Check if there is something to read on STDIN in Golang

Question

I need a command line utility to behave different if some string is piped into its STDIN. Here's some minimal example:

package main // file test.go

import (
    "fmt"
    "io/ioutil"
    "os"
)

func main() {
    bytes, _ := ioutil.ReadAll(os.Stdin)

    if len(bytes) > 0 {
        fmt.Println("Something on STDIN: " + string(bytes))
    } else {
        fmt.Println("Nothing on STDIN")
    }
}

This works fine if you call it like that:

echo foo | go run test.go

If test.go is called without anything on STDIN, the thing stucks at...

bytes, _ := ioutil.ReadAll(os.Stdin)

... waiting for EOF.

What do I need to do to get this going?

Thanks in advance!

Answer 1

I solved this by using os.ModeCharDevice:

stat, _ := os.Stdin.Stat()
if (stat.Mode() & os.ModeCharDevice) == 0 {
    fmt.Println("data is being piped to stdin")
} else {
    fmt.Println("stdin is from a terminal")
}

Answer 2

Use the IsTerminal function from code.google.com/p/go.crypto/ssh/terminal (which was exp/terminal) or the Isatty function from github.com/andrew-d/go-termutil which is a much more focussed package.

If stdin is a terminal/tty then you aren't being piped stuff and you can do something different.

Here is an example

package main

import (
    "fmt"
    termutil "github.com/andrew-d/go-termutil"
    "io"
    "os"
)

func main() {
    if termutil.Isatty(os.Stdin.Fd()) {
        fmt.Println("Nothing on STDIN")
    } else {
        fmt.Println("Something on STDIN")
        io.Copy(os.Stdout, os.Stdin)
    }
}

Testing

$ ./isatty 
Nothing on STDIN
$ echo "hello" | ./isatty 
Something on STDIN
hello
$ (sleep 1 ; echo "hello") | ./isatty 
Something on STDIN
hello

Answer 3

If none of the above works for you, try this way:

stat, err := os.Stdin.Stat()
if err != nil {
    return nil, fmt.Errorf("you have an error in stdin:%s", err)
}
if (stat.Mode() & os.ModeNamedPipe) == 0 {
    return nil, errors.New("you should pass smth to stdin")
}

It worked for me in both darwin (Mac OS) and linux (Ubuntu).