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The container is running and you need to look inside: a file, a variable, run one of the application’s commands. The short answer: docker exec opens a command line (or runs a command) inside a container that is already up, and docker cp copies files between the server and the container. They are for looking and diagnosing. They are not for maintaining the application: whatever you change in there is lost when the container is recreated.
The commands you use most
| To… | Command |
| Open a command line in the container | docker exec -it name sh (Alpine images only have sh; others may have bash). Leave with exit. |
| Run a single command and leave | docker exec name ls -la /app |
| Enter as root, if the image runs as another user | docker exec -u root -it name sh |
| Do the same in a Compose service | docker compose exec service sh (in the project folder) |
| Copy a file from the container to the server | docker cp name:/app/app.log ./app.log |
| Copy from the server into the container | docker cp ./config.json name:/app/config.json |
| Watch usage live | docker stats (memory and processor per container) |
| See a container’s processes | docker top name |
Diagnosing, step by step
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What you change inside the container is lost. If you edit a file with exec and then recreate the container (docker compose up -d with a new image, for example), the change disappears. The real fix is made in the code or the configuration, and the image rebuilt, or the file is mounted from outside: volumes and bind mounts. To update without losing data: updating a Docker application.
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Mind the secrets on screen. docker exec name env prints every variable, including passwords and keys. Do not paste that output into a support ticket or a forum without removing whatever is secret.
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Whoever can use docker controls the server. Belonging to the docker group is, in practice, being root. Give that access only to those who should have it: Docker precautions.
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The container runs, but the server stopped answering or SSH will not let you in? That is ours: tell us what you see. Open a support ticket |
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SEE ALSO Your first container, and what to look at when it will not start Docker volumes or bind mounts: where your data lives and how to back it up |
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