Integrating logrus with cobra

Sébastien Le Gall — Fri, 18 Jan 2019 08:29:15 +0000

You can find a working example on my GitHub: https://github.com/seblegall/blog-scripts/tree/master/logrus-cobra

I recently had to refactor the way one of my app print logs. My idea was to use logrus as it is a very well known lib to produce logs with Go.

Logrus let you print nicely color-coded and structured logs and is completely API compatible with the standard library logger.

My first question was about log level and how to setup it once across all sub-directory/package of my project.

Under the hood, logrus instantiate a new variable log. Once done, calling the logrus.SetLevel() function will store the level directly in this newly instantiated struct.

Thus, we have a global struct, available across all sub-package making the level automatically shared without having to use dependency injection.

If you need to set a different level in a sub-package (for example), you just need to instantiate a new logger :

var log = logrus.New()

//...

log.Debug("debug log")

More about logrus here : https://github.com/sirupsen/logrus

Then I thought about adding a flag to my binary, letting the user set its own log level.

My application is using cobra, so I had to decided where to read the flag and where to init the logger by setting the level, depending on the flag value.

My first thought was to create a SetLevel() func which could be called in each cobra sub-command run function. But I found a better approach by looking at the source code of Skaffold

Cobra has a cool feature called pre/post run. It's basically a function that will be executed right before the command start, or right after.

The same way you can defined "persistent" flags in cobra commands (which is a way to create inheritance between commands and sub-commands), the framework also offers a PersistentPreRun function.

That was the perfect place to put my log init as it is :

Defined once for all sub-commands (which is a behaviour you probably want when creating a verbose flag.)
A place where flags are already defined and read. So I could make a good use of the flag value set by the user.

Here is how it goes :

package cmd

import (
    "io"
    "os"

    "github.com/sirupsen/logrus"
    "github.com/spf13/cobra"
)

//The verbose flag value
var v string

var rootCmd = &cobra.Command{
    Use:   "mycmd",
    Short: "mycmd is a test project to integrate logrus",
}

//NewMyCmd return the root cobra command
func NewMyCmd() *cobra.Command {
    //Here is where we define the PreRun func, using the verbose flag value
    //We use the standard output for logs.
    rootCmd.PersistentPreRunE = func(cmd *cobra.Command, args []string) error {
        if err := setUpLogs(os.Stdout, v); err != nil {
            return err
        }
        return nil
    }

    //Here is where we bind the verbose flag
    //Default value is the warn level
    rootCmd.PersistentFlags().StringVarP(&v, "verbosity", "v", logrus.WarnLevel.String(), "Log level (debug, info, warn, error, fatal, panic")

    return rootCmd
}

//setUpLogs set the log output ans the log level
func setUpLogs(out io.Writer, level string) error {
    logrus.SetOutput(out)
    lvl, err := logrus.ParseLevel(level)
    if err != nil {
        return err
    }
    logrus.SetLevel(lvl)
    return nil
}

Once the PersistentPreRun func defined, you call easily create a sub-command which will use the logger.

var subCmd = &cobra.Command{
    Use:   "subcmd",
    Short: "subcmd is a subcommand of the main cmd",
    Run: func(cmd *cobra.Command, args []string) {
        runSubCmd()
    },
}

//...
//Inside the NewMyCmd() func
rootCmd.AddCommand(NewSubCmd())
//...

//NewSubCmd return the a sub cobra command
func NewSubCmd() *cobra.Command {
    return subCmd
}

func runSubCmd() {
    logrus.Debug("debug log")
}

This will produce :

$ go run main.go subcmd -v debug
DEBU[0000] debug log

By default, logrus is configured on Info level and the default value for my flag is Warn. Son the result proves it works ;-)

You are done !

Forem: Sébastien Le Gall

Integrating logrus with cobra