1. Overview
This tutorial is going to cover how to attach and detach from Docker container. It could be necessary either to view its ongoing output or to control it interactively and then detach from it when we’re done.
2. Attach to a Running Docker Container
To attach to a running Docker container, we can use the docker attach command which has the syntax as the following:
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docker attach [OPTIONS] CONTAINER |
Here are some command’s options:
| Name, shorthand | Default | Description |
| –detach-keys | Table | Override the key sequence for detaching a container |
| –no-stdin | false | Do not attach STDIN |
| –sig-proxy | true | Proxy all received signals to the process |
Example:
We can attach to the same contained process multiple times simultaneously, screen sharing style, or quickly view the progress of our detached process.
Let’s say we have an opensuse docker and start it in background as the following:
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<span class="hljs-comment"> docker run -d --name attachdemo opensuse /usr/bin/top -b</span> |
To attach to the above running container, we can issue the following command:
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[vagrant@localhost ~]$ docker attach attachdemo |
Then, it will let us connect with the attachdemo container, and we can see the output of the top command.
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[vagrant@localhost ~]$ docker attach attachdemo top - 14:32:57 up 1:25, 0 users, load average: 0.00, 0.02, 0.05 Tasks: 1 total, 1 running, 0 sleeping, 0 stopped, 0 zombie %Cpu(s): 0.3 us, 0.3 sy, 0.0 ni, 99.3 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st KiB Mem: 2842268 total, 421528 used, 2420740 free, 44892 buffers KiB Swap: 1572860 total, 0 used, 1572860 free. 224344 cached Mem PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 1 root 20 0 19648 1236 956 R 0.000 0.043 0:00.03 top |
3. Detach from a Docker Container
3.1. Detach from a Docker container and stop it
To detach from a Docker container and stop it, we can use CTRL-c key sequence which will send SIGKILL to the container. If –sig-proxy is true (the default),CTRL-c will send a SIGINT to the container.
3.2. Detach from a Docker container and leave it running
In general, to detach from a Docker container and leave it running, we can use the CTRL-p CTRL-q key sequence.
Points to note:
To detach from a Docker container, we often use CTRL-c, this often causes the container to be stopped. To work around, we can use the -sig-proxy is false when attach a running container.
Example:
Let’s start a Docker container:
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<span class="hljs-comment"> docker run -d --name attachdemo opensuse /usr/bin/top -b</span> |
And then attach to it with -sig-proxy
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docker attach --sig-proxy=false attachdemo |
Let’s use CTRL-c to detach from the container and then check its status:
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docker ps |
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CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 4585c85708a2 opensuse "/usr/bin/top -b" 31 seconds ago Up 30 seconds attachdemo |
We can see the container is still alive.
4. Get Into a Docker Container
When we attach to a running Docker container, we often need to control, monitor it or just simply for debugging purpose. In such a case, to execute an interactive bash shell on the container is most likely preferred. To do that, we can use the docker exec command which is used to run a command in a running container.
Example:
Let’s create a Docker container:
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docker run -d --name attachdemo opensuse /usr/bin/top -b |
And then execute an interactive bash shell on the above container:
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docker exec -it attachdemo bash |
Let’s see the terminal:
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[vagrant@localhost ~]$ docker exec -it attachdemo bash bash-4.3# |
We now can do whatever with the bash and detach from a Docker container and leave it running by CTRL-c.
5. Conclusion
We just got through how to attach to a running Docker container, how to detach from a Docker container and how to get into a Docker container by executing an interactive bash shell on the container. Those are useful when we often work with Docker.
Here are other tutorials for your reference:
How To Remove Unused Docker Images, Containers, Volumes, and Networks
How To Pull A Docker Image And Run A Container
Copy Files, Folders from Host to Docker Container and Vice Versa
Install Docker on Ubuntu 16.04, 15.10, 14.04 Step By Step
Introduction to Docker Compose
Vagrant Docker Provider Tutorial
How to Get IP Address of a Docker Container