Stdin Line Filter

A line filter is a common type of program that reads input on stdin, processes it, and then prints some derived result to stdout. grep and sed are common line filters.

Examples Filter
Here’s an example line filter in Go that writes a capitalized version of all input text. You can use this pattern to write your own Go line filters.
package main

import (
    "bufio"
    "fmt"
    "os"
    "strings"
)

func main() {

    // Wrapping the unbuffered os.Stdin with a buffered scanner gives us a convenient
    // Scan method that advances the scanner to the next token;
    // which is the next line in the default scanner.
    scanner := bufio.NewScanner(os.Stdin)

    for scanner.Scan() {

        ucl := strings.ToUpper(scanner.Text())
        // Write out the uppercased line.
        fmt.Println(ucl)
    }

    if err := scanner.Err(); err != nil {
        fmt.Fprintln(os.Stderr, "error:", err)
        os.Exit(1)
    }
}