A brief introduction to Docker start command:

The 'docker start' is a Docker order to begin at least one halted compartment. We can similerly utilize this holder to begin the compartment that we have made utilizing the 'docker make' order or the compartments that are in 'made' status because the 'docker make' order makes the compartment yet doesn't begin naturally. For instance, assuming we have made a naninx compartment and uncovered port 80 remotely yet that holder will serve until it is begun utilizing the 'docker start' order. This order has likewise a couple of banners to get the intelligent control center, re-establish from the designated spot, append STDOUT/STDERR, and so forth.

Here's how the start command works in Docker:

The 'docker start' works similarly to different orders work in Docker. At the point when we run this order from the CLI with compartment name passed as a contention, Docker CLI settles on an API decision to the Docker daemon and daemon begins that holder assuming the compartment is in the halted state or the made state.In the case where the compartments aren't designed before running, we won't be able to port, name, or append volumes.This order is essentially going to begin the holder, be that as it may, we can utilize a couple of banners to control not many things like interfacing with the compartment whenever it is begun, appending STDOUT/STDERR, and so on.

Let’s understand more about this command with a few examples.

Example #1 Start One or Multiple Stopped Containers

Step 1: Let’s create an nginx container using the below command and access the default page using curl:

docker run -d -p 80:80 --name my-Nginx nginx: alpine

Step 2: Now, stop this container using the ‘docker stop’ command and try to access localhost:

Step 3: Let’s start the container again but this time using the ‘docker start’ command as we want to just start the stopped container, don’t want to create a new one.

In the above example, we can see that the default page is again accessible after starting the container.

Step 4: Let’s assume we have multiple containers in exited status that we want to start using a single command. We can do so as shown below:

docker start <container_name1> <container_name2> <container_name3>

docker start $(docker ps -q -f “status=exited”)

In the above snapshot, we can see that we have 3 stopped containers and all stopped containers have been successfully started.

Note: The command shown in the snapshot will only work if the container status is ‘exited’, we have to use the first command mentioned above if the container is in a different state or change the status in the command.

Example #2 Create a Container and then start it

Step 1: Let’s create a container using the below command:

docker create -p 80:80 –name my-nginx2 nginx: alpine

Step 2: Now, start the container as below and try to access it again:

Example #3 Attach the Container STDOUT/STDERR while starting it

Step 1:  Let’s create a ‘hello-world’ container as shown below and try to start it without any flag:

docker start <container_ID or container_name>

Step 2: We can see that there is no output on the screen after starting the container. Let’s create another container and start it with the ‘-a’ or ‘–attach’ flag:

docker start -a <container_ID or container_name>

In the above snapshot, we can see that we got the output on the STDOUT when we run the ‘docker start’ command with the ‘-a’ option. If the container throws an error when it starts, we get the error as well.

Example #4 Start Container with the Checkpoint

Step 1: As a first step , we need to install CRIU (Checkpoint/Restore in Userspace) to work with Docker checkpoints. We can simply install CRIU using the below command:

Step 2: Docker checkpoint only works in experimental mode for Docker, so let’s enable experimental mode by editing daemon.json file as shown below and restart the docker service:

Step 3: Let’s run a container using the ‘ubuntu’ Docker image using the below command:

docker run -d --name checkpoint-cont --security-opt seccomp:unconfined ubuntu  /bin/sh -c 'i=0; while true; do echo $i; i=$(expr $i + 1); sleep 5; done'

In the above example, we have created a container named ‘checkpoint-cont’ and we can see in the logs that it starts printing from 0 every 5 seconds.

Step 4: Let’s create a checkpoint named ‘checkpoint-01’ for this container using the below command:

docker checkpoint create checkpoint-cont checkpoint-01

In the above snapshot, we can see that the container has already printed to 10 before creating a checkpoint.

Step 5: Finally, start the container using the ‘docker start’ command with the ‘–checkpoint’ option as shown below:

We will discuss Docker's Start command in this topic. We will be able to understand how it works step by step.

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