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The Go standard library provides straightforward
tools for outputting logs from Go programs, with
the log package for
free-form output and the
log/slog package for
structured output.
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package main
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import (
"bytes"
"fmt"
"log"
"os"
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"log/slog"
)
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func main() {
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Simply invoking functions like Println from the
log package uses the standard logger, which
is already pre-configured for reasonable logging
output to os.Stderr. Additional methods like
Fatal* or Panic* will exit the program after
logging.
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log.Println("standard logger")
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Loggers can be configured with flags to set
their output format. By default, the standard
logger has the log.Ldate and log.Ltime flags
set, and these are collected in log.LstdFlags.
We can change its flags to emit time with
microsecond accuracy, for example.
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log.SetFlags(log.LstdFlags | log.Lmicroseconds)
log.Println("with micro")
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It also supports emitting the file name and
line from which the log function is called.
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log.SetFlags(log.LstdFlags | log.Lshortfile)
log.Println("with file/line")
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It may be useful to create a custom logger and
pass it around. When creating a new logger, we
can set a prefix to distinguish its output
from other loggers.
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mylog := log.New(os.Stdout, "my:", log.LstdFlags)
mylog.Println("from mylog")
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We can set the prefix
on existing loggers (including the standard one)
with the SetPrefix method.
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mylog.SetPrefix("ohmy:")
mylog.Println("from mylog")
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Loggers can have custom output targets;
any io.Writer works.
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var buf bytes.Buffer
buflog := log.New(&buf, "buf:", log.LstdFlags)
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This call writes the log output into buf.
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buflog.Println("hello")
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This will actually show it on standard output.
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fmt.Print("from buflog:", buf.String())
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The slog package provides
structured log output. For example, logging
in JSON format is straightforward.
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jsonHandler := slog.NewJSONHandler(os.Stderr, nil)
myslog := slog.New(jsonHandler)
myslog.Info("hi there")
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In addition to the message, slog output can
contain an arbitrary number of key=value
pairs.
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myslog.Info("hello again", "key", "val", "age", 25)
}
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Sample output; the date and time
emitted will depend on when the example ran.
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$ go run logging.go
2023/08/22 10:45:16 standard logger
2023/08/22 10:45:16.904141 with micro
2023/08/22 10:45:16 logging.go:40: with file/line
my:2023/08/22 10:45:16 from mylog
ohmy:2023/08/22 10:45:16 from mylog
from buflog:buf:2023/08/22 10:45:16 hello
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These are wrapped for clarity of presentation
on the website; in reality they are emitted
on a single line.
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{"time":"2023-08-22T10:45:16.904166391-07:00",
"level":"INFO","msg":"hi there"}
{"time":"2023-08-22T10:45:16.904178985-07:00",
"level":"INFO","msg":"hello again",
"key":"val","age":25}
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