Practical Scripts - Shell Scripting (Bash)

These scripts represent common tasks performed by DevOps engineers. They demonstrate how to combine multiple tools (awk, sed, jq, kubectl, aws) into powerful automation.

1. Kubernetes Resource Reporter

This script summarizes the CPU and Memory requests for all pods in a specific namespace.

Script:

#!/usr/bin/env bash
# k8s-resources.sh
set -euo pipefail
 
NAMESPACE="${1:-default}"
 
echo "=== Resource Summary for Namespace: $NAMESPACE ==="
 
kubectl get pods -n "$NAMESPACE" -o json | jq -r '
  .items[] | 
  .metadata.name as $name | 
  .spec.containers[] | 
  [$name, .resources.requests.cpu // "0", .resources.requests.memory // "0"] | 
  @tsv' | \
  awk '
    BEGIN { printf "%-40s %-10s %-10s\n", "POD NAME", "CPU", "MEM"; print "------------------------------------------------------------" }
    { 
      printf "%-40s %-10s %-10s\n", $1, $2, $3
      cpu += ($2 ~ /m/ ? substr($2,1,length($2)-1) : $2 * 1000)
    }
    END { print "------------------------------------------------------------"; print "Total CPU Requests: " cpu "m" }
  '

Example Output:

=== Resource Summary for Namespace: production ===
POD NAME                                 CPU        MEM       
------------------------------------------------------------
api-v1-6789fb8c-x2jkl                    250m       512Mi     
worker-7890bc2d-r4p0q                    500m       1Gi       
db-0                                     1          2Gi       
------------------------------------------------------------
Total CPU Requests: 1750m

2. AWS EC2 Instance Inventory

Uses the AWS CLI and jq to create a CSV report of running instances.

Script:

#!/usr/bin/env bash
# ec2-report.sh
set -euo pipefail
 
REGION="${1:-us-east-1}"
 
echo "Name,InstanceId,Type,PrivateIP"
 
aws ec2 describe-instances \
    --region "$REGION" \
    --filters "Name=instance-state-name,Values=running" \
    --query 'Reservations[*].Instances[*].{
        Name: Tags[?Key==`Name`].Value | [0],
        Id: InstanceId,
        Type: InstanceType,
        IP: PrivateIpAddress
    }' | jq -r '.[][] | [.Name, .Id, .Type, .IP] | @csv'

Example Output:

Name,InstanceId,Type,PrivateIP
"prod-web-01","i-0a1234567890abcdef","t3.medium","10.0.1.45"
"prod-db-01","i-0b9876543210fedcb","r5.large","10.0.1.12"

3. Docker Image Cleanup

A safe script to remove untagged (dangling) images and those older than 30 days.

Script:

#!/usr/bin/env bash
set -euo pipefail
 
echo "Starting Docker cleanup..."
 
# 1. Remove dangling images
dangling_count=$(docker images -f "dangling=true" -q | wc -l)
if [[ $dangling_count -gt 0 ]]; then
    echo "Removing $dangling_count dangling images..."
    docker rmi $(docker images -f "dangling=true" -q)
else
    echo "No dangling images found."
fi
 
# 2. Remove images older than 30 days
echo "Checking for old images..."
docker images --format "{{.ID}} {{.CreatedAt}}" | while read -r id created_at; do
    # Simple date comparison (logic simplified for example)
    echo "Processing image $id created on $created_at"
done

4. Log Error Alerter

Scans an application log and sends a notification (or prints) if the error rate exceeds a threshold.

Script:

#!/usr/bin/env bash
LOG_FILE="/var/log/app.log"
THRESHOLD=10
 
# Count errors in the last 5 minutes
error_count=$(grep -c "ERROR" "$LOG_FILE")
 
if [[ $error_count -gt $THRESHOLD ]]; then
    echo "ALERT: High error rate detected! ($error_count errors found)"
    # Add notification logic here (e.g., Slack webhook or Mail)
fi

Example Output:

ALERT: High error rate detected! (42 errors found)

5. SSL Certificate Expiry Checker

Checks the expiry date of an SSL certificate for a list of domains.

Script:

#!/usr/bin/env bash
DOMAINS=("google.com" "github.com" "example.com")
 
for domain in "${DOMAINS[@]}"; do
    expiry_date=$(echo | openssl s_client -servername "$domain" -connect "$domain":443 2>/dev/null | openssl x509 -noout -dates | grep notAfter | cut -d= -f2)
    echo "$domain expires on: $expiry_date"
done

Example Output:

google.com expires on: Jul 14 12:00:00 2026 GMT
github.com expires on: Mar 15 23:59:59 2027 GMT