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

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1 Answer

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.

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