Go by Example: 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.	$ echo 'hello' > /tmp/lines $ echo 'filter' >> /tmp/lines
Then use the line filter to get uppercase lines.	$ cat /tmp/lines \| go run line-filters.go HELLO FILTER

Previous example: File Name Parsing.