go - Correct on input but panic on output in golang -

trying write bit of go, create sort of cat function in golang:

package main  import (         "fmt"         "os"         "io/ioutil"         "log" )  func main() {         // part ask question , input         fmt.print("which file read?: ")         var input string         fmt.scanln(&input)         fmt.print(input)          // part give output         f, err := os.open(os.args[1]) // open file                                                if err != nil {                 log.fatalln("my program broken")         }          defer f.close() // close things open                                                    bs, err := ioutil.readall(f)         if err != nil {                 log.fatalln("my program broken")         }          // part print output         fmt.printf("input", bs) // %s convert directly in string result                    } 

but go panic on execution , not find more explicit information.

what have done wrong?

how can more information error terminal?

$ go run gocat.go file read?: gocat.go gocat.gopanic: runtime error: index out of range

goroutine 1 [running]: panic(0x4b1840, 0xc42000a0e0) /usr/lib/go/src/runtime/panic.go:500 +0x1a1 main.main() /home/user/git/go-experimentations/gocat/gocat.go:23 +0x4ba exit status 2

i imagine found examples online , don't quite understand going on.

from os package's documentation.

args hold command-line arguments, starting program name.

since started program without arguments , os.args slice, program panics when access out of slice's bounds. (like trying access element of array not exist)

it looks trying prompt input here

var input string fmt.scanln(&input) fmt.print(input) 

all need replace

f, err := os.open(os.args[1]) 

with:

f, err := os.open(input) 

your last print statement incorrect.

it should be:

// part print output fmt.printf("input \n %s", string(bs)) 

you need cast bs []byte string , need add %s indicate fmt.printf string(bs) should placed in format string