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cheatsheet

Check files or folders size (human-readable sizes)

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show size of all items in current directory (sort by size)
du -sh * | sort -hr
ls -lah

Find which process is using a specific port 8080:

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lsof -i :8080

lists processes listening on TCP port 8082, showing clean output

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lsof -nP -iTCP:8080 -sTCP:LISTEN
Option What It Does
lsof Lists open files (including network sockets)
-n Don’t resolve IPs to hostnames (faster, no DNS delay)
-P Don’t convert port numbers to names (e.g. show 8082 instead of us-cli)
-iTCP:8082 Filter: only show TCP connections on port 8082
-sTCP:LISTEN Filter: only show listening sockets (not established connections)

detailed information about the process with PID ( using wide output (no truncation).)

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ps -p 123 -ww

-ww: Show full command line without cutting off arguments (double w means no truncation at all)

lists all running Java processes with their PID and main class or jar file.

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jps -l

shows which IP/port the Java process(pid=12345) is listening on

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lsof -nP -p 12345 -iTCP -sTCP:LISTEN

List all open network ports (Java or not)

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lsof -nP -iTCP -sTCP:LISTEN
or 
netstat -anp tcp

show both TCP and/or UDP

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lsof -i
lsof -iTCP
lsof -iUDP

Check UDP Connectivity via nc (netcat): see the message, UDP is open.

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listener: listen on my port 1234
nc -u -l -p 1234

publisher: send msg to listener ip:port
echo "test" | nc -u <listener_public_ip> 1234

storage

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# Check total size of current folder
du -sh .

# See size of each subfolder
du -sh *

# Sort folders by size (reverse -hr)
du -sh * | sort -h


# Check disk free space (overall, not folder)
df -h .

IP

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# Check mac local/private ip
ipconfig getifaddr en0

Check what service/process/task are using the port

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ps aux | grep PID

docker

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# Show running containers
docker ps
docker ps --format "table {{.Names}}\t{{.Status}}\t{{.Image}}"

# Show ALL containers (running + stopped)
docker ps -a

# Check logs (to confirm it’s alive)
docker logs grafana
docker logs -f grafana

# See what ports are exposed
docker port grafana
or
docker ps | grep grafana

Quick external test in the same laptop (local ports exposed?)

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# find your public IP
curl ifconfig.me
-> 1.2.3.4

# Try to open a TCP connection to <PUBLIC_IP> on port 1234 and tell me if it succeeds.
same laptop:
1. Tries to reach its own public IP
2. That traffic goes to the router
Router decides:
  ❌ drop it (no port forward) → SAFE
  ✅ forward it to laptop:1234 → EXPOSED
But important caveat:
Some routers do not support NAT loopback. nc from the same laptop will always fail, Even if the port is actually exposed to the internet

The REAL definitive test:
Use a device NOT on your Wi-Fi
http://1.2.3.4:1234