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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.

Monitoring CPU and memory usage helps in identifying performance bottlenecks.

Commands:

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.

    htop 

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

    vmstat -s
     
       980492 K total memory
       480208 K used memory
       132040 K active memory
       582364 K inactive memory
        67924 K free memory
        21724 K buffer memory
       410636 K swap cache
            0 K total swap

    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.

Commands:

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 

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

    vmstat -s
     
       980492 K total memory
       480208 K used memory
       132040 K active memory
       582364 K inactive memory
        67924 K free memory
        21724 K buffer memory
       410636 K swap cache
            0 K total swap

 free -h # Displays memory usage in "human-readable" format

    free -h
               total        used        free      shared  buff/cache   available
Mem:           7.7G        1.9G        3.9G        487M        1.9G        5.0G
Swap:          2.0G          0B        2.0G

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

df -h # Shows disk space  free -h # Displays memory usage in "human-readable" format

     freedf -h

Filesystem      Size  Used Avail Use% Mounted on
udev      total       3.8G     0  3.8G   0% /dev
tmpfs   used        free783M  1.7M  781M  shared 1% buff/cache   available
Mem:run
/dev/sda2       233G   25G 7.7G 197G  12%  /
tmpfs   1.9G        3.9G   39M  3.8G   487M 1% /dev/shm
tmpfs       1.9G    5.0M  4.0K  5.0G
Swap:0M      1% /run/lock
tmpfs    2.0G       783M   0B80K  783M      2.0G

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

...

1% /run/user/1000

du -sh /path/to/directory # Displays the size of a specific directory.

 du  df -h

Filesystem      Size  Used Avail Use% Mounted on
udev            3.8G     0  3.8G   0% /dev
tmpfs           783M  1.7M  781M   1% /run-sh /path/to/directory

1.1G    /path/to/directory

fdisk -l # List all partitions

 fdisk -l
Disk /dev/sda: 20 GiB, 21474836480 bytes, 41943040 sectors
Disk model: QEMU HARDDISK    
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: XXXXXX-XXX-XXX-XXXX-XXXXXXXXXX

Device     Start      End  Sectors Size Type
/dev/sda1   2048     4095     2048   1M BIOS boot
/dev/sda2   4096 41940607 41936512  20G Linux filesystem

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

 iostat
Linux 5.15.0-112-generic (testhost)    06/21/24  233G   25G  197G  12% /
tmpfs           3.9G_x86_64_   39M  3.8G   1% /dev/shm
tmpfs  (1 CPU)

avg-cpu:  %user   %nice      5.0M  4.0K  5.0M   1% /run/lock
tmpfs%system %iowait  %steal   %idle
           1.68   783M 0.11  80K  783M1.41   1% /run/user/1000

du -sh /path/to/directory # Displays the size of a specific directory.

 du -sh /path/to/directory

1.1G    /path/to/directory

 0.14    1.16   95.50

Managing processes and services is essential for maintaining system stability.

ps aux # Lists all running processesfdisk -l # List all partitions

 fdiskps -l
Disk /dev/sda: 20 GiB, 21474836480 bytes, 41943040 sectors
Disk model: QEMU HARDDISK    
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: XXXXXX-XXX-XXX-XXXX-XXXXXXXXXX

Device     Start      End  Sectors Size Type
/dev/sda1   2048     4095     2048   1M BIOS boot
/dev/sda2   4096 41940607 41936512  20G Linux filesystem

Managing processes and services is essential for maintaining system stability.

1. ps aux # Lists all running processes
   

 ps aux

USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  0.0  0.0 168800 10944aux

USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  0.0  0.0 168800 10944 ?        Ss   Jun12   0:02 /sbin/init
root         2  0.0  0.0      0     0 ?        S    Jun12   0:00 [kthreadd]
root         3  0.0  0.0      0     0 ?        SsI<   Jun12   0:02 /sbin/init00 [rcu_gp]
root         24  0.0  0.0      0     0 ?        S I<   Jun12   0:00 [kthreadd]
root         3  0.0  0.0      0     0 ?        I<   Jun12   0:00 [rcu_gp]
root         4  0.0  0.0      0     0 ?        I<   Jun12   0:00 [rcu_par_gp]
...

systemctl status service_name # Checks the status of a service

rcu_par_gp]
...

systemctl status service_name # Checks the status of a service

 systemctl status apache2

● apache2.service - The Apache HTTP Server
     Loaded: loaded (/lib/systemd/system/apache2.service; enabled; vendor preset: enabled)
     Active: active (running) since Wed 2024-05-29 16:06:04 UTC; 2 weeks 5 days ago
       Docs: https://httpd.apache.org/docs/2.4/
    Process: 1000205 ExecReload=/usr/sbin/apachectl graceful (code=exited, status=0/SUCCESS)
   Main PID: 40385 (apache2)
      Tasks: 55 (limit: 1013

 systemctl status apache2

● apache2.service - The Apache HTTP Server
     Loaded: loaded (/lib/systemd/system/apache2.service; enabled; vendor preset: enabled)
     ActiveMemory: active (running) since Wed 2024-05-29 16:06:04 UTC; 2 weeks 5 days ago8.8M
        CPU: 1min 55.654s
       DocsCGroup: https://httpd.apache.org/docs/2.4/system.slice/apache2.service
    Process: 1000205 ExecReload=         ├─  40385 /usr/sbin/apachectlapache2 graceful (code=exited, status=0/SUCCESS)
-k start
    Main PID: 40385 (apache2)
      Tasks: 55 (limit: 1013)
     Memory: 8.8M
        CPU: 1min 55.654s
     CGroup: /system.slice/apache2.service
             ├─  40385 /usr/sbin/apache2 -k start
             ├─1000209 /usr/sbin/apache2 -k start
├─1000209 /usr/sbin/apache2 -k start

systemctl restart service_name # Restarts a service.If a service is not behaving correctly, stopped responding, restarting 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

 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.Linux log files are stored in plain-text and can be found in the /var/log directory

Essential Linux Log Files:

/var/log/syslog or /var/log/messages - Stores all activity data across the global system. Activity for Redhat-based systems, such as CentOS or Rhel, are stored in messages, while Ubuntu and other Debian-based systems are stored in syslog.

/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.

/var/log/apt directory - on Debian-based systems we can find the logs for the apt package manager on Debian-based systems, directory contains several log files that provide important information about apt-related activities on the system.

...

Linux log files are stored in plain-text and can be found in the /var/log directory

journalctl -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:

1. System logs:

/var/log/syslog or /var/log/messages - Stores all activity data across the global system. Activity for Redhat-based (CentOS or Rhel) stored in messages, while Debian-based (Ubuntu) systems are stored in syslog.

/var/log/dmesg
dmesg allows reviewing messages stored in the Linux ring buffer, providing insights into hardware errors and startup issues. How To Use dmesg

 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
 revision 0

2. Service Logs:

/var/log/daemon.log - Tracks services running in the background that perform important tasks, but has no graphical output

journalctl # Views systemd logs and allows filtering Linux system logs to extract relevant information for monitoring and troubleshooting. How To Use journalctl

...

 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.

tail /var/log/syslog # Shows the last part of the logs, where problems usually lie.

...

...

 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.

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.

4. 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

6. Event Logs:

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

var/log/lastlog - Information about the last logins for all users. This binary file can be read by command lastlog.

.home-banner {
                    background: #459df0;
                    color: #fff;
                    font-size: 20px;
                    padding: 20px;
                    }
                    .home-banner h2 {
                    color: #fff;
                    }
                    .title-box {
                    border: 1px none #459df0;
                    padding: 10px;
                    }
                    .title-box > h2 {
                    background: #459df0;
                    bottom: 10px;
                    color: #fff;
                    margin-left: -10px;
                    margin-right: -10px;
                    padding: 2px 10px;
                    position: relative;
                    }

...