These’re the basic Docker commands we should really know:
1. run
The Docker run command is used to run a container from an image. Let’s run the following command:
docker run nginx
Running the preceding command will run an instance of the nginx application on the docker host if the image already exists. If the image is not present on the host, Docker will go out to docker-hub and pull that image down. This process is only done the first time for the subsequent executions. The same image will be reused if we run the preceding command again.
2. pull
When we ran the Docker run command earlier, it downloaded the nginx image as it couldn’t find one locally. What if we simply want to download the image and keep it on our host so the next time we run the Docker run command we don’t need to wait for it to download?
For the answer, we can use the Docker pull command. It will only pull the image and not run the container. Let’s run the following command:
docker pull ubuntu
By running the preceding command, Docker will only pulls the Ubuntu image from docker-hub and will not create any container using that image.
3. images
As we have discussed earlier, Docker will download image from docker-hub when we create a container for the first time or when we run pull command.
If we want to list all images that available in our host, we can run images command as follow:
docker images
The output of the preceding should be a list of available images on our host. For instance:
REPOSITORY TAG IMAGE ID CREATED SIZE
nginx latest 35c43ace9216 20 hours ago 133MB
ubuntu latest f63181f19b2f 4 weeks ago 72.9MB
4. ps
The Docker ps command will list all running containers and some basic information about them such as the container ID, the name of the image we used to run the containers, the current status and the name of the container. Each container automatically gets a random ID and name created for it by Docker.
Now, let’s run the following command:
docker ps
When I run the preceding command, the response in my machine is as follow:
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
de9a6c766f31 nginx "/docker-entrypoint.…" 11 seconds ago Up 10 seconds 80/tcp funny_hermann
As we can see, the Docker create a random name for container nginx, which is funny_hermann
To see all containers that is running or not, use the -a option. The command should be as follow:
docker ps -a
The preceding command will output all running containers as well as previously stopped or exited containers.
5. stop
The Docker stop command will stop a running container. When we use the stop command, we have to provide either the container ID or the container name. If we’re not sure of the name, run the docker ps command to get it.
Here the example command to stop nginx container we have created when running docker run nginx:
docker stop de9a6
NOTE: de9a6 is the container ID that we get by running docker ps command. We don’t need to provide completed container ID, instead the first few characters alone just so it is different from the other container ID on the host.
After we stop the container and we run docker ps -a command, the container we’ve just stopped is appear as Exited state.
6. rm
After we stop a container and will not use it anymore, we can delete it so it’s not consume space. To do that, we can use rm command to remove the stopped or exited container permanently. The command is as follow:
docker rm de9a6
Note that the preceding command need either the container ID or the container name (as we also use in stop command). Use docker ps -a to get the container name or ID.
If rm command run successfully, it will response the container ID we have provided in the command.
7. rmi
The Docker rmi command is used to remove image we are not using anymore. We can see the list of available images on our host by running docker images command.
To remove an image that we’re no longer plan to use, run the rmi command as follow:
docker rmi f6318
The preceding command will permanently remove ubuntu image from our host since its ID begins with f6318.
REMEMBER: We must ensure that no containers are running off of that image before attempting to remove the image. We have to stop and delete all dependent containers to be able to delete an image.
8. exec
Now, let’s talk about exec command. But, before that, we need to understand more about the flow of the docker.
Suppose we run a docker container from an Ubuntu image by running the following command:
docker run ubuntu
By running the preceding command, Docker runs an instance of ubuntu image and exits immediately. If we list the running containers by running docker ps command, we won’t see the container running.
However, if we list all containers including those that are stopped by running docker ps -a command, we will see that the new container we ran is in an exited stage.
Why can it happen?
Containers are not meant to host an operating system. Containers are meant to run a specific task or process such as to host an instance of a web server or application server or a database or simply to carry some kind of computation or analysis task. Once the task is complete the container exits. A container only lives as long as the process inside it is alive.
If the web service inside the container is stopped or crash then the container exits. This is why when we run a container from an Ubuntu image it stops immediately because Ubuntu is just an image of an operating system that is used as the base image for other applications. There is no process or application running in it by default.
If the image isn’t running any service as is the case with Ubuntu, we can instruct Docker to run a process with the docker run command for example a sleep command with a duration of five seconds as follow:
docker run ubuntu sleep 5000
By running the preceding command, when the container starts it runs sleep command and goes into sleep for five seconds. After that, the sleep command exits and the container stops.
But what if we would like to execute a command on a running container? (For instance, when we run docker run ubuntu sleep 100000 command which is a running Ubuntu container that sleeps for 100 second).
For that purpose, we can use exec command. Suppose we would like to see the contents of a file inside this particular container, we can use the docker exec command to execute a command on our docker container. For instance, running the following command:
docker exec trusting_chatelet cat /etc/hosts
NOTE: We can find the container name (trusting_chatelet) by using docker ps -a command.
By running the preceding command, Docker will print the contents of the /etc/hosts file on the console.
9 & 10. attach and dettach
By default, when we run docker run command, Docker will run the container in the foreground or in an attached mode meaning we will be attached to the console or the standard out of the docker container and we will see the output of the web service on your screen.
For instance, let’s run the following command:
docker run ubuntu sleep 100
After running the preceding command, we won’t be able to do anything else on this console other than view the output (although the sleep command doesn’t produce) until this docker container stops.
Another option is to run the docker container in the detached mode by providing the -d option. Now, let’s run the preceding command again and provide -d option as follow:
docker run -d ubuntu sleep 100
This will run the docker container in the background mode and we will be back to our prompt immediately.
To view the running container, we can docker ps command. The response of mine is as follow:
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
7a9647438e0b ubuntu "sleep 100" 10 seconds ago Up 9 seconds zealous_wescoff
Now if we would like to attach back to the running container later run docker attach command and specify the name or container ID as follow:
docker attach 7a9