Run Simple Linux Commands in Docker
When a docker image is run, it becomes a docker container.
Docker containers can be started, stopped, restarted, and removed. In this exercise we will note what happens to a container during each of these phases.
Containers can be started, stopped, and removed using the docker command.
Docker Command
docker help
Run an ubuntu container
docker run --rm ubuntu echo 'hello'
In this command, the ubuntu image is being run.
The entrypoint for the ubuntu image is being overridden with the command echo hello.
The --rm option tells docker to delete the container when the process termintes. By default, stopped containers are not deleted. It is possible to view their log files. In most examples in this tutorial, we will utilize the --rm option.
Run ubuntu with the default entrypoint
The default entrypoint for ubuntu is to run a shell.
docker run --rm ubuntu
Note that nothing happened. A shell requires a terminal. You must indicate to docker to provide a terminal.
docker run -it --rm ubuntu
You can issue commands in your your container.
root@91abfa3e77ba:/# ls
bin boot dev etc home lib lib32 lib64 libx32 media mnt opt proc root run sbin srv sys tmp usr var
root@91abfa3e77ba:/# hostname
91abfa3e77ba
root@91abfa3e77ba:/# uname -a
Linux 91abfa3e77ba 5.4.39-linuxkit #1 SMP Fri May 8 23:03:06 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
Type exit to terminate your container.
root@91abfa3e77ba:/# exit
When you wish to network your containers, you can assign names (--name name) to the containers to make them easy to locate. The -d option indicates that the container should be run in the background.
docker run -it --rm --name mycontainer -d ubuntu
The docker ps command can be used to see your running containers.
docker ps -a
Note the results. You can refer to your running container by the container id or by the name you assigned to the container.
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
8bb9eb68e3e9 ubuntu "/bin/bash" 2 minutes ago Up 2 minutes mycontainer
The docker exec lets you connect to a running container.
Run docker exec to attach to your container
docker exec -it mycontainer bash
Browse the /tmp directory, note that it is empty
root@8bb9eb68e3e9:/# ls /tmp
root@8bb9eb68e3e9:/#
Create a file in the /tmp directory
root@8bb9eb68e3e9:/# echo "hello from docker exec" > /tmp/hello.txt
root@8bb9eb68e3e9:/# ls /tmp
hello.txt
Type exit to terminate your terminal session.
root@8bb9eb68e3e9:/# exit
Verify that the file is still present in the container
docker exec -it mycontainer ls /tmp
hello.txt
Since the container had been run in the background, we need to stop it explicitly.
docker stop mycontainer
mycontainer
Note that the container is no longer running.
docker ps -a
Result
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
Restart your container
docker run -it --rm --name mycontainer ubuntu
Note that when a new container is created, the /tmp directory is empty.
root@c5eb44f52795:/# ls /tmp
root@c5eb44f52795:/#
Type exit to terminate your container.
root@c5eb44f52795:/# exit
Stopping and starting a container without --rm
Restart your container without --rm
docker run -it --name mycontainer ubuntu
Create a test file
root@f3c626708043:/# echo "hello" > /tmp/new_hello.txt
Type exit to terminate your container.
root@f3c626708043:/# exit
Run docker ps to view your containers.
docker ps -a
Note that mycontainer is stopped.
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
f3c626708043 ubuntu "/bin/bash" 39 seconds ago Exited (0) 17 seconds ago mycontainer
Run docker start to restart your container.
docker start mycontainer
Run docker ps to view your containers.
docker ps -a
Note that mycontainer is restarted.
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
f3c626708043 ubuntu "/bin/bash" 3 minutes ago Up 2 seconds mycontainer
Run docker exec to reconnect to your container.
docker exec -it mycontainer bash
List the contents of /tmp. Note that the test file is still present.
root@f3c626708043:/# ls /tmp
new_hello.txt
Type exit to terminate your exec session.
root@f3c626708043:/# exit
Run docker ps to view your containers.
docker ps -a
Note that mycontainer is still running.
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
f3c626708043 ubuntu "/bin/bash" 3 minutes ago Up 2 seconds mycontainer
Run docker stop to stop the container.
docker stop mycontainer
Run docker ps to view your containers.
docker ps -a
Note that mycontainer is now stopped.
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
f3c626708043 ubuntu "/bin/bash" 6 minutes ago Exited (0) 3 seconds ago mycontainer
Run docker rm to delete the container.
docker rm mycontainer
Run docker ps to view your containers.
docker ps -a
Note that mycontainer is no longer present.
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
While base images such as ubuntu can be used to solve many problems, docker provides a convenient way to run pre-built application containers.