Random ports
- Docker & Kubernetes course
- Beginner
- Lesson 46/82
- 4 min
Learn to assign random ports with -P.
Random port
If no host port is written (-p 80), Docker picks a free high port. -P (uppercase) publishes all EXPOSEd ports in the image to random ports:
$ docker run -d --name w1 -p 80 nginx
$ docker port w1
80/tcp -> 0.0.0.0:32768
$ docker run -d --name w2 -P nginx
$ docker port w2
80/tcp -> 0.0.0.0:32769When is it used?
Random ports are handy for opening many copies of the same service without conflicts (tests, CI). The chosen port is then read with a script:
docker run -d --name test-db -P postgres:16
PORT=$(docker port test-db 5432 | cut -d: -f2)
echo "Bağlantı: localhost:$PORT"Practice Test database
Write two commands: start a container named test-db from postgres:16 in the background with POSTGRES_PASSWORD=test, publishing all EXPOSEd ports randomly; then show which host port 5432 is mapped to.
Starter code
# buraya yazSample solution
docker run -d --name test-db -e POSTGRES_PASSWORD=test -P postgres:16
docker port test-db 5432Expected output
7a8b9c0d1e2f 0.0.0.0:32770
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Which ports does the -P flag publish?
- Only 80
- None
- All host ports
- All ports declared with EXPOSE in the image Correct answer
Explanation: This is a concrete effect of EXPOSE.
2 Publish all EXPOSEd ports randomly.
Question
docker run -d ____ nginx
Answer
docker run -d -P nginxExplanation: Uppercase P means "publish all".
3 Match.
| Term | Meaning |
|---|---|
-p 80 |
Random host port |
-P |
All EXPOSE ports |
32768 |
Chosen high port |
docker port |
Find the chosen one |