hslog

A thin, non-hiding wrapper around Go's log/slog.

hslog.New hands you a plain *slog.Logger; hslog.NewHandler hands you a plain slog.Handler. Everything you can do with slog still works — this package only adds a bit of convenient pre-configuration you'd otherwise copy between projects.

See the documentation.

What it adds

• A --log-format flag value (hslog.Format, implements flag.Value) accepting:

  value	meaning
  auto	detect at runtime (see below)
  [color-]jsonl	slog-native JSONL (colour variant tints level/msg)
  [color-]journal	<severity> prefixed, systemd-journal friendly
  [color-]syslog	<priority> prefixed, RFC 5424 style
  [color-]plain	human-readable logfmt-ish

• Auto detection for auto:

  • journal if started by systemd and our output is the journal (JOURNAL_STREAM matches the writer's device/inode);
  • syslog if the writer is a socket (e.g. inherited /dev/log);
  • plain otherwise — colourised when the writer is a colour-capable terminal (NO_COLOR and TERM=dumb are honoured).

• Default level colours, dimmed for DEBUG up to bold red for ERROR and any application level above it (e.g. a CRIT at slog.LevelError+4).

Nothing is hidden

hslog.Options embeds slog.HandlerOptions, so Level, AddSource and ReplaceAttr behave exactly as in the standard library. Plain jsonl output is produced by the stdlib slog.JSONHandler; color-jsonl only injects ANSI around the level and msg values (strip the escapes and it parses again — but prefer plain jsonl for machines).

Usage

package main

import (
	"flag"
	"log/slog"
	"os"

	"go.schlittermann.de/heiko/hslog"
)

func main() {
	format := hslog.AutoFormat()
	flag.Var(&format, "log-format", hslog.FlagUsage)
	flag.Parse()

	log := hslog.New(os.Stderr, format, &hslog.Options{
		HandlerOptions: slog.HandlerOptions{Level: slog.LevelDebug},
	})
	slog.SetDefault(log)

	slog.Info("service started", "port", 8080)
}

Custom levels and colours:

const LevelCrit = slog.LevelError + 4

pal := hslog.DefaultPalette() // tweak pal.LevelColor / pal.Key / pal.Message
log := hslog.New(os.Stderr, hslog.FormatColorPlain, &hslog.Options{
	Palette: &pal,
})
log.Log(context.Background(), LevelCrit, "meltdown", "reactor", 4)

License

Apache-2.0. See LICENSE.