CompTIA Linux+ (XK0-005) Domain 3: Scripting, Automation, and Scheduling

Key Bash Scripting Concepts:

• Shebang: Scripts begin with #!/bin/bash (or other interpreter path) to specify the interpreter.
• Variables: Store data (e.g., MY_VAR="Hello World"; access with $MY_VAR or ${MY_VAR}).
• Command Substitution: Capture command output (e.g., CURRENT_DATE=$(date +%F)).
• Positional Parameters: Access script arguments ($1, $2, ..., $@ for all, $# for count).
• Conditional Statements:
  • if [ condition ]; then ... elif [ condition ]; then ... else ... fi
  • Test conditions for files (-f, -d, -e), strings (=, !=, -z, -n), and numbers (-eq, -ne, -gt, -lt).
• Loops:
  • for item in list; do ... done
  • while [ condition ]; do ... done
  • until [ condition ]; do ... done
• Functions: Define reusable blocks of code (e.g., my_function() { commands; }).
• Input/Output Redirection: > (overwrite), >> (append), < (input), 2> (stderr), | (pipe).
• Exit Status: Check command success with $? (0 for success, non-zero for failure). Use exit n to end script with status n.
• Make scripts executable: chmod +x script_name.sh.

Cron & Crontab:

Cron is a daemon for running scheduled tasks (cron jobs). Users manage their cron jobs using the crontab command.

Crontab Syntax (Minute Hour DayOfMonth Month DayOfWeek Command):

# Example: Run /opt/scripts/backup.sh every day at 2:30 AM
30 2 * * * /opt/scripts/backup.sh

• Edit crontab: crontab -e
• List crontab: crontab -l
• Remove crontab: crontab -r
• System-wide cron jobs can be placed in /etc/crontab or files within /etc/cron.d/, /etc/cron.hourly/, /etc/cron.daily/, etc.

At Command:

Schedules a command to be run once at a specified future time.

# Example: Run a command in 10 minutes
at now + 10 minutes
/usr/local/bin/my_one_time_script.sh
# Press Ctrl+D to finish

• List pending jobs: atq
• Remove a job: atrm

Systemd Timers:

A modern and more flexible alternative to cron, integrated with systemd. Timers are defined in .timer files and trigger associated .service files.

Example mybackup.timer:

[Unit]
Description=Run mybackup service daily

[Timer]
OnCalendar=daily
Persistent=true # Run on next boot if missed

[Install]
WantedBy=timers.target

Example mybackup.service:

[Unit]
Description=My Backup Script

[Service]
Type=oneshot
ExecStart=/opt/scripts/backup.sh

• Enable and start: sudo systemctl enable mybackup.timer && sudo systemctl start mybackup.timer
• List timers: systemctl list-timers --all

Python:

Widely used for its readability, extensive libraries (e.g., os, subprocess, requests, boto3 for AWS), and suitability for tasks ranging from simple scripts to large applications.

#!/usr/bin/env python3
import os
import datetime

backup_dir = "/mnt/backups"
source_dir = "/home/user/data"
timestamp = datetime.datetime.now().strftime("%Y-%m-%d_%H-%M-%S")
backup_file = f"{backup_dir}/data_backup_{timestamp}.tar.gz"

# Simple example of a backup command
os.system(f"tar -czf {backup_file} {source_dir}")
print(f"Backup created: {backup_file}")

Execute with: python3 script_name.py or ./script_name.py (if executable and shebang is set).

Ansible:

An agentless automation tool that uses SSH (by default) to connect to managed nodes. Configurations are defined in YAML-based "playbooks."

• Inventory: Defines managed hosts (e.g., /etc/ansible/hosts).
• Modules: Small programs that perform specific tasks (e.g., apt, yum, service, copy, template).
• Playbooks: YAML files that orchestrate tasks using modules against hosts in the inventory.
• Roles: A way to organize playbooks into reusable components.

Example Ansible Playbook (install_nginx.yml):

- name: Install and configure Nginx
  hosts: webservers
  become: yes # Equivalent to sudo
  tasks:
    - name: Install Nginx package
      ansible.builtin.apt: # Using FQCN (Fully Qualified Collection Name)
        name: nginx
        state: present
        update_cache: yes

    - name: Ensure Nginx service is started and enabled
      ansible.builtin.service:
        name: nginx
        state: started
        enabled: yes

Run playbook: ansible-playbook -i inventory_file install_nginx.yml