I am bit confused with commands in kubectl. I am not sure when I can use the commands directly like
command: ["command"] or -- some_command
vs
command: [/bin/sh, -c, "command"] or -- /bin/sh -c some_command
I am bit confused with commands in kubectl. I am not sure when I can use the commands directly
Thankfully the distinction is easy(?): every command: is fed into the exec system call (or its golang equivalent); so if your container contains a binary that the kernel can successfully execute, you are welcome to use it in command:; if it is a shell built-in, shell alias, or otherwise requires sh (or python or whatever) to execute, then you must be explicit to the container runtime about that distinction
If it helps any, the command: syntax of kubernetes container:s are the equivalent of ENTRYPOINT ["",""] line of Dockerfile, not CMD ["", ""] and for sure not ENTRYPOINT echo this is fed to /bin/sh for you.
At a low level, every (Unix/Linux) command is invoked as a series of "words". If you type a command into your shell, the shell does some preprocessing and then creates the "words" and runs the command. In Kubernetes command: (and args:) there isn't a shell involved, unless you explicitly supply one.
I would default to using the list form unless you specifically need shell features.
<!-- language: lang-yaml -->command: # overrides Docker ENTRYPOINT
- the_command
- --an-argument
- --another
- valueIf you use list form, you must explicitly list out each word. You may use either YAML block list syntax as above or flow list syntax [command, arg1, arg2]. If there are embedded spaces in a single item [command, --option value] then those spaces are included in a single command-line option as if you quoted it, which frequently confuses programs.
You can explicitly invoke a shell if you need to:
<!-- language: lang-yaml -->command:
- sh
- -c
- the_command --an-argument --another valueThis command is in exactly three words, sh, the option -c, and the shell command. The shell will process this command in the usual way and execute it.
You need the shell form only if you're doing something more complicated than running a simple command with fixed arguments. Running multiple sequential commands c1 && c2 or environment variable expansion c1 "$OPTION" are probably the most common ones, but any standard Bourne shell syntax would be acceptable here (redirects, pipelines, ...).