Go by Example: Reading Files

Reading and writing files are basic tasks needed for many Go programs. First we’ll look at some examples of reading files.
	package main
	import ( "bufio" "fmt" "io" "io/ioutil" "os" )
Reading files requires checking most calls for errors. This helper will streamline our error checks below.	func check(e error) { if e != nil { panic(e) } }
	func main() {
Perhaps the most basic file reading task is slurping a file’s entire contents into memory.	dat, err := ioutil.ReadFile("/tmp/dat") check(err) fmt.Print(string(dat))
You’ll often want more control over how and what parts of a file are read. For these tasks, start by `Open`ing a file to obtain an `os.File` value.	f, err := os.Open("/tmp/dat") check(err)
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) check(err) fmt.Printf("%d bytes: %s\n", n1, string(b1))
You can also `Seek` to a known location in the file and `Read` from there.	o2, err := f.Seek(6, 0) check(err) b2 := make([]byte, 2) n2, err := f.Read(b2) check(err) 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) check(err) b3 := make([]byte, 2) n3, err := io.ReadAtLeast(f, b3, 2) check(err) 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) check(err)
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) check(err) fmt.Printf("5 bytes: %s\n", string(b4))
Close the file when you’re done (usually this would be scheduled immediately after `Open`ing with `defer`).	f.Close()
	}

	$ echo "hello" > /tmp/dat $ echo "go" >> /tmp/dat $ go run reading-files.go hello go 5 bytes: hello 2 bytes @ 6: go 2 bytes @ 6: go 5 bytes: hello
Next we’ll look at writing files.

Previous example: Base64 Encoding.

Next example: Writing Files.