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4.02 Commands and Arguments in Docker & Kubernetes

This guide explains how commands and arguments work in both Docker and Kubernetes in a simple way.

It focuses on:

  • How containers decide what to run at startup
  • How CMD and ENTRYPOINT behave in Docker
  • How Kubernetes command and args override image behavior
  • Correct patterns and common mistakes

🐳 Commands and Arguments in Docker

Docker containers run one main process. When that process exits, the container stops.

🎯 Core Concept — Container = Main Process

A container is not a virtual machine. It does not boot an OS.
It runs a single primary process.

Lifecycle:

Container start → Main process runs → Process exits → Container stops

Startup command can come from:

  • Dockerfile CMD
  • Dockerfile ENTRYPOINT
  • Command passed in docker run

Note

If there is no long‑running process = container exits immediately.

▶️ Why docker run ubuntu Stops Immediately

docker run ubuntu

What happens:

  • Ubuntu image default command = bash
  • No interactive terminal attached
  • bash exits
  • container stops
docker ps
docker ps -a

Warning

If the startup command finishes, container status becomes Exited.

⏱️ Override Default Command at Runtime

Anything after the image name overrides Dockerfile CMD.

docker run ubuntu sleep 5

Final startup command:

sleep 5

Tip

Runtime command replaces CMD completely.

🧱 CMD — Default Command

CMD defines the default command + arguments if none are provided at runtime.

CMD sleep 5

CMD ["sleep", "5"]

Success

Always prefer JSON form — safer argument parsing.

❌ Wrong CMD JSON Format

CMD ["sleep 5"]

Bug

Executable and arguments must be separate array elements.

Correct:

CMD ["sleep","5"]

🔁 CMD Is Replaced by Runtime Command

Dockerfile:

CMD ["sleep","5"]

Runtime:

docker run myimg sleep 10

Final command:

sleep 10

Note

Runtime command fully replaces CMD.

🚪 ENTRYPOINT — Fixed Executable

ENTRYPOINT defines the base executable.

Runtime values are appended as arguments.

ENTRYPOINT ["sleep"]

docker run myimg 10

Final command:

sleep 10

Abstract

ENTRYPOINT = fixed program
CMD = default parameters

⚠️ ENTRYPOINT Without Default Args

ENTRYPOINT ["sleep"]

docker run myimg

Result:

sleep: missing operand

Warning

ENTRYPOINT needs arguments — provide via CMD or runtime.

✅ Best Practice — ENTRYPOINT + CMD

FROM ubuntu
ENTRYPOINT ["sleep"]
CMD ["5"]

docker run myimg

Runs → sleep 5

docker run myimg 20

Runs → sleep 20

Success

ENTRYPOINT + CMD = best pattern for flexible containers.

🔧 Override ENTRYPOINT at Runtime

docker run --entrypoint sleep2.0 myimg 30

Tip

Useful for debugging or temporary overrides.

🧭 Docker Startup Resolution Order

docker run command
      ↓
Replaces CMD
      ↓
Appended to ENTRYPOINT (if present)
      ↓
Final executed process

📊 CMD vs ENTRYPOINT — Quick Compare

Behavior CMD ENTRYPOINT
Default executable ✅ ✅
Default args ✅ ❌
Runtime command Replaces Appends
Best for Defaults Fixed program

☸️ Commands and Arguments in Kubernetes

Kubernetes lets you override container startup behavior using:

  • command
  • args

These map directly to Docker behavior.

🎯 Core Idea — Pod Startup Control

Container startup inside a Pod can come from:

  • Docker ENTRYPOINT
  • Docker CMD
  • Pod command
  • Pod args

Note

Kubernetes can override both ENTRYPOINT and CMD.

🧱 Mapping — Docker vs Kubernetes

Docker Kubernetes
ENTRYPOINT ➡️ command
CMD➡️ args

Tip

command = executable
args = parameters

▶️ Default Behavior (No Pod Overrides)

Dockerfile:

ENTRYPOINT ["sleep"]
CMD ["5"]

Pod:

containers:
  - name: sleeper
    image: ubuntu-sleeper

Final command:

sleep 5

Success

Image defaults are used.

🔢 Override Only Arguments (args)

Equivalent to:

docker run ubuntu-sleeper 10

Pod:

containers:
  - name: sleeper
    image: ubuntu-sleeper
    args: ["10"]

Final command:

sleep 10

Note

args overrides Docker CMD only.

🔁 Override Executable (command)

Equivalent to Docker:

docker run --entrypoint sleep2.0 ubuntu-sleeper 10

Pod:

containers:
  - name: sleeper
    image: ubuntu-sleeper
    command: ["sleep2.0"]
    args: ["10"]

Final command:

sleep2.0 10

Warning

command replaces Docker ENTRYPOINT.

🧭 Kubernetes Override Flow

Dockerfile ENTRYPOINT + CMD
        ↓
Image default command
        ↓
Pod args → replaces CMD
Pod command → replaces ENTRYPOINT

⚠️ Common Mistake

Wrong:

command: ["sleep 10"]

Bug

Must be split into executable + argument.

Correct:

command: ["sleep"]
args: ["10"]

🧪 Minimal Test Pod

apiVersion: v1
kind: Pod
metadata:
  name: demo
spec:
  containers:
    - name: sleeper
      image: ubuntu
      command: ["sleep"]
      args: ["15"]

🧠 Exam Checks

Question

Which Pod field overrides Docker ENTRYPOINT?

command

Question

Which Pod field overrides Docker CMD?

args

Question

Are command and args arrays?

Yes — always list form.

✅ Final Summary

Summary

  • Containers run one main process
  • Docker CMD = default args
  • Docker ENTRYPOINT = fixed executable
  • Runtime command replaces CMD
  • Runtime args append to ENTRYPOINT
  • Kubernetes command ↔ ENTRYPOINT
  • Kubernetes args ↔ CMD
  • Always use JSON/list form