Bash Scripting for DevOps

# CHAPTER 17

Bash Scripting for DevOps #

1. Introduction #

2. Learning Objectives #

• Understand the role of Bash within a modern CI/CD pipeline.

• Write a script to automate Git repository updates (git pull).

• Write a script to automate the building and running of Docker containers.

• Safely manage sensitive environment variables (secrets) in deployment pipelines.

• Architect an automated, zero-downtime deployment workflow.

3. Bash in the CI/CD Pipeline #

1. 1. Build: Compiles the code (e.g., npm install).

1. 2. Test: Runs automated tests (e.g., npm test). If the test returns a non-zero exit code ($?), the script uses set -e to abort the pipeline.

1. 3. Deploy: Pushes the code to the production server.

4. Git Deployment Automation #

#!/bin/bash
set -e

APP_DIR="/var/www/my_app"
REPO_BRANCH="main"

echo "=== INITIATING DEPLOYMENT ==="

# 1. Navigate to the app directory
cd "$APP_DIR" || exit 1

# 2. Pull the latest code from GitHub
echo "Pulling latest code from branch: $REPO_BRANCH..."
git fetch origin
git reset --hard "origin/$REPO_BRANCH"

# 3. Apply changes (e.g., restart the web server to load new code)
echo "Restarting application service..."
systemctl restart my_app_service

echo "=== DEPLOYMENT SUCCESSFUL ==="

*(This script completely replaces the archaic practice of manually dragging and dropping files via FTP).*

5. Docker Container Automation #

#!/bin/bash
set -e

IMAGE_NAME="my_web_app"
CONTAINER_NAME="production_web"
PORT="8080"

echo "1. Building new Docker image..."
docker build -t "$IMAGE_NAME:latest" .

echo "2. Stopping existing container..."
# Use || true so the script doesn't crash if the container doesn't exist yet
docker stop "$CONTAINER_NAME" 2>/dev/null || true
docker rm "$CONTAINER_NAME" 2>/dev/null || true

echo "3. Launching new container..."
docker run -d --name "$CONTAINER_NAME" -p "$PORT:80" "$IMAGE_NAME:latest"

echo "Deployment complete. Application running on port $PORT."

6. Managing Secrets (.env files) #

The .env file:

DB_USER="admin"
DB_PASS="SuperSecret123!"

The Bash Script:

#!/bin/bash

# Source the environment file
source /secure/path/.env

# The variables are now safely loaded into memory!
echo "Connecting to database as $DB_USER..."
# Execute database connection using $DB_PASS...

7. Diagrams/Visual Suggestions #

• Arrow 1: To a Docker icon (Building image).

• Arrow 2: To an npm icon (Running tests).

• Arrow 3: To an AWS Cloud icon (Deploying container).

8. Best Practices #

• Immutable Infrastructure: When scripting deployments, do not write scripts that attempt to surgically fix a broken server (e.g., deleting bad files one by one). In modern DevOps, if a server is broken, you write a script to completely destroy the server and instantly deploy a brand-new, fresh clone. This is the philosophy of Docker and cloud computing. Treat servers like cattle, not pets.

9. Common Mistakes #

• Running git clone instead of git pull: Beginners often write deployment scripts that attempt to git clone the repository every single time. This fails on the second deployment because the directory already exists. You must clone the repository manually once, and then design your script to use git pull or git reset to update the existing code.

10. Mini Project: Build a "Deploy" Wrapper Tool #

1. 1. nano deploy_local.sh

1. 2. Write the code:

#!/bin/bash
set -e

# Validation
if [ "$#" -ne 1 ]; then
    echo "Usage: ./deploy_local.sh [branch_name]"
    exit 1
fi

BRANCH=$1
APP_DIR="/opt/dev_environment"

echo "[*] Checking out branch: $BRANCH"
cd "$APP_DIR"
git checkout "$BRANCH"
git pull origin "$BRANCH"

echo "[*] Tearing down old environment..."
docker-compose down

echo "[*] Rebuilding containers..."
docker-compose up -d --build

echo "[SUCCESS] Environment is live and tracking branch '$BRANCH'."

1. 3. Developers can now type ./deploy_local.sh feature-login to instantly spin up an entire testing infrastructure.

11. Practice Exercises #

1. 1. Explain the philosophical shift from traditional system administration (manually configuring an Apache web server) to modern DevOps CI/CD methodologies (Dockerizing an application via a Bash script).

1. 2. Detail the critical security vulnerability of hardcoding a database password directly into a deploy.sh script, and walk through the architectural solution utilizing a .env file.

12. MCQs with Answers #

13. Interview Questions #

• Q: A developer merges new code into the GitHub main branch. Walk me through the conceptual steps of how a CI/CD server utilizes a Bash script to transform that raw code into a live Docker container running on a production web server.

• Q: You are auditing a deployment script and notice the command git pull origin main. Explain why a professional DevOps engineer would replace this with git fetch followed by git reset --hard origin/main in a fully automated environment.

• Q: Explain the necessity of the set -e directive within the context of a Continuous Integration testing script. If a unit test fails, how does the Bash interpreter communicate that failure back to the Jenkins CI/CD dashboard?

14. FAQs #

15. Summary #

16. Next Chapter Recommendation #