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 ${MY_VAR}). See GNU Bash Shell Parameters.
• Command Substitution: Capture command output (e.g., CURRENT_DATE=$(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). See Bash Conditional Expressions.
• 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; }). See Shell Functions documentation.
• Input/Output Redirection: > (overwrite), >> (append), < (input), 2> (stderr), | (pipe). See Redirections.
• 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. For detailed syntax, see the crontab(5) man page.

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.
• Use Crontab Guru to validate and explain cron expressions.

At Command:

Schedules a command to be run once at a specified future time. See the at(1) man page for complete usage.

# 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. See the systemd.timer documentation.

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:

Ansible is 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). See Ansible Inventory Guide.
• Modules: Small programs that perform specific tasks (e.g., apt, yum, service, copy, template). Browse the Module Index.
• Playbooks: YAML files that orchestrate tasks using modules against hosts in the inventory. See Playbook Guide.
• Roles: A way to organize playbooks into reusable components. See Ansible Roles documentation.

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