slogan is a logger library for Golang.

Features :

10 Levels from "silent" to "trace", including POSIX levels Configurable format Configurable colors on individual items Easy elapsed time

Programming language: Go
License: MIT License
Tags: Utilities     Logging     Golang    

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slogan is a logger library for Golang.

Features :

  • 10 Levels from "silent" to "trace", including POSIX levels
  • Configurable format
  • Configurable colors on individual items
  • Easy elapsed time


Declare use

import (

An alias (here 'log') can be used by setting it before import :

import (
    log  "github.com/crownedgrouse/slogan"


```go main.go package main

import ( log "github.com/crownedgrouse/slogan" )

func main() {


// Set verbosity
log.Trace(map[string]string{"this is a": "map"})
log.Debug("A debug message")
log.Info("An informative message")
log.Notice("A notification")
log.Warning("A warning")
log.Error("An Error")
log.Critical("A critical message")
log.Alert("An alert")
log.Emergency("An Emergency")


Will produce (color not visible in this example):

$ go build $ ./main debug OS:linux ARCH:386 CPU:4 COMPILER:gc ROOT:/home/eric/git/goroot trace map[string]string %v: map[this is a:map]

%v+: map[this is a:map]

%#v: map[string]string{"this is a":"map"} debug A debug message info An informative message notice A notification warning A warning error An Error debug Immediate exit with code 4 $ echo $? 4

## Utilities ##

### Show Runtime infos ###

Runtime informations can be easily shown as a debug level with a uniq call.

   debug     OS:linux ARCH:386 CPU:4 COMPILER:gc ROOT:/home/eric/git/goroot

Trace Go values

Call to Trace/1 will produce a trace log made of several lines. First line with 'trace' level and type of the value given. Below is written three usual ways to display Go values (%v, %v+ and %#v) separated with an empty line. This format can be overrriden by your own preference (See 'Configuring/Formats' below).


Time elapsed

Display how many time elapsed since program start or since last call to ElapsedTime() .

    // Show time elapsed since last call to ElapsedTime() or since beginning at first call
    // Show time elapsed since beginning. This will reset start time reference

Output will be a notice :

  notice    All done in : 296.538µs


slogan can be configured at beginning of your program (and also at any time inside your program).


Tags can be changed by overwritting tags map, with GetTags/0 and SetTags/1.

var tags = [10]string{
        "",          // Prefix
        "emergency", // 1
        "alert    ", // 2
        "critical ", // 3
        "error    ", // 4
        "warning  ", // 5
        "notice   ", // 6
        "info     ", // 7
        "debug    ", // 8
        "trace    ", // 9


Default output is on STDERR. Output can be set in a file by passing File Descriptor to "slogan".

    f, err := os.OpenFile("/var/log/myown.log", os.O_RDWR|os.O_CREATE, 0755)
    if err != nil {
        log.Critical("Cannot open file")

Color will be disabled if output is not a Terminal unless forcing it.


slogan is using legacy "log" package underneath. SetFlags can be used to change "log" parameters.

For instance to show caller and line number in code :

    slogan.SetFlags(slogan.Lshortfile)  // No need to import "log". Same constants used in "slogan".

Will produce something like below :

   debug     main.go:17     A debug message
   info      main.go:18     An informative message
   notice    main.go:20     A warning
   error     main.go:21     An Error

as well date/time information can be set this way.

Set a prefix to any log :

    log.SetPrefix("===> ")


Considering Warning as Error (and potentialy exit) :


Set or change verbosity from 0 (silent) to 9 (debug) :

slogan.SetVerbosity(0)              // Silent totally logs
slogan.SetVerbosity(slogan.Lsilent) // Same but using "slogan" Levels constant

By setting verbosity, all logs with level lower or equal will be generated (if no immediate exit on error was set and no error occured) :

log.SetExitOnError(true) // Exit if log level reach Error or worst.

If the case, the error message is generated and a debug level may appear, depending current verbosity, indicating that an immediate exit occured, and telling what is the program exit code. The exit code is equal to the level reached by the last fatal error, i.e 1 (emergency) to 4 (error) , or even 5 if warning considered error.


Formats can be configured by settings new "Sprintf" values to the three arguments passed to slogan functions :

  • tag %[1]s
  • log %[2]s
  • caller (path:line) %[3]s (only if caller is required in "log" parameters)
// Get current map
formats := slogan.GetFormats()
// Override format for caller : set caller in first
formats["caller"]= "%-25[3]s %[1]s %[2]s "
slogan.SetFlags(log.Lshortfile) // caller is required to be shown

Default formats are :

var formats = map[string]string{
    "fatal"   : "Immediate exit with code %d",                        // immediate exit on error format
    "trace"   : "%[1]T\n %%v: %[1]v\n\n%%v+: %+[1]v\n\n%%#v: %#[1]v", // multiline trace format
    "empty"   : "%#v",                                                // trace format for empty variable (avoid unuseful multiline)
    "runtime" : "OS:%s ARCH:%s CPU:%d COMPILER:%s ROOT:%s",           // runtime infos format
    "default" : "   %[1]s %[2]s",                                     // default log format
    "caller"  : "   %[1]s %[3]s\t %[2]s",                             // default log format with caller (where)
    "where"   : "%s:%d",                                              // format for caller location path:linenumber
    "alldone" : "All done in : %s",                                   // all done time format
    "elapsed" : "Elapsed time : %s",                                  // elapsed time format


Color will be disabled if output is not a terminal. This can be avoid however by calling SetForceColor/1 .

Colors can be changed by overwritting colors map, with GetColors/0 and SetColors/1.

See here for possible colors and other output (reverse, underlining, etc.)

var colors = map[int]string{
    10: "Underline",    // Caller
    9:  "DarkGray",     // trace
    8:  "DarkGray",     // debug
    7:  "Purple",       // info
    6:  "Green",        // notice
    5:  "Yellow",       // warning
    4:  "LightRed",     // error
    3:  "Red",          // critical
    2:  "BLightRed",    // alert
    1:  "BRed",         // emergency
    0:  "",             // Prefix

As well colorization of elements (called 'parts') in log line can be tuned by changing parts map, with GetParts/0 and SetParts/1

var parts = map[string]bool{
    "caller": true,            // colorize caller (event if it is underlining)
    "tag":    true,            // colorize tag
    "log":    false,           // do not colorize log entry
    "prefix": false,           // do not colorize prefix