DevOps Classroom Series – 02/Feb/2020

Docker Essential Command line

• Executing the commands inside the running container

# Check the contents of root directory in tomcat container

docker container run --name tomcat1 -d tomcat:8

docker container exec tomcat1 ls /

# check the process list
docker container exec tomcat1 ps -eaf

# executing commands in the interactive mode
docker container exec -it tomcat1 /bin/bash

• Starting and stopping containers

docker container stop tomcat1
docker container ls
# tomcat1 shouldn't be in the list
docker container ls -a

# start the container
docker container start tomcat1

• Getting to know the instructions led to the creation of docker image

docker image history <imagename>:<tag>

Dockerfile instructions

• WORKDIR: workdir changes the default directory of the container
  • Create a Dockerfile and build the image with tag demo:0.1

FROM openjdk:8

RUN mkdir /app && cd /app && wget https://war-jar-files.s3-us-west-2.amazonaws.com/spring-petclinic-2.2.0.BUILD-SNAPSHOT.jar
WORKDIR /app
EXPOSE 8080
CMD ["java", "-jar", "spring-petclinic-2.2.0.BUILD-SNAPSHOT.jar"]

* create a container in the interactive mode to check the work directory

docker container run -it demo:0.1 /bin/bash

pwd

• USER: By default docker container runs as a root user, if you want to create a user and run as that user

USER qt

Docker Single Host networking and Port forwarding

• To acces the application running inside the container we establish port forwarding between host port and container port
• Now lets examine one docker command

docker container run --name tomcatwn1 -d -p 8081:8080 tomcat:8

docker container run --name jenkinswn1 -d -p 8082:8080 jenkins

• Lets run this command

docker container run --name jenkinswn2 -d -p 8081:8080 jenkins

docker: Error response from daemon: driver failed programming external connectivity on endpoint jenkinswn2 (1b1e
dac2c349d064d7b5a50ff2e883ebb426d1da1b7afd61e6e2935bc5d9d919): Bind for 0.0.0.0:8081 failed: port is already allocated.

• To resolve the situation try use -P which will allocate any free port of the docker host to docker container

docker container run --name jenkinswn3 -d -P jenkins

• One scenario for container communications with in the container network
  • By default, docker containers (in linux) run on bridge network
  • Lets try to create two container c1 and c2.

docker container run --name c1 -d alpine sleep 1d
docker container run --name c2 -d alpine sleep 1d

# inspect the network
docker network inspect bridge

# make a note ip addresses of c1 and c2

* Login into c1 container and ping c2 by name and ipaddress

docker container exec -it c1 /bin/sh

ping -c 4 172.17.0.3
# it works

# ping by name of the continer

ping -c 4 c2
# it fails

• Observeration: In the default bridge network, ip address is resolved but not the host name