: buffer-size, hence you could use the implementation of io.Reader.Read from *os.File

func main() {     file, err := os.Open("file.txt")     if err != nil {         log.Fatal(err)     }     defer func() {         if err = file.Close(); err != nil {             log.Fatal(err)         }     }()       buf := make([]byte, 32*1024) // define your buffer size here.      for {         n, err := file.Read(buf)          if n > 0 {             fmt.Print(buf[:n]) // your read buffer.         }          if err == io.EOF {             break         }         if err != nil {             log.Printf("read %d bytes: %v", n, err)             break         }     }  } 

Otherwise, you could also use the standard util package: bufio, try Scanner. A Scanner reads your file in tokens: separator.

By default, scanner advances the token by newline (of course you can customise how scanner should tokenise your file, learn from here the bufio test).

package main  import (     "fmt"     "os"     "log"     "bufio" )  func main() {     file, err := os.Open("file.txt")     if err != nil {         log.Fatal(err)     }     defer func() {         if err = file.Close(); err != nil {             log.Fatal(err)         }     }()      scanner := bufio.NewScanner(file)      for scanner.Scan() {             // internally, it advances token based on sperator         fmt.Println(scanner.Text())  // token in unicode-char         fmt.Println(scanner.Bytes()) // token in bytes      } } 

Lastly, I would also like to reference you to this awesome site: go-lang file cheatsheet. It encompassed pretty much everything related to working with files in go-lang, hope you'll find it useful.