os.Stdin and os.Stdout
- Go course
- Intermediate
- Lesson 37/80
- 4 min
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)
}
}Practice 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.
Starter code
package main
import (
"fmt"
"io"
"os"
"strings"
)
func main() {
// buraya yaz
}Sample 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)
}
}Expected output
GO
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 In <code>./app | grep hata</code>, which stream does grep see?
- Only stdout Correct answer
- Only stderr
- Both
- Neither
Explanation: A pipe (|) connects only stdout; to pass stderr too, 2>&1 is added.
2 Error stream
fmt.Fprintln(os.____, "dosya bulunamadı")fmt.Fprintln(os.Stderr, "dosya bulunamadı")Explanation: Error messages are written to os.Stderr, keeping them separate from normal output.
3 Stream and use
| Term | Meaning |
|---|---|
os.Stdin |
Input |
os.Stdout |
Result output |
os.Stderr |
Errors and warnings |
os.Exit(1) |
Failing exit |