Line Filters

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. Here's an example line filter in Go that writes a capitalized version of all input text.

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() {

Text returns the current token, here the next line, from the input.

		ucl := strings.ToUpper(scanner.Text())

Write out the uppercased line.

		fmt.Println(ucl)
	}

Check for errors during Scan. End of file is expected and not reported by Scan as an error.

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

$ printf 'hello\nfilter\n' | go run ./72_line_filters
HELLO
FILTER

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