os.Stdin and os.Stdout

Learn the standard input and output streams.

os.Stdin and os.Stdout

Every process starts with three standard streams: os.Stdin (input), os.Stdout (normal output) and os.Stderr (errors and diagnostics). All are *os.File, so they are Readers and Writers.

fmt.Fprintln(os.Stdout, "sonuç: 42")
fmt.Fprintln(os.Stderr, "uyarı: önbellek boş")

Writing errors to stderr keeps the output clean when it is redirected with ./app > cikti.txt.

Exit codes

Command-line tools report success with the exit code: 0 means success, anything else an error. os.Exit(1) exits immediately without running defers. That's why the common pattern puts the work in a run() error function and only evaluates the result in main:

func main() {
    if err := run(os.Stdin, os.Stdout); err != nil {
        fmt.Fprintln(os.Stderr, err)
        os.Exit(1)
    }
}

Exercise The run pattern

Write run(in io.Reader, out io.Writer) error: read all the input with io.ReadAll, convert it to upper case and write it to out. In main, call it with strings.NewReader("go") and os.Stdout.

Show example solution

package main

import (
    "fmt"
    "io"
    "os"
    "strings"
)

func run(in io.Reader, out io.Writer) error {
    b, err := io.ReadAll(in)
    if err != nil {
        return err
    }
    _, err = fmt.Fprintln(out, strings.ToUpper(string(b)))
    return err
}

func main() {
    if err := run(strings.NewReader("go"), os.Stdout); err != nil {
        fmt.Fprintln(os.Stderr, err)
        os.Exit(1)
    }
}

Test yourself

In <code>./app | grep hata</code>, which stream does grep see?
- Only stdout
- Only stderr
- Both
- Neither
Show answer

Only stdout — A pipe (|) connects only stdout; to pass stderr too, 2>&1 is added.

Error stream

fmt.Fprintln(os.____, "dosya bulunamadı")

Show answer

fmt.Fprintln(os.Stderr, "dosya bulunamadı")

Stream and use

os.Stdin

Input
os.Stdout

Result output
os.Stderr

Errors and warnings
os.Exit(1)

Failing exit

Next lesson: bufio.Scanner

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