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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 (very important)
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