Docker Cheatsheet

Images

Inspect image content (with or without starting container)

If no container of the image exists (running or stopped), create it from the image (here: my_image). It won’t be started. Run:
$ docker create --name my_container my_image
Hint

You may also reference the image ID, instead of the name.
Export the container contents into a tar archive:
$ docker export my_container > my_container.tar
Hint

This also works for running or stopped containers.

Extract the image archive:

$ mkdir my_container
$ tar -xf my_container.tar -C ./my_container

You may now inspect all the content inside the ./my_container directory.

exec

Make `docker exec` return same exit code as executed command

Problem:

When a command, being executed on a running container, returns a exit code other than zero (0), the docker exec command itself still returns the exit code 0.

Cause:

The docker exec command ran successfully, as the container was reachable and the command could be triggered. When running docker exec like this:

$ docker exec -it CONTAINER_NAME COMMAND [ARGUMENTS...]
# for example
$ docker exec -it some_container ls -l /not/here
# output:
# ls: cannot access '/not/here': No such file or directory
$ echo $?
# 0 -> shows exit code 0 (OK)

Solution:

The command itself must call another bash instance to execute the command. If that instance returns a non-zero exit code, the docker exec command will also return a non-zero exit code:

$ docker exec -it CONTAINER_NAME /bin/bash -c 'COMMAND [ARGUMENTS...]'
# for example
$ docker exec -it some_container /bin/bash 'ls -l /not/here'
# output:
# ls: cannot access '/not/here': No such file or directory
$ echo $?
# 2 -> shows exit code 2 (NOK)

Attention

The command argument for the -c option must be put in single quotes, not double quotes.

Hint

You may need to replace /bin/bash by another shell executable, like sh, /bin/sh or /bin/zsh - whatever shell is available on the Docker container. Make sure, that shell supports the -c option.