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System Information

Understanding your system's hardware and software configuration

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    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.

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CPU and Memory Diagnostics

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.

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    htop

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

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    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

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    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

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Disk Usage and Health

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

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

    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.

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CPU and Memory Diagnostics

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

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    htop

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

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    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

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    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 ).

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    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

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Disk Usage and Health

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

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

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   df -h

Filesystem      Size  Used Avail Use% Mounted on
udev            3.8G     0  3.8G   0% /dev
tmpfs           783M  1.7M  781M   1% /run
/dev/sda2       233G   25G  197G  12% /
tmpfs

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   df -h

Filesystem      Size  Used Avail Use% Mounted on
udev            3.8G9G     039M  3.8G   0%1% /dev/shm
tmpfs           783M5.0M  14.7M0K  781M5.0M   1% /run
/dev/sda2/lock
tmpfs        233G   25G783M  197G 80K 12% /
tmpfs783M   1% /run/user/1000

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

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 du -sh /path/to/directory

1.1G     3.9G   39M  3.8G   1% /dev/shm
tmpfs           5.0M  4.0K  5.0M   1% /run/lock
tmpfs           783M   80K  783M   1% /run/user/1000

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

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 du -sh /path/to/directory

1.1G    /path/to/directory

/path/to/directory

fdisk -l # List all partitions

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 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

fdisk -l # List all partitionsiostat # 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 ).

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 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 filesystemiostat
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

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Process and Service Management

Managing processes and services is essential for maintaining system stability.

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 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)
     Memory: 8.8M
        CPU: 1013)
 1min 55.654s
     CGroup: /system.slice/apache2.service
             ├─  Memory: 8.8M
  40385 /usr/sbin/apache2 -k start
      CPU: 1min 55.654s
     CGroup:├─1000209 /system.slice/apache2.service
             ├─  40385 /usr/sbin/apache2 -k start
             ├─1000209 /usr/sbin/apache2 -k start
usr/sbin/apache2 -k start

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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.

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systemctl restart service_name # Restarts a service.

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)

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Logs and Monitoring

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

 14:46:00 2024] piix4_smbus 0000:00:01.3: SMBus Host Controller at 0x700, revision 0

2. Service 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.

/var/log/daemon.log - 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.' 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.

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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System Information

CPU and Memory Diagnostics

Disk Usage and Health

CPU and Memory Diagnostics

Disk Usage and Health

Process and Service Management

Logs and Monitoring