log package - log - Go Packages

Package log implements a simple logging package. It defines a type, Logger, with methods for formatting output. It also has a predefined 'standard' Logger accessible through helper functions Print[f|ln], Fatal[f|ln], and Panic[f|ln], which are easier to use than creating a Logger manually. That logger writes to standard error and prints the date and time of each logged message. Every log message is output on a separate line: if the message being printed does not end in a newline, the logger will add one. The Fatal functions call os.Exit(1) after writing the log message. The Panic functions call panic after writing the log message.

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const (
	Ldate         = 1 << iota     
	Ltime                         
	Lmicroseconds                 
	Llongfile                     
	Lshortfile                    
	LUTC                          
	Lmsgprefix                    
	LstdFlags     = Ldate | Ltime 
)

These flags define which text to prefix to each log entry generated by the Logger. Bits are or'ed together to control what's printed. With the exception of the Lmsgprefix flag, there is no control over the order they appear (the order listed here) or the format they present (as described in the comments). The prefix is followed by a colon only when Llongfile or Lshortfile is specified. For example, flags Ldate | Ltime (or LstdFlags) produce,

2009/01/23 01:23:23 message

while flags Ldate | Ltime | Lmicroseconds | Llongfile produce,

2009/01/23 01:23:23.123123 /a/b/c/d.go:23: message

This section is empty.

Fatal is equivalent to Print followed by a call to os.Exit(1).

Fatalf is equivalent to Printf followed by a call to os.Exit(1).

Fatalln is equivalent to Println followed by a call to os.Exit(1).

Flags returns the output flags for the standard logger. The flag bits are Ldate, Ltime, and so on.

Output writes the output for a logging event. The string s contains the text to print after the prefix specified by the flags of the Logger. A newline is appended if the last character of s is not already a newline. Calldepth is the count of the number of frames to skip when computing the file name and line number if Llongfile or Lshortfile is set; a value of 1 will print the details for the caller of Output.

Panic is equivalent to Print followed by a call to panic().

Panicf is equivalent to Printf followed by a call to panic().

Panicln is equivalent to Println followed by a call to panic().

Prefix returns the output prefix for the standard logger.

Print calls Output to print to the standard logger. Arguments are handled in the manner of fmt.Print.

Printf calls Output to print to the standard logger. Arguments are handled in the manner of fmt.Printf.

Println calls Output to print to the standard logger. Arguments are handled in the manner of fmt.Println.

SetFlags sets the output flags for the standard logger. The flag bits are Ldate, Ltime, and so on.

SetOutput sets the output destination for the standard logger.

SetPrefix sets the output prefix for the standard logger.

Writer returns the output destination for the standard logger.

A Logger represents an active logging object that generates lines of output to an io.Writer. Each logging operation makes a single call to the Writer's Write method. A Logger can be used simultaneously from multiple goroutines; it guarantees to serialize access to the Writer.

package main

import (
	"bytes"
	"fmt"
	"log"
)

func main() {
	var (
		buf    bytes.Buffer
		logger = log.New(&buf, "logger: ", log.Lshortfile)
	)

	logger.Print("Hello, log file!")

	fmt.Print(&buf)
}

Output:
logger: example_test.go:19: Hello, log file!

Default returns the standard logger used by the package-level output functions.

New creates a new Logger. The out variable sets the destination to which log data will be written. The prefix appears at the beginning of each generated log line, or after the log header if the Lmsgprefix flag is provided. The flag argument defines the logging properties.

func (l *Logger) Fatal(v ...any)

Fatal is equivalent to l.Print() followed by a call to os.Exit(1).

Fatalf is equivalent to l.Printf() followed by a call to os.Exit(1).

func (l *Logger) Fatalln(v ...any)

Fatalln is equivalent to l.Println() followed by a call to os.Exit(1).

func (l *Logger) Flags() int

Flags returns the output flags for the logger. The flag bits are Ldate, Ltime, and so on.

Output writes the output for a logging event. The string s contains the text to print after the prefix specified by the flags of the Logger. A newline is appended if the last character of s is not already a newline. Calldepth is used to recover the PC and is provided for generality, although at the moment on all pre-defined paths it will be 2.

package main

import (
	"bytes"
	"fmt"
	"log"
)

func main() {
	var (
		buf    bytes.Buffer
		logger = log.New(&buf, "INFO: ", log.Lshortfile)

		infof = func(info string) {
			logger.Output(2, info)
		}
	)

	infof("Hello world")

	fmt.Print(&buf)
}

Output:
INFO: example_test.go:36: Hello world

func (l *Logger) Panic(v ...any)

Panic is equivalent to l.Print() followed by a call to panic().

Panicf is equivalent to l.Printf() followed by a call to panic().

func (l *Logger) Panicln(v ...any)

Panicln is equivalent to l.Println() followed by a call to panic().

Prefix returns the output prefix for the logger.

func (l *Logger) Print(v ...any)

Print calls l.Output to print to the logger. Arguments are handled in the manner of fmt.Print.

Printf calls l.Output to print to the logger. Arguments are handled in the manner of fmt.Printf.

func (l *Logger) Println(v ...any)

Println calls l.Output to print to the logger. Arguments are handled in the manner of fmt.Println.

func (l *Logger) SetFlags(flag int)

SetFlags sets the output flags for the logger. The flag bits are Ldate, Ltime, and so on.

SetOutput sets the output destination for the logger.

func (l *Logger) SetPrefix(prefix string)

SetPrefix sets the output prefix for the logger.

Writer returns the output destination for the logger.