How do I write to a file inside a running docker container?

Question

With cat I can read from a file to stdout. This way I can for example pipe a file out of a docker container:

docker exec my_container cat file > file_on_host

When I want to do the opposite, I would need a command that reads from stdin and saves to a file. Is there such a command?

docker exec my_container ??? file < file_on_host

Answer 1

tee can do that:

docker exec -i my_container tee file < file_on_host

The -i is necessary for stdin to work.

It will also output the file, which can be avoided by redirecting to the host's /dev/null:

docker exec -i my_container tee file < file_on_host > /dev/null

Answer 2

Similar to your approach,

docker exec -i my_container dd of=file < file_on_host

which gives you a nice status summary and doesn't write the data to stdout.

There are probably a few other options, e.g., cp /dev/stdin file (which might not work, depending on whether your container's OS supports /dev/stdin) and sh -c "cat > file".

Answer 3

Actually seems to be easier with :

docker cp localfile.txt <CONTAINER ID>:/home/docker/data/

Answer 4

I must not have understood the question, because you can either use plain old redirection like you did the first time

docker exec my_container < file_on_host > file

or use cat (again, with redirection).