Starting a shell in the Docker Alpine container
To start an interactive shell for the Ubuntu image we can run:
ole@T:~$ docker run -it --rm ubuntu
root@1a6721e1fb64:/# ls
bin boot dev etc home lib lib64 media mnt opt proc root run sbin srv sys tmp usr var
But when this is run for the Alpine Docker image, the following results:
ole@T:~$ docker run -it --rm alpine
Error response from daemon: No command specified
What is the command for starting an interactive shell in an Alpine base container?
ole@T:~$ docker run -it --rm alpine /bin/ash
(inside container) / #
Options used above:
/bin/ash is Ash (Almquist Shell) provided by BusyBox
--rm Automatically remove the container when it exits (docker run --help)
-i Interactive mode (Keep STDIN open even if not attached)
-t Allocate a pseudo-TTY
Usually, an Alpine Linux image doesn't contain bash, Instead you can use /bin/ash, /bin/sh, ash or only sh.
/bin/ash
docker run -it --rm alpine /bin/ash
/bin/sh
docker run -it --rm alpine /bin/sh
ash
docker run -it --rm alpine ash
sh
docker run -it --rm alpine sh
I hope this information helps you.
/bin/ash and ash? Similar for /bin/sh and sh.
ash is only a symbolic link to /bin/ash. Some linux distributions doesn't have symbolic links.
Nowadays, Alpine images will boot directly into /bin/sh by default, without having to specify a shell to execute:
$ sudo docker run -it --rm alpine
/ # echo $0
/bin/sh
This is since the alpine image Dockerfiles now contain a CMD command, that specifies the shell to execute when the container starts: CMD ["/bin/sh"].
In older Alpine image versions (pre-2017), the CMD command was not used, since Docker used to create an additional layer for CMD which caused the image size to increase. This is something that the Alpine image developers wanted to avoid. In recent Docker versions (1.10+), CMD no longer occupies a layer, and so it was added to alpine images. Therefore, as long as CMD is not overridden, recent Alpine images will boot into /bin/sh.
For reference, see the following commit to the official Alpine Dockerfiles by Glider Labs:
https://github.com/gliderlabs/docker-alpine/commit/ddc19dd95ceb3584ced58be0b8d7e9169d04c7a3#diff-db3dfdee92c17cf53a96578d4900cb5b
In case the container is already running:
docker exec -it container_id_or_name ash
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-doption.ashand notbash, which, not being very familiar with Alpine, is what got me and prevented me from accessing the container earlier.