Infrastructure as Code Concepts

Category: DevOps and System Tools
Type: Linux Commands
Generated on: 2025-07-10 03:22:28
For: System Administration, Development & Technical Interviews

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Infrastructure as Code Concepts Cheatsheet (Linux Commands - DevOps & System Tools)

This cheatsheet covers essential Linux commands frequently used in Infrastructure as Code (IaC) and DevOps workflows. It aims to be a quick reference for sysadmins and developers alike.

1. Command Overview:

• ssh: Secure Shell; remotely access and manage servers. Essential for IaC execution on remote machines.
• scp: Secure Copy; securely transfer files between systems. Used for deploying configurations and artifacts.
• rsync: Remote Sync; efficient file synchronization. Ideal for replicating configurations and backups.
• sed: Stream EDitor; perform text transformations. Used extensively for configuration templating and automation.
• awk: Pattern scanning and processing language; powerful text processing. Used for data extraction and reporting.
• grep: Global Regular Expression Print; search for patterns in files. Used for log analysis and configuration validation.
• find: Locate files based on criteria. Used for managing files across a system.
• xargs: Build and execute command lines from standard input. Used to chain commands and process large lists of files.
• curl: Transfer data with URLs; used for API interactions and downloading resources.
• jq: JSON processor; parse, filter, and transform JSON data. Essential for working with APIs.
• systemctl: Manage systemd services; control services on modern Linux systems.
• docker: Containerization platform; build, run, and manage containers.
• kubectl: Kubernetes command-line tool; manage Kubernetes clusters.
• ansible: Automation engine; orchestrate configuration management and application deployment.
• terraform: Infrastructure as Code tool; define and manage infrastructure across multiple cloud providers.

2. Basic Syntax:

• ssh: ssh [user@]host [command]
• scp: scp [options] source destination
• rsync: rsync [options] source destination
• sed: sed [options] 'command' file
• awk: awk '{action}' file
• grep: grep [options] pattern file
• find: find path [expression]
• xargs: command | xargs [options] command
• curl: curl [options] URL
• jq: jq [options] 'filter' file
• systemctl: systemctl [command] service
• docker: docker [command] [options] image
• kubectl: kubectl [command] [options] resource
• ansible: ansible [host-pattern] -m module -a 'arguments'
• terraform: terraform [command]

3. Practical Examples:

• ssh:

  # Connect to server 'webserver' as user 'deploy'
  ssh deploy@webserver
  
  # Execute a command on the remote server and exit
  ssh deploy@webserver "uptime"

  Output:

  14:32:01 up 10 days, 1:22,  1 user,  load average: 0.01, 0.02, 0.00

• scp:

  # Copy a file to a remote server
  scp local_file.txt deploy@webserver:/tmp/
  
  # Copy a directory recursively
  scp -r local_directory deploy@webserver:/tmp/

• rsync:

  # Sync a local directory to a remote server, recursively and with compression
  rsync -avz local_directory deploy@webserver:/tmp/
  
  # Sync a remote directory to a local directory
  rsync -avz deploy@webserver:/tmp/remote_directory ./local_directory

• sed:

  # Replace all occurrences of 'old_string' with 'new_string' in a file
  sed 's/old_string/new_string/g' input.txt > output.txt
  
  # Replace in-place (be careful!)
  sed -i 's/old_string/new_string/g' input.txt
  
  # Replace based on a regular expression
  sed 's/pattern/replacement/g' file.txt

• awk:

  # Print the first column of each line
  awk '{print $1}' file.txt
  
  # Print lines where the second column is greater than 10
  awk '$2 > 10 {print}' file.txt
  
  # Sum the values in the third column
  awk '{sum += $3} END {print sum}' file.txt

• grep:

  # Search for a pattern in a file
  grep "error" logfile.txt
  
  # Search recursively in a directory
  grep -r "error" /var/log/
  
  # Case-insensitive search
  grep -i "error" logfile.txt

• find:

  # Find all files in the current directory
  find . -type f
  
  # Find all files modified in the last 24 hours
  find . -type f -mtime -1
  
  # Find all files with a specific name
  find . -name "important.txt"
  
  # Find all files and execute a command on them
  find . -type f -exec chmod 644 {} \;  #WARNING: Be careful with this.

• xargs:

  # Find all files and delete them (WARNING: VERY DANGEROUS!)
  find . -name "*.tmp" -print0 | xargs -0 rm -f
  
  # Find all files and compress them
  find . -name "*.txt" -print0 | xargs -0 gzip

• curl:

  # Download a file
  curl -o output.txt https://example.com/file.txt
  
  # Send a POST request with data
  curl -d "param1=value1&param2=value2" https://example.com/api
  
  # Get data from an API with authentication
  curl -u user:password https://example.com/api

• jq:

  # Pretty print JSON data
  curl -s https://api.example.com/data | jq .
  
  # Extract a specific field
  curl -s https://api.example.com/data | jq '.name'
  
  # Filter data based on a condition
  curl -s https://api.example.com/data | jq '.[] | select(.status == "active")'

• systemctl:

  # Start a service
  sudo systemctl start nginx
  
  # Stop a service
  sudo systemctl stop nginx
  
  # Restart a service
  sudo systemctl restart nginx
  
  # Check the status of a service
  systemctl status nginx
  
  # Enable a service to start on boot
  sudo systemctl enable nginx
  
  # Disable a service from starting on boot
  sudo systemctl disable nginx

• docker:

  # Pull an image
  docker pull ubuntu:latest
  
  # Run a container
  docker run -d -p 80:80 nginx
  
  # List running containers
  docker ps
  
  # Stop a container
  docker stop <container_id>
  
  # Remove a container
  docker rm <container_id>
  
  # Build an image from a Dockerfile
  docker build -t my-app .

• kubectl:

  # Get all pods
  kubectl get pods
  
  # Get details about a specific pod
  kubectl describe pod <pod_name>
  
  # Apply a Kubernetes configuration file
  kubectl apply -f deployment.yaml
  
  # Delete a resource
  kubectl delete deployment <deployment_name>
  
  # Get logs from a pod
  kubectl logs <pod_name>

• ansible:

  # Ping all hosts defined in the inventory file
  ansible all -m ping
  
  # Execute a shell command on all hosts
  ansible all -m shell -a "uptime"
  
  # Copy a file to all hosts
  ansible all -m copy -a "src=local_file.txt dest=/tmp/"
  
  # Run a playbook
  ansible-playbook my_playbook.yml

• terraform:

  # Initialize Terraform
  terraform init
  
  # Plan the changes
  terraform plan
  
  # Apply the changes
  terraform apply
  
  # Destroy the infrastructure
  terraform destroy

4. Common Options:

• ssh:

  • -i: Specify identity (private key) file.
  • -p: Specify port number.
  • -v: Verbose mode for debugging.
  • -N: Do not execute a remote command (port forwarding).
  • -L: Local port forwarding.
  • -R: Remote port forwarding.

• scp:

  • -r: Recursive copy (for directories).
  • -p: Preserve modification times, access times, and modes.
  • -C: Enable compression.
  • -q: Quiet mode.

• rsync:

  • -a: Archive mode (recursive, preserves permissions, etc.).
  • -v: Verbose mode.
  • -z: Enable compression.
  • -P: Show progress and keep partially transferred files if interrupted.
  • --delete: Delete extraneous files on the destination.
  • --exclude: Exclude files or directories.

• sed:

  • -i: Edit file in place. USE WITH CAUTION!
  • -n: Suppress automatic printing of pattern space.
  • -e: Execute multiple commands.
  • -r: Use extended regular expressions.

• awk:

  • -F: Specify field separator.
  • -v: Assign a variable.

• grep:

  • -i: Case-insensitive search.
  • -r: Recursive search.
  • -n: Show line numbers.
  • -v: Invert match (show lines not matching).
  • -c: Count the number of matching lines.

• find:

  • -type: Specify file type (e.g., f for file, d for directory).
  • -name: Specify file name.
  • -mtime: Specify modification time (in days).
  • -exec: Execute a command on found files.
  • -delete: Delete found files. USE WITH EXTREME CAUTION!

• xargs:

  • -n: Maximum number of arguments per command line.
  • -I: Replace occurrences of a placeholder in the command.
  • -0: Expect input items to be terminated by a null character (for handling filenames with spaces).

• curl:

  • -o: Specify output file.
  • -d: Send POST data.
  • -H: Add a custom header.
  • -u: Specify username and password.
  • -X: Specify request method (e.g., POST, PUT, DELETE).
  • -s: Silent mode.

• jq:

  • .: Represents the entire input.
  • []: Array/object value iterator.
  • |: Pipe output from one filter to another.
  • .key: Access object properties.
  • [index]: Access array elements.

• systemctl:

  • start: Start a service.
  • stop: Stop a service.
  • restart: Restart a service.
  • status: Check the status of a service.
  • enable: Enable a service to start on boot.
  • disable: Disable a service from starting on boot.

• docker:

  • run: Run a container.
  • ps: List containers.
  • stop: Stop a container.
  • rm: Remove a container.
  • build: Build an image from a Dockerfile.
  • pull: Pull an image from a registry.

• kubectl:

  • get: Get resources.
  • describe: Get details about a resource.
  • apply: Apply a configuration file.
  • delete: Delete a resource.
  • logs: Get logs from a pod.

• ansible:

  • -m: Specify the module to use.
  • -a: Specify arguments for the module.
  • -k: Ask for SSH password.
  • -i: Specify inventory file.

• terraform:

  • init: Initialize Terraform.
  • plan: Plan the changes.
  • apply: Apply the changes.
  • destroy: Destroy the infrastructure.

5. Advanced Usage:

• ssh: Port forwarding for secure tunneling:

  # Local port forwarding: Access a service on the remote server via localhost
  ssh -L 8080:localhost:80 deploy@webserver
  
  # Remote port forwarding: Allow others to access a service on your local machine via the remote server
  ssh -R 8080:localhost:80 deploy@webserver

• sed: Complex substitutions with backreferences:

  # Swap the first and second words on each line
  sed 's/\(\w\+\) \(\w\+\)/\2 \1/' file.txt

• awk: Using custom functions and arrays:

  # Calculate the average of numbers in a file
  awk '{sum += $1; count++} END {print sum/count}' numbers.txt

• find + xargs: Parallel execution for faster processing:

  # Find all files and compress them in parallel using multiple cores
  find . -name "*.txt" -print0 | xargs -0 -P 4 gzip  # -P 4 uses 4 parallel processes

• curl + jq: Chaining to extract specific data from APIs:

  # Get the public IP address of the current machine using an external API
  curl -s https://api.ipify.org?format=json | jq -r .ip

• ansible: Using with_items to iterate over a list of items:

  - name: Create users
    user:
      name: "{{ item }}"
      state: present
    loop:
      - user1
      - user2
      - user3

• terraform: Using variables and modules for reusable infrastructure:

  variable "region" {
    type    = string
    default = "us-east-1"
  }
  
  module "ec2_instance" {
    source = "./modules/ec2"
    ami    = "ami-0c55b4cdcec567a0a" # Example AMI ID
    region = var.region
  }

6. Tips & Tricks:

• Shell Aliases: Define aliases for frequently used commands to save time and reduce typing errors. Example: alias k=kubectl
• Bash History: Use Ctrl+R to search your command history.
• Tab Completion: Use Tab to auto-complete commands, file names, and options.
• Pipe to less: Pipe long output to less for easier viewing (command | less). Use Spacebar to go down one page, q to quit.
• tee: Use tee to both display output on the screen and save it to a file (command | tee output.txt).
• history | grep <command>: Search your command history for a specific command.

7. Troubleshooting:

• ssh Connection Refused: Ensure the SSH service is running on the remote server and that the firewall is not blocking connections on port 22 (or the custom SSH port).
• scp Permission Denied: Verify you have the necessary permissions to write to the destination directory on the remote server.
• sed Incorrect Substitution: Double-check your regular expressions and escape characters. Use a testing tool like regex101.com to validate your regex.
• find Unexpected Results: Carefully review your search criteria and ensure the path is correct.
• curl Connection Errors: Check your network connection and ensure the URL is correct.
• jq Invalid JSON: Verify the JSON data is valid using a validator like jsonlint.com.
• systemctl Service Fails to Start: Check the service logs for errors using journalctl -u <service_name>.
• docker Container Fails to Start: Check the container logs for errors using docker logs <container_id>.
• kubectl Resource Not Found: Ensure the resource exists in the specified namespace and that you have the correct permissions.
• ansible Host Unreachable: Verify the host is reachable via SSH and that the inventory file is correctly configured. Check SSH key permissions.
• terraform State Corruption: Use terraform state pull and terraform state push to back up and restore your Terraform state. Consider using remote state storage like S3 or Azure Blob Storage.

8. Related Commands:

• Package Management: apt, yum, dnf, pacman (for installing and managing software packages).
• Networking: ifconfig, ip, netstat, ss, ping, traceroute (for network configuration and troubleshooting).
• File Management: ls, cd, mkdir, rm, cp, mv, chmod, chown (for basic file operations).
• Process Management: ps, top, htop, kill, pkill (for monitoring and managing processes).
• User Management: useradd, userdel, usermod, passwd (for managing user accounts).
• Disk Management: df, du, fdisk, mkfs (for managing disk space and partitions).
• Cloud-Specific CLIs: aws, gcloud, az (for interacting with cloud services).

This cheatsheet provides a solid foundation for using Linux commands in Infrastructure as Code workflows. Remember to always test your commands in a safe environment before applying them to production systems. Always double-check destructive commands like rm -rf and find -delete.