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Golang Read File

  • Reading and writing files are basic tasks needed for many Go programs. First we’ll look at some examples of reading files.
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package main

	import (
// Reading files requires checking most calls for errors. This helper will streamline our error checks below.
	func check(e error) {
    if e != nil {

	func main() {
// Perhaps the most basic file reading task is slurping a file’s entire contents into memory.
	    dat, err := ioutil.ReadFile("/tmp/dat")
// You’ll often want more control over how and what parts of a file are read. For these tasks, start by Opening a file to obtain an os.File value.
	    f, err := os.Open("/tmp/dat")
// Read some bytes from the beginning of the file. Allow up to 5 to be read but also note how many actually were read.
	    b1 := make([]byte, 5)
    n1, err := f.Read(b1)
    fmt.Printf("%d bytes: %s\n", n1, string(b1))
// You can also Seek to a known location in the file and Readfrom there.
	    o2, err := f.Seek(6, 0)
    b2 := make([]byte, 2)
    n2, err := f.Read(b2)
    fmt.Printf("%d bytes @ %d: %s\n", n2, o2, string(b2))
// The io package provides some functions that may be helpful for file reading. For example, reads like the ones above can be more robustly implemented with ReadAtLeast.
	    o3, err := f.Seek(6, 0)
    b3 := make([]byte, 2)
    n3, err := io.ReadAtLeast(f, b3, 2)
    fmt.Printf("%d bytes @ %d: %s\n", n3, o3, string(b3))

// There is no built-in rewind, but Seek(0, 0) accomplishes this.
	    _, err = f.Seek(0, 0)
// The bufio package implements a buffered reader that may be useful both for its efficiency with many small reads and because of the additional reading methods it provides.
	    r4 := bufio.NewReader(f)
    b4, err := r4.Peek(5)
    fmt.Printf("5 bytes: %s\n", string(b4))
// Close the file when you’re done (usually this would be scheduled immediately after Opening with defer).


	$ echo "hello" > /tmp/dat
$ echo "go" >>   /tmp/dat
$ go run reading-files.go 
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output of the above go program

5 bytes: hello
2 bytes @ 6: go
2 bytes @ 6: go
5 bytes: hello

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