Golang : Not able to grep log.Println() output
Problem:
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 (
"fmt"
"io/ioutil"
// "log"
"net/http"
"os"
)
func main() {
// http.Get() can handle gzipped data response
// automagically
resp, err := http.Get("https://golang.org")
if err != nil {
fmt.Println(err)
os.Exit(1)
}
defer resp.Body.Close()
htmlData, err := ioutil.ReadAll(resp.Body)
if err != nil {
fmt.Println(err)
os.Exit(1)
}
//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">
the content of this page is licensed under the
</div><!-- #page -->
By Adam Ng
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