What are the ways to enter the docker container?

There are many ways to enter a Docker container. Here we focus on several commonly used methods of entering a Docker container.

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Some of the more common ways to enter a Docker container are as follows:

• Use docker attach

• Use SSH

• Use nsenter

• Use exec

1. Use docker attach to enter the Docker container

Docker provides the attach command to enter the Docker container.

Next we create a daemon Docker container, and then use the docker attach command to enter the container.

$ sudo docker run -itd ubuntu:14.04 /bin/bash

Then we use docker ps to view the container information, and then use docker attach to enter the container

$ sudo docker attach 44fc0f0582d9

We can see that we have entered the container.

But there is a problem with using this command. When multiple windows use this command to enter the container at the same time, all windows will be displayed simultaneously. If one window is blocked, other windows will no longer be able to operate.

For this reason, the docker attach command is not suitable for production environments. You can use this command when developing your own applications.

2. Use SSH to enter the Docker container

After excluding using the docker attach command to enter the container in the production environment, I believe the first thing that everyone thinks of is ssh. Install SSH Server in the image (or container), so that multiple people can enter the container without interfering with each other. I believe everyone does this in the current production environment (without using Docker). However, it is not recommended to use ssh to enter the Docker container after using it.

3. Use nsenter to enter the Docker container

If the above two methods are not suitable, there is a more convenient method, which is to use nsenter to enter the Docker container.

After understanding what nsenter is, the system will install the nsenter we need into the host by default.

If it is not installed, just follow the steps below to install it (note that it is the host and not the container or Mirror)

The specific installation command is as follows:

$ wget https://www.kernel.org/pub/linux/utils/util-linux/v2.24/util-linux-2.24.tar.gz  
$ tar -xzvf util-linux-2.24.tar.gz  
$ cd util-linux-2.24/  
$ ./configure --without-ncurses  
$ make nsenter  
$ sudo cp nsenter /usr/local/bin

After installing nsenter, you can check the use of this command.

#nsenter can access the namespace of another process. So in order to connect to a container we also need to get the PID of the first process of the container. You can use the docker inspect command to get the PID.

The docker inspect command is used as follows:

$ sudo docker inspect --help

The inspect command can display information about an image or container hierarchically. For example, we currently have a running container.

You can use docker inspect to view the details of the container.

$ sudo docker inspect 44fc0f0582d9

Since there is a lot of information, only part of it is captured here for display. If you want to display the first PID of the container, you can use the following method

$ sudo docker inspect -f {{.State.Pid}} 44fc0f0582d9

After getting the PID of the process, we can use the nsenter command to access the container.

$ sudo nsenter --target 3326 --mount --uts --ipc --net --pid  
$ sudo nsenter --target 3326 --mount --uts --ipc --net --pid

The 3326 is the PID of the process just obtained.

4. Use docker exec to enter the Docker container

In addition to the above methods, docker also provides a new command exec after version 1.3.X for entering the container. This The method is relatively simpler. Let’s take a look at the use of this command:

$ sudo docker exec --help

Next we use this command to enter an already running container

$ sudo docker ps  
$ sudo docker exec -it 775c7c9ee1e1 /bin/bash

Related recommendations: docker tutorial

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