First argument is not passed to image in kubernetes deployments - kubernetes

Question

I have a docker image with below entrypoint.
ENTRYPOINT ["sh", "-c", "python3 -m myapp ${*}"]
I tried to pass arguments to this image in my kubernetes deployments so that ${*} is replaced with them, but after checking the logs it seem that the first argument was ignored.
I tried to reproduce the result regardless of image, and applied below pod:
apiVersion: v1
kind: Pod
metadata:
name: test
spec:
containers:
- name: test
image: postgres # or any image you may like
command: ["bash -c /bin/echo ${*}"]
args:
- sth
- serve
- arg
when I check the logs, I just see serve arg, and sth is completely ignored.
Any idea on what went wrong or what should I do to pass arguments to exec-style entrypoints instead?

Answer 1

First, your command has quoting problems -- you are effectively running bash -c echo.
Second, you need to closely read the documentation for the -c option (emphasis mine):
If the -c option is present, then commands are read from
the first non-option argument command_string. If there
are arguments after the command_string, the first argument
is assigned to $0 and any remaining arguments are assigned
to the positional parameters. The assignment to $0 sets
the name of the shell, which is used in warning and error
messages.
So you want:
command: ["bash", "-c", "echo ${*}", "bash"]
Given your pod definition, this would set $0 to bash, and then $1 to sth, $2 to serve, and $3 to arg.

Answer 2

There are some subtleties around using sh -c here. For the examples you show, it's not necessary. The important things to remember are that the ENTRYPOINT and CMD are combined together into a single command (or, in Kubernetes, command: and args:), and that sh -c generally takes only a single string argument and acts on it.
The examples you show don't use any shell functionality and you can break the commands into their constituent words as YAML list items.
command:
- /bin/echo
- sth
- serve
- arg
For the Dockerfile case, there is a pattern of using ENTRYPOINT to specify a command and CMD for its arguments, which parallels Kubernetes's syntax here. For this to work well, I'd avoid sh -c (including the implicit sh -c from the ENTRYPOINT shell form); just provide the first set of words in JSON-array form.
ENTRYPOINT ["python", "-m", "myapp"]
# don't override command:, the image's ENTRYPOINT is right, but do add
args:
- foo
- bar
- baz
(If your entrypoint setup is complex enough to require shell operators, it's typically easier to write and debug to move it into a dedicated script and make that script be the ENTRYPOINT or CMD, rather than trying to figure out sh -c semantics and YAML quoting.)