Command line log viewer with a web UI

Leño is a JSON lines log viewer with a web UI. Think of it as local Kibana/Sumo Logic for development. Works great with Pino logging library, any other app that logs into JSON, or nginx/nginx-ingress access logs.

Installation

Download the latest binary for your platform from the releases page and place it somewhere on your $PATH.

Or build from source (requires Go 1.21+ and Node.js for the Svelte build step):

git clone https://github.com/suda/leno
cd leno
npm install
make build

This produces a single ./leno binary with the web UI embedded — no runtime dependencies.

Usage

Pipe your app's output to leno:

$ ./myapp | leno
Leno running on http://localhost:3000

Now open http://localhost:3000 to see logs stream in real time.

Custom port

$ LENO_PORT=8080 ./myapp | leno
Leno running on http://localhost:8080

Example with Pino

$ node server.js | leno

Nginx / nginx-ingress access logs

Use --log-format=nginx to parse nginx access logs (both the standard combined log format and the extended nginx-ingress format). Each line is converted to a JSON object before display.

# Local nginx
$ tail -f /var/log/nginx/access.log | leno --log-format=nginx

# Kubernetes nginx-ingress
$ kubectl logs -n nginx -l app.kubernetes.io/name=ingress-nginx \
    --all-containers=true -f | leno --log-format=nginx

The following fields are extracted:

Field	Type	Description
remote_addr	string	Client IP address
remote_user	string	Authenticated user (- if none)
time	string	Request time in RFC 3339 UTC
method	string	HTTP method
path	string	Request path
http_version	string	HTTP version
status	number	HTTP response status code
body_bytes	number	Bytes sent to client
http_referer	string	Referer header
http_user_agent	string	User-Agent header
request_length	number	(nginx-ingress) Request size in bytes
request_time	number	(nginx-ingress) Request processing time in seconds
upstream_name	string	(nginx-ingress) Upstream service name
upstream_addr	string	(nginx-ingress) Upstream pod address
upstream_response_time	string	(nginx-ingress) Upstream response time(s)
upstream_status	number	(nginx-ingress) Upstream response status code
request_id	string	(nginx-ingress) Unique request ID

Lines that don't match either format are passed through as-is, so mixed log streams (e.g. nginx error lines alongside access lines) are handled gracefully.

Generate fake logs for testing

$ ./scripts/generate-fake-logs.sh | leno

Building from source

Requirements: Go 1.21+, Node.js (build only)

# Install JS build dependencies
npm install

# Build Svelte app and compile Go binary
make build        # produces ./leno

# Development (Svelte hot-reload)
make dev          # runs rollup in watch mode; run ./leno separately

# Clean build artifacts
make clean

Credits

Leño's logo is based on log by Smalllike from the Noun Project.