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.
// 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)

    // Text returns the current token, here the next line, from the input.
    for scanner.Scan() {

        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)
    }
}

To try out our line filter, first make a file with a few lowercase lines.

Then use the line filter to get uppercase lines.

$ echo 'hello'   > /tmp/lines
$ echo 'filter' >> /tmp/lines

$ cat /tmp/lines | go run line-filters.go
HELLO
FILTER