Go by Example: Writing Files

Writing files in Go follows similar patterns to the ones we saw earlier for reading.

 
package main
 
import (
    "bufio"
    "fmt"
    "os"
)
 
func check(e error) {
    if e != nil {
        panic(e)
    }
}
 
func main() {

To start, here’s how to dump a string (or just bytes) into a file.

 
    d1 := []byte("hello\ngo\n")
    err := os.WriteFile("/tmp/dat1", d1, 0644)
    check(err)

For more granular writes, open a file for writing.

 
    f, err := os.Create("/tmp/dat2")
    check(err)

It’s idiomatic to defer a Close immediately after opening a file.

 
    defer f.Close()

You can Write byte slices as you’d expect.

 
    d2 := []byte{115, 111, 109, 101, 10}
    n2, err := f.Write(d2)
    check(err)
    fmt.Printf("wrote %d bytes\n", n2)

A WriteString is also available.

 
    n3, err := f.WriteString("writes\n")
    check(err)
    fmt.Printf("wrote %d bytes\n", n3)

Issue a Sync to flush writes to stable storage.

 
    f.Sync()

bufio provides buffered writers in addition to the buffered readers we saw earlier.

 
    w := bufio.NewWriter(f)
    n4, err := w.WriteString("buffered\n")
    check(err)
    fmt.Printf("wrote %d bytes\n", n4)

Use Flush to ensure all buffered operations have been applied to the underlying writer.

 
    w.Flush()
 
}

Try running the file-writing code.

 
$ go run writing-files.go 
wrote 5 bytes
wrote 7 bytes
wrote 9 bytes

Then check the contents of the written files.

 
$ cat /tmp/dat1
hello
go
$ cat /tmp/dat2
some
writes
buffered

Next we’ll look at applying some of the file I/O ideas we’ve just seen to the stdin and stdout streams.

Next example: Line Filters.

by Mark McGranaghan and Eli Bendersky | source | license