Golang : Not able to grep log.Println() output


You have a Golang program that uses log.Println() to capture text output generated by your program and you want to pipe the output to grep command. Mysteriously, it is not working as expected. What's going on?

Solution and notes:

The underlying code behind log.Println() uses fmt.Sprintln() (see https://golang.org/src/log/log.go?s=8792:8822#L284) while fmt.Println() uses Fprintln() function. (see https://golang.org/src/fmt/print.go?s=7388:7437#L246)

The difference is that Fprintln() writes to io.Writer, while Sprintln() does not.

io.Writer will pipe text output to grep command via the Unix/Linux's |.

It is ok to use log.Println() if you expect the log messages to be short or one line. Do not use log.Println() to capture text output if it is going to be long - such as HTML.

Run the code example below and try to grep the lines with page generated by log.Println() versus fmt.Println(). You will see the differences immediately.

 package main

 import (
 // "log"

 func main() {

 // http.Get() can handle gzipped data response
 // automagically

 resp, err := http.Get("https://golang.org")

 if err != nil {

 defer resp.Body.Close()

 htmlData, err := ioutil.ReadAll(resp.Body)

 if err != nil {

 //log.Println(string(htmlData)) //<-- grep will not work

 fmt.Println(string(htmlData)) //<-- grep will work


Output with fmt.Println():

./grepalog | grep "page"

<div id="page">

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</div><!-- #page -->

By Adam Ng

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