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LFCS: Essential Commands (20%)

This sectiondocument covers essential Linux commands for Git, services, system performance, application constraints, disk troubleshooting, and SSL management.

Goals

Essential Commands 20%

    Basic Git Operations Create, configure, and troubleshoot services Monitor and troubleshoot system performance and services Determine application and service specific constraints Troubleshoot diskspace issues Work with SSL certificates

    1. Basic Git Operations

    Learn

    • Clone repositories, check status, commit, and push.

    Commands

    # Clone repository
git clone <url>

# Check status
git status

# Add files and commit
git add <file>
git commit -m "message"

# Push to remote
git push origin main

# Pull latest changes
git pull

    Exercises

    1. Clone a public repository.
    2. Make a change, commit it, and push to a forked repo.

    2. Create, Configure, Troubleshoot Services

    Learn

    • Use systemctl to manage services.
    • Check logs for troubleshooting.

    Commands

    # Start, stop, restart service
sudo systemctl start <service>
sudo systemctl stop <service>
sudo systemctl restart <service>

# Enable/disable at boot
sudo systemctl enable <service>
sudo systemctl disable <service>

# Check logs
journalctl -u <service>

    Exercises

    1. Enable and start a service.
    2. Check the last 20 lines of the service log.

    3. Monitor & Troubleshoot System Performance

    Learn

    • Check CPU, memory, disk usage.
    • Identify resource-consuming processes.

    Commands

    # CPU and memory
top
htop
vmstat 2 5

# Disk usage
df -h
du -sh /path/to/dir

# Check running processes
ps aux | sort -nk 3 | tail -10

    Exercises

    1. Identify top 5 processes consuming CPU.
    2. Check disk usage of /var/log.

    4. Application & Service Constraints

    Learn

    • Resource limits for users or services.
    • Configure ulimit.

    Commands

    # Check limits
ulimit -a

# Set temporary limit
ulimit -n 4096

# Persistent limits
sudo nano /etc/security/limits.conf
# user soft nofile 4096
# user hard nofile 8192

    Exercises

    1. Increase the open file limit for a user temporarily.
    2. Make the change permanent.

    5. Troubleshoot Diskspace Issues

    Learn

    • Find large files or directories.
    • Free space using cleanup tools.

    Commands

    # Find large directories
du -ah / | sort -rh | head -20

# Remove old logs
sudo journalctl --vacuum-time=7d

    Exercises

    1. Identify top 10 largest files.
    2. Clear old logs to free space.

    6. Work with SSL Certificates

    Learn

    • Generate, verify, and check certificates.
    • Use OpenSSL.

    Commands

    # Generate self-signed certificate
openssl req -x509 -nodes -days 365 -newkey rsa:2048 -keyout server.key -out server.crt

# Verify certificate
openssl x509 -in server.crt -text -noout

# Check SSL connection
openssl s_client -connect example.com:443

    Exercises

    1. Generate a self-signed certificate.
    2. Inspect and verify the certificate using OpenSSL.

    Exam Tips

    • Practice using systemctl, top, df, du frequently.
    • Know basic Git workflows.
    • Be comfortable troubleshooting disk space and service issues.
    • Understand SSL certificate basics with OpenSSL.
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