Alauda Container Platform

Understanding Startup Commands

TOC

Overview

Startup commands in Kubernetes define the primary executable that runs when a container starts. They correspond to the command field in Kubernetes Pod specifications and override the default ENTRYPOINT instruction defined in container images. Startup commands provide complete control over what process runs inside your containers.

Core Concepts

What are Startup Commands?

Startup commands are:

  • The primary executable that runs when a container starts
  • Override the ENTRYPOINT instruction in container images
  • Define the main process (PID 1) inside the container
  • Work in conjunction with parameters (args) to form the complete command line

Relationship with Dockerfile and Parameters

Understanding the relationship between Dockerfile instructions and Kubernetes fields:

DockerfileKubernetesPurpose
ENTRYPOINTcommandDefines the executable
CMDargsProvides default arguments
# Dockerfile example
FROM ubuntu:20.04
ENTRYPOINT ["/usr/bin/myapp"]
CMD ["--config=/etc/default.conf"]
# Kubernetes override
apiVersion: v1
kind: Pod
spec:
  containers:
  - name: myapp
    image: myapp:latest
    command: ["/usr/bin/myapp"]
    args: ["--config=/etc/custom.conf", "--debug"]

Command vs Args Interaction

ScenarioContainer ImageKubernetes SpecResulting Command
DefaultENTRYPOINT + CMD(none)ENTRYPOINT + CMD
Override args onlyENTRYPOINT + CMDargs: ["new-args"]ENTRYPOINT + new-args
Override command onlyENTRYPOINT + CMDcommand: ["new-cmd"]new-cmd
Override bothENTRYPOINT + CMDcommand: ["new-cmd"]
args: ["new-args"]		new-cmd + new-args

Use Cases and Scenarios

1. Custom Application Startup

Run different applications using the same base image:

apiVersion: v1
kind: Pod
metadata:
  name: web-server
spec:
  containers:
  - name: nginx
    image: ubuntu:20.04
    command: ["/usr/sbin/nginx"]
    args: ["-g", "daemon off;", "-c", "/etc/nginx/nginx.conf"]

2. Debugging and Troubleshooting

Override the default command to start a shell for debugging:

apiVersion: v1
kind: Pod
metadata:
  name: debug-pod
spec:
  containers:
  - name: debug
    image: myapp:latest
    command: ["/bin/bash"]
    args: ["-c", "sleep 3600"]

3. Initialization Scripts

Run custom initialization before starting the main application:

apiVersion: v1
kind: Pod
spec:
  containers:
  - name: app
    image: myapp:latest
    command: ["/bin/sh"]
    args:
    - "-c"
    - |
      echo "Initializing application..."
      /scripts/init.sh
      echo "Starting main application..."
      exec /usr/bin/myapp --config=/etc/app.conf

4. Multi-Purpose Images

Use the same image for different purposes:

# Web server
apiVersion: apps/v1
kind: Deployment
metadata:
  name: web
spec:
  template:
    spec:
      containers:
      - name: web
        image: myapp:latest
        command: ["/usr/bin/myapp"]
        args: ["server", "--port=8080"]

---
# Background worker
apiVersion: apps/v1
kind: Deployment
metadata:
  name: worker
spec:
  template:
    spec:
      containers:
      - name: worker
        image: myapp:latest
        command: ["/usr/bin/myapp"]
        args: ["worker", "--queue=tasks"]

---
# Database migration
apiVersion: batch/v1
kind: Job
metadata:
  name: migrate
spec:
  template:
    spec:
      containers:
      - name: migrate
        image: myapp:latest
        command: ["/usr/bin/myapp"]
        args: ["migrate", "--up"]
      restartPolicy: Never

CLI Examples and Practical Usage

Using kubectl run

# Override command completely
kubectl run debug --image=nginx:alpine --command -- /bin/sh -c "sleep 3600"

# Run interactive shell
kubectl run -it debug --image=ubuntu:20.04 --restart=Never --command -- /bin/bash

# Custom application startup
kubectl run myapp --image=myapp:latest --command -- /usr/local/bin/start.sh --config=/etc/app.conf

# One-time task
kubectl run task --image=busybox --restart=Never --command -- /bin/sh -c "echo 'Task completed'"

Using kubectl create job

# Create a job with custom command
kubectl create job backup --image=postgres:13 --dry-run=client -o yaml -- pg_dump -h db.example.com mydb > backup.yaml

# Apply the job
kubectl apply -f backup.yaml

Complex Startup Command Examples

Multi-Step Initialization

apiVersion: v1
kind: Pod
metadata:
  name: complex-init
spec:
  containers:
  - name: app
    image: myapp:latest
    command: ["/bin/bash"]
    args:
    - "-c"
    - |
      set -e
      echo "Step 1: Checking dependencies..."
      /scripts/check-deps.sh

      echo "Step 2: Setting up configuration..."
      /scripts/setup-config.sh

      echo "Step 3: Running database migrations..."
      /scripts/migrate.sh

      echo "Step 4: Starting application..."
      exec /usr/bin/myapp --config=/etc/app/config.yaml
    volumeMounts:
    - name: scripts
      mountPath: /scripts
    - name: config
      mountPath: /etc/app
  volumes:
  - name: scripts
    configMap:
      name: init-scripts
      defaultMode: 0755
  - name: config
    configMap:
      name: app-config

Conditional Startup Logic

apiVersion: apps/v1
kind: Deployment
metadata:
  name: conditional-app
spec:
  template:
    spec:
      containers:
      - name: app
        image: myapp:latest
        command: ["/bin/sh"]
        args:
        - "-c"
        - |
          if [ "$APP_MODE" = "worker" ]; then
            exec /usr/bin/myapp worker --queue=$QUEUE_NAME
          elif [ "$APP_MODE" = "scheduler" ]; then
            exec /usr/bin/myapp scheduler --interval=60
          else
            exec /usr/bin/myapp server --port=8080
          fi
        env:
        - name: APP_MODE
          value: "server"
        - name: QUEUE_NAME
          value: "default"

Best Practices

1. Signal Handling and Graceful Shutdown

# ✅ Proper signal handling
apiVersion: v1
kind: Pod
spec:
  containers:
  - name: app
    image: myapp:latest
    command: ["/bin/bash"]
    args:
    - "-c"
    - |
      # Trap SIGTERM for graceful shutdown
      trap 'echo "Received SIGTERM, shutting down gracefully..."; kill -TERM $PID; wait $PID' TERM

      # Start the main application in background
      /usr/bin/myapp --config=/etc/app.conf &
      PID=$!

      # Wait for the process
      wait $PID

2. Error Handling and Logging

apiVersion: v1
kind: Pod
spec:
  containers:
  - name: app
    image: myapp:latest
    command: ["/bin/bash"]
    args:
    - "-c"
    - |
      set -euo pipefail  # Exit on error, undefined vars, pipe failures

      log() {
        echo "[$(date '+%Y-%m-%d %H:%M:%S')] $*" >&2
      }

      log "Starting application initialization..."

      if ! /scripts/health-check.sh; then
        log "ERROR: Health check failed"
        exit 1
      fi

      log "Starting main application..."
      exec /usr/bin/myapp --config=/etc/app.conf

3. Security Considerations

# ✅ Run as non-root user
apiVersion: v1
kind: Pod
spec:
  securityContext:
    runAsNonRoot: true
    runAsUser: 1000
    runAsGroup: 1000
  containers:
  - name: app
    image: myapp:latest
    command: ["/usr/bin/myapp"]
    args: ["--config=/etc/app.conf"]
    securityContext:
      allowPrivilegeEscalation: false
      readOnlyRootFilesystem: true
      capabilities:
        drop:
        - ALL

4. Resource Management

apiVersion: v1
kind: Pod
spec:
  containers:
  - name: app
    image: myapp:latest
    command: ["/usr/bin/myapp"]
    args: ["--config=/etc/app.conf"]
    resources:
      requests:
        memory: "64Mi"
        cpu: "250m"
      limits:
        memory: "128Mi"
        cpu: "500m"

Advanced Usage Patterns

1. Init Containers with Custom Commands

apiVersion: v1
kind: Pod
spec:
  initContainers:
  - name: setup
    image: busybox
    command: ["/bin/sh"]
    args:
    - "-c"
    - |
      echo "Setting up shared data..."
      mkdir -p /shared/data
      echo "Setup complete" > /shared/data/status
    volumeMounts:
    - name: shared-data
      mountPath: /shared
  containers:
  - name: app
    image: myapp:latest
    command: ["/bin/sh"]
    args:
    - "-c"
    - |
      while [ ! -f /shared/data/status ]; do
        echo "Waiting for setup to complete..."
        sleep 1
      done
      echo "Starting application..."
      exec /usr/bin/myapp
    volumeMounts:
    - name: shared-data
      mountPath: /shared
  volumes:
  - name: shared-data
    emptyDir: {}

2. Sidecar Containers with Different Commands

apiVersion: v1
kind: Pod
spec:
  containers:
  # Main application
  - name: app
    image: myapp:latest
    command: ["/usr/bin/myapp"]
    args: ["--config=/etc/app.conf"]

  # Log shipper sidecar
  - name: log-shipper
    image: fluent/fluent-bit:latest
    command: ["/fluent-bit/bin/fluent-bit"]
    args: ["--config=/fluent-bit/etc/fluent-bit.conf"]

  # Metrics exporter sidecar
  - name: metrics
    image: prom/node-exporter:latest
    command: ["/bin/node_exporter"]
    args: ["--path.rootfs=/host"]

3. Job Patterns with Custom Commands

# Backup job
apiVersion: batch/v1
kind: Job
metadata:
  name: database-backup
spec:
  template:
    spec:
      containers:
      - name: backup
        image: postgres:13
        command: ["/bin/bash"]
        args:
        - "-c"
        - |
          set -e
          echo "Starting backup at $(date)"
          pg_dump -h $DB_HOST -U $DB_USER $DB_NAME > /backup/dump-$(date +%Y%m%d-%H%M%S).sql
          echo "Backup completed at $(date)"
        env:
        - name: DB_HOST
          value: "postgres.example.com"
        - name: DB_USER
          value: "backup_user"
        - name: DB_NAME
          value: "myapp"
        volumeMounts:
        - name: backup-storage
          mountPath: /backup
      restartPolicy: Never
      volumes:
      - name: backup-storage
        persistentVolumeClaim:
          claimName: backup-pvc

Startup commands provide complete control over container execution in Kubernetes. By understanding how to properly configure and use startup commands, you can create flexible, maintainable, and robust containerized applications that meet your specific requirements.