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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.
|
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.
|
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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