Docker Basic Commands

Creating and run a container from an image

docker run hello-world

Here, docker is the docker-client name, run is create and run the container and hello-world is the docker image name.

With this command we also run the start-up script. That start-up script is responsible to print the text Hello from Docker!.

Override startup or default command

Anytime we execute docker run with an image,

To override the default startup command, we can pass our command as 4th parameter.

docker run busybox echo hi there

We will get the output

hi there

Here, busybox is the image and echo hi there is our over ride start up command.

We use busybox image instead of hello-world because busy box has the program like echo.

Listing container in local machine

Check the currently running container.

docker ps

If there is no docker container is running the list will be empty.

Let's run a container for long time and check it docker ps works.

docker run busybox ping google.com

This will run a significant time to measure the latency. In the meantime let's run the listing command

docker ps

Now we should see the busybox is running along with it's other properties.

To check all the containers ever been created in the local machine, we can use a flag all.

docker pas --all

This will output all the containers we been created in local machine.

Restart stopped docker container

We can restart the docker container in future at any point.

To do so, we need the container_id. We can get the container_id by following command

docker ps --all

In the first column of the table, contains container_id.

Now we can start the stopped docker using the following commands

docker start -a cee8f8c62478

Here -a watch out the output of the container and print it in the console.

Removing stopped containers

We can see all the containers we run before by

docker ps --all

To remove all these containers along with the build file we can use the following

docker system prune

With docker system prune, we have to re-download all the images we downloaded earlier

Retrieve the logs of a container

In some scenario, we might need to see the output of the docker container.

For example,

# get ready the busybox container that will print `Hi there!`
docker create busybox echo Hi there!
# start the container
docker start container_id

Now the docker provide the output Hi there, but since we did not use -a flag, the output will not be printed in the console.

To see logs, we can use the following commands

docker logs container_id

This will give us the output Hi there!.

docker logs does not re-run or restart the docker container. It just get the logs emitted from the container.

Stopping a container

To stop a docker container we can use the following method

When we use the stop command a SIGTERM signal is passed to the primary process of the container. It gives the process 10 seconds to close the process and do the post process, like save files, do logging etc. If the primary process does not close within 10 seconds, it then pass SIGKILL signal that kill the process immediately.

The kill command pass SIGKILL signal that kill the process immediately.

SIGTERM stands for Terminate Signal. SIGKILL stands for Kill Signal.

Multiple commands in container

Let's say we need to tweak the redis-cli. In this case, if redis server is installed in my local machine, in one terminal, we can start the redis server. In another terminal, we can start the redis-cli.

With docker, if we install the redis, we can start the redis server by the startup command. But in this case, if we go to another terminal window and try to start the redis-cli, we can not access it. Because, from a regular machine terminal window, we can not access the redis server that is running in a isolated namespace.

Example :

Lets run the redis container by

docker run redis

This will install the redis server and start the server also.

Now If we go to another terminal and try to access redis-cli, we will get an error.

Solution :

To execute the redis-cli inside the container we need to follow the skeleton,

docker exec -it container_id redis-cli

This will execute the redis-cli command inside the container and with -it flag, we can pass the command through the terminal and get the emitted output by the container inside our terminal.

In this way we can interact with redis server like

127.0.0.1:6379> set myVal 5
127.0.0.1:6379> get MyVal

Here the exec stands for Execute. Without -it the redis-cli will run inside the container but we can not interact using host machine terminal.

Each process running in the docker or virtual linux environment contains STDIN, STDOUT, STDERR. The STDIN stands for Standard In, STDOUT stands for Standard Output and STDERR stands for Standard Error. -it is combined of two standalone flag. -i stands connect with processes STDIN and -t ensure formatted text.

Start a terminal/shell of container context along with docker start

May be we do not want to always use the exec command to open a terminal for a container every time we want to execute some command inside the container.

To open a terminal inside the container we can use the following command

docker exec -it container_id sh

Now a terminal inside the container will be appeared and we can execute all the unix command there. For example we can run the previous redis-cli command here and interact with redis-server.

sh is program execute inside the container, known as command processor. Similar programs are zsh, powershell, bash etc. Traditionally most of the container has sh program included.

If we want to run the terminal inside the container on startup, we can use the following

docker run -it image_name sh

This will start the container and also start a terminal in the container context.

This sh program will prevent other startup command supposed to be run during the container start.