Test basic file operations in go. Copy a file using the package os & io.

test_file_io.go

package main

import (
    "io"
    "os"
)

func main() {
    // Open a file for reading
    // The associated file descriptor has mode O_RDONLY.
    fi, err := os.Open("input.txt")
    if err != nil {
        panic(err)
    }
    defer fi.Close()

    // Create creates the named file mode 0666 (before umask),
    // truncating it if it already exists.
    // The associated file descriptor has mode O_RDWR.
    fo, err := os.Create("output.txt")
    if err != nil {
        panic(err)
    }
    defer fo.Close()

    // Make a slice as buffer.
    buf := make([]byte, 1024)

    for {
        n, err := fi.Read(buf)
        if err != nil && err != io.EOF {
            panic(err)
        }

        if n == 0 {  // EOF
            break
        }

        if n2, err := fo.Write(buf[:n]); err != nil {
            panic(err)
        } else if n2 != n {
            panic("error in writing")
        }
    }
}

test_io_copy.go

package main

import (
    "io"
    "os"
    "log"
)

func main () {
    // open input stream
    r, err := os.Open("input.txt")
    if err != nil {
        panic(err)
    }
    defer r.Close()

    // open output stream
    w, err := os.Create("output.txt")
    if err != nil {
        panic(err)
    }
    defer w.Close()

    // func Copy(dst Writer, src Reader) (written int64, err error)
    // Copy copies from src to dst until either EOF is reached on
    // src or an error occurs.
    n, err := io.Copy(w, r)
    if err != nil {
        panic(err)
    }

    log.Printf("Copied %v bytes\n", n)
}

test_ioutil.go

package main

import (
    "io/ioutil"
)

func main() {
    b, err := ioutil.ReadFile("input.txt")
    if err != nil { panic(err) }

    err = ioutil.WriteFile("output.txt", b, 0644)
    if err != nil { panic(err) }
}