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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.
    uptime # Shows how long the system has been running and load averages.

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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

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           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

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

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 ).

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 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

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Process and Service Management

Managing processes and services is essential for maintaining system stability.

ps aux # Lists all running processes

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 ps aux

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 ?        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

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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: 1min 55.654s
     CGroup: /system.slice/apache2.service
             ├─  40385 /usr/sbin/apache2 -k start
             ├─1000209 /usr/sbin/apache2 -k start

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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.

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

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

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

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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/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

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 journalctl

-- Logs begin at Tue 2024-06-18 10:55:23 UTC, end at Tue 2024-06-18 12:06:19 UTC. --
Jun 18 10:55:23 hostname kernel: Linux version 5.4.0-74-generic (buildd@lcy01-amd64-013) (gcc version 9.3.0 (Ubuntu 9.3.0-17ubuntu1~20.04)) #83-Ubuntu SMP Thu May 6 10:34:06 UTC 2021 (Ubuntu 5.4.0-74.83-generic 5.4.101)
Jun 18 10:55:23 hostname kernel: Command line: BOOT_IMAGE=/boot/vmlinuz-5.4.0-74-generic root=UUID=5d7f5dcd-1234-5678-9abc-def012345678 ro quiet splash vt.handoff=7
...

journalctl -u service_name # Views Logs for a Specific Service

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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.

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.

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System Information

CPU and Memory Diagnostics

Disk Usage and Health

Process and Service Management

Logs and Monitoring