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" "io" "os" "strings" )
	func main() {
Wrapping the unbuffered `os.Stdin` with a buffered reader gives us a convenient `ReadString` method that we’ll use to read input line-by-line.	rdr := bufio.NewReader(os.Stdin) out := os.Stdout
`ReadString` returns the next string from the input up to the given separator byte. We give the newline byte `'\n'` as our separator so we’ll get successive input lines.	for { switch line, err := rdr.ReadString('\n'); err {
If the read succeeded (the read `err` is nil), write out out the uppercased line. Check for an error on this write as we do on the read.	case nil: ucl := strings.ToUpper(line) if _, err = out.WriteString(ucl); err != nil { fmt.Fprintln(os.Stderr, "error:", err) os.Exit(1) }
The `EOF` error is expected when we reach the end of input, so exit gracefully in that case.	case io.EOF: os.Exit(0)
Otherwise there’s a problem; print the error and exit with non-zero status.	default: 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