Basic Logging in Go

PublishedDecember 9, 2018

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Srinivasan Rangarajan

Software Engineer with an entrepreneurship gene. Love talking about code, products, UI/UX and how to build startups.

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Pro Golang Dev

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Basic Logging

Lets see a quick example of how to log when you get an error.

package main import ( "errors" "fmt" "log" ) func div(a, b float64) (ret float64, err error) { if b == 0 { return 0.0, errors.New("can't divide by 0") } return a / b, nil } func main() { ret, err := div(1.0, 0.0) if err != nil { log.Fatal(err) } fmt.Println(ret) }

We have a simple function which divides a number by another. But if the second number is 0, we return an error value saying, we can't divide by 0.

In the main function, we check for this error and use the log.Fatal(err) function call to display the error and exit out. log.Fatal always calls os.Exit(1) after printing the error message.

Running this program prints this error message:

2018/12/09 22:48:51 can't divide by 0

Logging to a file

By default the standard logger, logs to standard error. But if you want to log to a different file instead, you would have to use the log.SetOutput(f) function. SetOutput takes in any value which satisfies the io.Writer Interface. Lets see a simple example which logs to a local file.

package main import ( "log" "os" ) func main() { f, err := os.OpenFile("app.log", os.O_WRONLY|os.O_CREATE|os.O_APPEND, 0644) if err != nil { log.Fatal(err) } defer f.Close() log.SetOutput(f) log.Println("Application started") // Do the rest }

We open a file called app.log in write only, append mode and create a new file if it doesn't exist. Then we use log.SetOutput to set this as the logging output file. After this, whenever we use log.Print, log.Fatal, log.Panic functions, it will push them to the app.log file.

This is a quick and easy way to setup logging in your go code. Takes the same number of characters as using a fmt.Println, but with the log package, you get much more control over where and how the output is stored.

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Basic Logging

Logging to a file