Kubernetes: using bash variable expansion in container entrypoint

Question

According to the [documentation][1] Kubernetes variables are expanded using the previous defined environment variables in the container using the syntax $(VAR_NAME). The variable can be used in the container's entrypoint.

For example:

env:
- name: MESSAGE
  value: "hello world"
command: ["/bin/echo"]
args: ["$(MESSAGE)"]

Is this possible though to use bash expansion aka ${Var1:-${Var2}} inside the container's entrypoint for the kubernetes environment variables E.g.

env:
- name: Var1
  value: "hello world"
- name: Var2
  value: "no hello"
command: ['bash', '-c', "echo ${Var1:-$Var2}"]

mdaniel · Accepted Answer · 2021-06-07 15:55:44Z

4

Is this possible though to use bash expansion aka ${Var1:-${Var2}} inside the container's entrypoint ?

Yes, by using

command: 
- /bin/bash
- "-c"
- "echo ${Var1:-${Var2}}"

but not otherwise -- kubernetes is not a wrapper for bash, it use the Linux exec system call to launch programs inside the container, and so the only way to get bash behavior is to launch bash

That's also why they chose $() syntax for their environment interpolation so it would be different from the ${} style that a shell would use, although this question comes up so much that one might wish they had not gone with $ anything to avoid further confusing folks

answered Jun 7, 2021 at 15:55

mdaniel

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3 Comments

alixander Over a year ago

Thanks daniel! I updateed the question. Will this work also if the variables are kubernetes variables? As stated in the documentation $() syntax is used to get the value of previous defined environment variables in the container

mdaniel Over a year ago

It took you longer to type out that comment and the question update than it would have to try it; yes, env vars are made available to the bash entrypoint, and will be evaluated in their normal bash-y way

Craig Ringer Over a year ago

Also note that this won't work in an os-less container.

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