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Understanding your system's hardware and software configuration

    uname -a # Displays kernel version and system architecture.
    lscpu  # Provides detailed information about the CPU.
    lsblk # Lists all available block devices.
    lsusb # Shows USB devices connected to the system.
    uptime # Shows how long the system has been running and load averages.Load average reflects the average number of processes waiting to run or use CPU resources over different time intervals, typically 1, 5, and 15 minutes.

Monitoring CPU and memory usage helps in identifying performance bottlenecks.

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htop is an interactive system monitor process viewer and process manager. It is designed as an alternative to the Unix program top. It shows a frequently updated list of the processes running on a computer, normally ordered by the amount of CPU usage.

Installation: on Ubuntu or Debian-based systems via command apt install htop or sudo yum install htop on CentOS

    htop 

 vmstat -s # Provides a detailed breakdown of memory usage and other related metrics since the last system boot. 

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mpstat # Reports CPU usage. ( requires install sysstat package on Ubuntu or Debian-based systems via command apt install sysstatinstall sysstat or sudo yum install sysstat on CentOS ).

    mpstat
Linux 5.15.0-112-generic (testhost)    06/21/24        _x86_64_        (1 CPU)

10:37:12     CPU    %usr   %nice    %sys %iowait    %irq   %soft  %steal  %guest  %gnice   %idle
10:37:12     all    1.68    0.11    1.38    0.14    0.00    0.04    1.16    0.00    0.00   95.49

Keeping track of disk usage and health is crucial for preventing data loss and ensuring efficient storage management.

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iostat # Reports CPU and I/O statistics ( requires install sysstat package on Ubuntu or Debian-based systems via command apt install sysstatinstall sysstat or sudo yum install sysstat on CentOS ).

 iostat
Linux 5.15.0-112-generic (testhost)    06/21/24        _x86_64_        (1 CPU)

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           1.68    0.11    1.41    0.14    1.16   95.50

Managing processes and services is essential for maintaining system stability.

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systemctl restart service_name # Restarts a service.

systemctlIf enablea service_name #is Enablesnot abehaving servicecorrectly, tostopped startresponding, atrestarting boot.
 
kill -9 process_id(PID) # Forces termination of a process.
(No output unless there is an error, the process is terminated forcefully)it can often resolve the issue without needing a full system reboot. Also, when you modify a service config file, using systemctl restart applies the changes immediately. 

systemctl enable service_name # Enables a service to start at boot.
 
kill -9 process_id(PID) # Forces termination of a process.
#(No output unless there is an error, the process is terminated forcefully)

Analyzing logs and monitoring system activities can help in identifying issues and tracking system performance.

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tail /var/log/syslog # Shows the last part of the logs, where problems usually liejournalctl -f # will show you the latest system logs and keep updating the display with new log entries as they are created, which is useful for monitoring live system activity.

Essential Linux Log Files:

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 dmesg -T 

[Wed May 29 14:46:00 2024] Run /init as init process
[Wed May 29 14:46:00 2024]   with arguments:
[Wed May 29 14:46:00 2024]     /init
[Wed May 29 14:46:00 2024]   with environment:
[Wed May 29 14:46:00 2024]     HOME=/
[Wed May 29 14:46:00 2024]     TERM=linux
[Wed May 29 14:46:00 2024]     BOOT_IMAGE=/boot/vmlinuz-5.15.0-97-generic
[Wed May 29 14:46:00 2024]     biosdevname=0
[Wed May 29 14:46:00 2024]     netcfg/do_not_use_netplan=true
[Wed May 29 14:46:00 2024] piix4_smbus 0000:00:01.3: SMBus Host Controller at 0x700, revision 0

2. Service Logs:

/var/log/authdaemon.log or /var/log/secure - Stores authentication logs, including both successful and failed logins and authentication methods. Debian/Ubuntu information is stored in /var/log/auth.log, while Redhat/CentrOS is stored in /var/log/secure./var/log/daemon.log - Tracks services running in the - Tracks services running in the background that perform important tasks, but has no graphical output

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 journalctl -u apache2

-- Logs begin at Tue 2024-06-18 10:55:23 UTC, end at Tue 2024-06-18 12:05:19 UTC. --
Jun 18 11:56:19 hostname systemd[1]: Starting The Apache HTTP Server...
Jun 18 11:56:19 hostname apache2[1342]: AH00558: apache2: Could not reliably determine the server's fully qualified domain name, using 127.0.1.1. Set the 'ServerName' directive globally to suppress this message
Jun 18 11:56:19 hostname systemd[1]: Started The Apache HTTP Server. 18 11:56:19 hostname systemd[1]: Started The Apache HTTP Server.

3. Authentication Logs:

/var/log/auth.log or /var/log/secure - Stores authentication logs, including both successful and failed logins and authentication methods. Debian/Ubuntu information is stored in /var/log/auth.log, while Redhat/CentrOS is stored in /var/log/secure.

43. Application Logs(examples):

/var/log/apache2/ - Apache HTTP server logs (access, error).

/var/log/mysql/ - MySQL database server logs.

5. Package Management Logs:

/var/log/apt directory - contains several log files that provide important information about apt-related activities on Debian-based systems.

/var/log/yum.log - Red Hat-based systems store the yum and dnf package manager logs, file provides information about the results of a specific command, such as any errors or warnings that were generated. For example, if a package update fails

46. Event Logs:

var/log/wtmp - Records of user logins and logouts.

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