startup.sh

Overview

`startup.sh` is a lightweight Bash script designed to perform a simple startup health check for a local service running on the host machine. Its primary function is to verify the availability of a service listening at `http://localhost:26657` by querying its `/status` endpoint. The script includes a conditional mechanism to disable the startup probe if a specific file (`/root/disable_startup`) exists, which allows for manual override or temporary bypass of the health check.

This script is typically used as a startup probe in containerized environments (e.g., Kubernetes) or init scripts to ensure that dependent services are available before proceeding with further application initialization or readiness signaling.

Script Breakdown and Explanation

#!/bin/bash

DISABLE_STARTUP_PROBE=/root/disable_startup

if [[ -f "$DISABLE_STARTUP_PROBE" ]]; then
  echo "startup probe disabled"
  exit 0
fi

curl -sf http://localhost:26657/status && exit 0 || exit 1

1. Shebang

#!/bin/bash

• Specifies the script interpreter as Bash. Ensures compatibility and correct execution in Unix-like environments.

2. Variable Declaration

DISABLE_STARTUP_PROBE=/root/disable_startup

• Defines a constant path variable that points to the file used to disable the startup probe check.

3. Disable Probe Check

if [[ -f "$DISABLE_STARTUP_PROBE" ]]; then
  echo "startup probe disabled"
  exit 0
fi

• Checks if the file /root/disable_startup exists.

• If it does, the script prints a message indicating the probe is disabled and exits successfully (exit 0).

• This allows operators or automated processes to temporarily disable the startup probe without modifying the script.

4. Health Check Using curl

curl -sf http://localhost:26657/status && exit 0 || exit 1

• Uses curl to silently (-s) and fail silently on server errors (-f) request the /status endpoint on localhost port 26657.

• If the request is successful (HTTP 2xx response), the script exits with a success status (exit 0).

• If the request fails (connection refused, timeout, or non-2xx response), it exits with failure (exit 1).

This simple mechanism provides a binary health indication that can be used by orchestrators or monitoring systems.

Usage Example

To use this script as a startup health check, you might:

1. Copy startup.sh into your container image or host system.

2. Set executable permissions:

   chmod +x startup.sh
   

3. Execute the script manually or configure it to run via container startup probes or init systems:

   ./startup.sh
   

4. If you want to disable the health check temporarily:

   touch /root/disable_startup
   ./startup.sh
   # Output: startup probe disabled
   

Important Implementation Details

• Disabling Mechanism: The use of a file as a "flag" to disable the probe is a lightweight and effective method to toggle behavior without changing code or configuration files.

• Health Endpoint: The script assumes the service provides a /status endpoint on localhost:26657. This endpoint should be implemented by the service to return a simple HTTP 200 status when healthy.

• Exit Codes: Exit codes conform to Unix conventions: 0 for success and 1 for failure. This behavior is critical for integration with container orchestrators that rely on exit codes to determine container readiness or liveness.

Interaction with Other System Components

• Service Under Test: The script depends on a local service listening on port 26657, likely part of the same application stack or pod/container.

• Container Orchestrators / Init Systems: The script is intended to be used as a startup probe or health check script. Orchestrators like Kubernetes can execute this script to decide if a container is ready to receive traffic or should be restarted.

• Operators: The presence of the /root/disable_startup file allows system operators or deployment scripts to disable the startup probe without modifying the script or container image.

Mermaid Flowchart Diagram

flowchart TD
    A[Start: Execute startup.sh] --> B{Check if /root/disable_startup exists}
    B -- Yes --> C[Print "startup probe disabled"]
    C --> D[Exit with 0 (success)]
    B -- No --> E[Execute curl -sf http://localhost:26657/status]
    E -- Success --> F[Exit with 0 (success)]
    E -- Failure --> G[Exit with 1 (failure)]

Summary

This script serves as a minimal but effective readiness or startup probe for a local service. It integrates smoothly into containerized environments or startup sequences, offering a simple disable mechanism and straightforward health check logic. Its compact design and clear exit codes make it ideal for automation and system orchestration workflows.