difference between docker attach and docker exec

Question

Both will be able to execute commands in container. Both could detach the container.

So what is the real difference between docker exec and docker attach?

Answer 1

Docker exec executes a new command / create a new process in the container’s environment, while docker attach just connects the standard input/output/error of the main process(with PID 1) inside the container to corresponding standard input/output/error of current terminal(the terminal you are using to run the command).

A container is an isolated environment, with some processes running in the environment. Specifically, a container has its own file system space and PID space that are isolated from host and other containers. When the container is started using “docker run –it …”, the main process will have a pseudo-tty and STDIN kept open. When attached in the tty mode, you can detach from the container (and leave it running) using a configurable key sequence. The default sequence is CTRL-p CTRL-q. You configure the key sequence using the --detach-keys option or a configuration file. You can reattach to a detached container with docker attach.

Docker exec just starts a new process, inside the container’s environment, that is, belongs to the PID space of the container.

For example, if you start your container using “docker run –dit XXX /bin/bash”,you can attach to the container(‘s main process) using two different terminals. While you are inputting in one terminal, you can see it appears in the other terminal, for both terminal are connected to same tty. Be careful that you are now in the main process of the container, if you type “exit”, you will exit the container(so be careful, using detach-keys to detach), and you will see both terminals exited. But if you run “docker exec –it XXX /bin/bash” in two terminals, you have started two new processes inside the container, and they are not related to each other and to the main process, and you can safely exit from them.