How Can I Detect if Input is Piped to STDIN in Go?

Detecting Input on STDIN in Golang

When developing command-line utilities, it's often necessary to distinguish between input being piped from another program or entered interactively. This article addresses how to check if data is present on STDIN in Go.

Example

Consider the following code:

package main

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")
    }
}

When invoked with piped input (e.g., echo foo | go run test.go), this code correctly identifies the input. However, if called interactively (without piped input), the program indefinitely waits at ioutil.ReadAll(os.Stdin).

Solution

To differentiate between piped and non-piped input, utilize os.ModeCharDevice. This mode identifies whether the input is from a terminal or not. If it's not from a terminal, it's likely piped.

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")
}

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