What happens when I run `docker start image_name`? - docker

I understand that docker start starts a container and docker run creates and starts a container given an image. But naively at first I just ran the command docker start image_name and it just outputs image_name to the console and no container is created and started.
Does docker start image_name do anything besides echo the name to the console? The doc is not very illustrative. If so, what a bad way to fail, better would have been to tell me, that that is not a container but an image and I should first create a container, but maybe I'm missing some useful action which docker start image_name does?

What happens when I run docker start image_name?
$ docker start --help
Usage: docker start [OPTIONS] CONTAINER [CONTAINER...]
Start one or more stopped containers
it just outputs image_name to the console and no container is created and started
a, --attach Attach STDOUT/STDERR and forward signals
The default is not to attach.
Does docker start image_name do anything besides echo the name to the console?
Yes it Start(-s) one or more stopped containers.
I'm missing some useful action which docker docker start image_name does?
It restarts the stopped container.
$ docker run --name work alpine sh -c 'echo important_work ; sleep 10 ; echo success'
important_work
success
$ docker start -a work
important_work
success
Most probably your container that you tested does nothing or expects interactive session to work, and without input it just exits.

Related

How to see the docker state instead of interacting with the shell?

I use following command to build web server
docker run --name webapp -p 8080:4000 mypyweb
When it stopped and I want to restart, I always use:
sudo docker start webapp && sudo docker exec -it webapp bash
But I can't see the server state as the first time:
Digest: sha256:e61b45be29f72fb119ec9f10ca660c3c54c6748cb0e02a412119fae3c8364ecd
Status: Downloaded newer image for ericgoebelbecker/stackify-tutorial:1.00
* Running on http://0.0.0.0:4000/ (Press CTRL+C to quit)
How can I see the state instead of interacting with the shell?
When you use docker run, the default behavior is to run the container detached. This runs in the background and is detached from your shell's stdin/out.
To run the container in the foreground and connected to stdin/out:
docker run --interactive --tty --publish=8080:4000 mypyweb
To docker start a container, similarly:
docker start --interactive --attach [CONTAINER]
NB --attach rather than -tty
You may list (all add --all) running containers:
docker container ls
E.g. I ran Nginx:
CONTAINER ID IMAGE PORTS NAMES
7cc4b4e1cfd6 nginx 0.0.0.0:8888->80/tcp nostalgic_thompson
NB You may use the NAME or any uniquely identifiable subset of the ID to reference the container
Then:
docker stop nostalgic_thompson
docker start --interative --attach 7cc4
You may check the container's logs (when running detached or from another shell) by grabbing the container's ID or NAMES
docker logs nostalgic_thompson
docker logs 7cc4
HTH!
Using docker exec is causing the shell to attach to the container. If you are comparing the behavior of docker run versus docker start, they behave differently, and it is confusing. Try this:
$ sudo docker start -a webapp
the -a flag tells docker to attach stdout/stderr and forward signals.
There are some other switches you can use with the start command (and a huge number for the run command). You can run docker [command] --help to get a summary of the options.
One other command that you might want to use is logs which will show the console output logs for a running container:
$ docker ps
[find the container ID]
$ docker logs [container ID]
If you think your container's misbehaving, it's often not wrong to just delete it and create a new one.
docker rm webapp
docker run --name webapp -p 8080:4000 mypyweb
Containers occasionally have more involved startup sequences and these can assume they're generally starting from a clean slate. It should also be extremely routine to delete and recreate a container; it's required for some basic tasks like upgrading the image underneath a container to a newer version or changing published ports or environment variables.
docker exec probably shouldn't be part of your core workflow, any more than you'd open a shell to interact with your Web browser. I generally don't tend to docker stop containers, except to immediately docker rm them.

docker run vs create+start: why are created containers different?

Related to
docker container started in Detached mode stopped after process execution
https://serverfault.com/questions/661909/the-right-way-to-keep-docker-container-started-when-it-used-for-periodic-tasks
I do understand the difference between docker run and create + start, but don't understand how the actual containers created in these two ways differ.
Say I create and run a container with
docker run -dit debian:testing-slim
and then stop it. The created container can later be started with
docker start silly_docker_name
and it'll run in the background, because the entry command for the image is bash.
But when a container is first created
docker create --name silly_name debian:testing-slim
and then started with
docker start silly_name
then it'll exit immediately. Why isn't bash started, or how come it exits in this case?
The difference for a container process that is a shell (like bash in your debian example) is that a shell started without a terminal+interactive "mode" exits without doing anything.
You can test this by changing the command of a create'd container to something that doesn't require a terminal:
$ docker create --name thedate debian date
Now if I run thedate container, each time I run it it outputs the date (in the logs) and exits. docker logs thedate will show this; one entry for each run.
To be explicit, your docker run command has flags -dit: detached, interactive (connect STDIN), and tty are all enabled.
If you want a similar approach with create & start, then you need to allocate a tty for the created container:
$ docker create -it --name ashell debian
Now if I start it, I ask to attach/interactively to it and I get the same behavior as run:
$ docker start -ai ashell
root#6e44e2ae8817:/#
NOTE: [25 Jan 2018] Edited to add the -i flag on create as a commenter noted that as originally written this did not work, as the container metadata did not have stdin connected at the create stage

How to run docker image as singleton

I'm new to docker.
I have an image that I want to run, but I want docker to see if that image is already running from another terminal...if it is running I don't want it to load another one...
is this something that can be done with docker?
if it helps, I'm running the docker with a privileged mode.
I've tried to search for singleton docker or something like that, but no luck.
updates-
1.working from ubuntu.
My scenario- from terminal X I run docker run Image_a
from terminal Y I run docker run Image_a
when trying to run from terminal Y, I want docker to check if there is already a docker running with Image_a, and the answer is true - I want docker not to run in terminal Y
You can use the following docker command to get all containers that running from specific image:
docker ps --filter ancestor="imagename:tag"
Example:
docker ps --filter ancestor="drone/drone:0.5"
Example Output:
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
3fb00087d4c1 drone/drone:0.5 "/drone agent" 6 days ago Up 26 minutes 8000/tcp drone_drone-agent_1
This approach uses docker api and docker daemon, so it doesnt matter if the run command executed in background or other terminal.
Aother approach:
If you have a single container form a single image:
Try naming your containers, You cant have 2 containers with the same name:
docker run --name uniquecontainer Image_a
Next time you run the above command you will get an error. Btw consider using -d so you dont have to switch terminals.
docker run -d --name uniquecontainer Image_a

How restart a stopped docker container

I launch a docker container from an image with the following command:
$ docker run -d myimage /bin/bash -c "mycommand"
When "mycommand" is finished, the container is stopped (I suppose it is stopped), but it is not deleted, because I can see it with this command:
$ docker ps -a
Is there any way to restart this container with the same parameters and keep data generated by mycommand?
Yes, when the initial command finish its execution then the container stops.
You can start a stopped container using:
docker start container_name
If you want to see the output of your command then you should add -ai options:
docker start -ai container_name
PS. there is a docker restart container_name but that is used to restart a running container - I believe that is not your case.
First, $ docker ps -a shows all containers (the ones that are running and the stopped ones), so that is the reason you are not seeing your stopped container listed.
Second, you can easily start a stopped container running:
$ docker start container_name
Once the container has been started, you can run your command by:
$ docker exec -it container_name bash -c "mycommand"
The stuff you create in your container will remain inside your container as long as it exists. If you want to keep data even if your container is removed you can use a volume.
It should be
$ docker restart container_id # OR
$ docker restart container_name
From the above picture we see one container is up and other status is Exited.
When a container is exited we can still start it back up, because a container stop doesn't mean that it's like dead or cannot be used again we can very easily stop and then start containers again at some point in the future. To start a container backup we can take it's ID and then execute docker start and paste the ID end.
sudo docker start container_id
command for exited container in the above picture will be.
sudo docker start -a bba606a95392
Out put:
By the way: While restarting a container we can not replace the default command, as soon as you started up with the default command is set for the container, for example if we start our container overriding the default command let's see what happened:
Docker is thinking we are trying to start and attach multiple container at the same time.
So when we up a container and let it exit, we can start it back up again which is going to reissue the default command that was used when the container was first created. It is part of docker container lifecycle.
Unfortunately, if you restart your VM/System and it shows
mysql-tls:5.7 "docker-entrypoint.s…" 18 hours ago Exited (255) 44 seconds ago
Answer :
Start the Container
docker start mysql
or
docker start your_container_name

Difference between Running and Starting a Docker container

In practice to start a container I do:
docker run a8asd8f9asdf0
If thats the case, what does:
docker start
do?
In the manual it says
Start one or more stopped containers
This is a very important question and the answer is very simple, but fundamental:
Run: create a new container of an image, and execute the container. You can create N clones of the same image. The command is:
docker run IMAGE_ID and not docker run CONTAINER_ID
Start: Launch a container previously stopped. For example, if you had stopped a database with the command docker stop CONTAINER_ID, you can relaunch the same container with the command docker start CONTAINER_ID, and the data and settings will be the same.
run runs an image
start starts a container.
The docker run doc does mention:
The docker run command first creates a writeable container layer over the specified image, and then starts it using the specified command.
That is, docker run is equivalent to the API /containers/create then /containers/(id)/start.
You do not run an existing container, you docker exec to it (since docker 1.3).
You can restart an exited container.
Explanation with an example:
Consider you have a game (iso) image in your computer.
When you run (mount your image as a virtual drive), a virtual drive is created with all the game contents in the virtual drive and the game installation file is automatically launched. [Running your docker image - creating a container and then starting it.]
But when you stop (similar to docker stop) it, the virtual drive still exists but stopping all the processes. [As the container exists till it is not deleted]
And when you do start (similar to docker start), from the virtual drive the games files start its execution. [starting the existing container]
In this example - The game image is your Docker image and virtual drive is your container.
run command creates a container from the image and then starts the root process on this container. Running it with run --rm flag would save you the trouble of removing the useless dead container afterward and would allow you to ignore the existence of docker start and docker remove altogether.
run command does a few different things:
docker run --name dname image_name bash -c "whoami"
Creates a Container from the image. At this point container would have an id, might have a name if one is given, will show up in docker ps
Starts/executes the root process of the container. In the code above that would execute bash -c "whoami". If one runs docker run --name dname image_name without a command to execute container would go into stopped state immediately.
Once the root process is finished, the container is stopped. At this point, it is pretty much useless. One can not execute anything anymore or resurrect the container. There are basically 2 ways out of stopped state: remove the container or create a checkpoint (i.e. an image) out of stopped container to run something else. One has to run docker remove before launching container under the same name.
How to remove container once it is stopped automatically? Add an --rm flag to run command:
docker run --rm --name dname image_name bash -c "whoami"
How to execute multiple commands in a single container? By preventing that root process from dying. This can be done by running some useless command at start with --detached flag and then using "execute" to run actual commands:
docker run --rm -d --name dname image_name tail -f /dev/null
docker exec dname bash -c "whoami"
docker exec dname bash -c "echo 'Nnice'"
Why do we need docker stop then? To stop this lingering container that we launched in the previous snippet with the endless command tail -f /dev/null.
daniele3004's answer is already pretty good.
Just a quick and dirty formula for people like me who mixes up run and start from time to time:
docker run [...] = docker pull [...] + docker start [...]
It would have been wiser to name the command "new" instead of "run".
Run creates a container instance of an existing (or downloadable) image and starts it.

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