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System Information

Understanding your system's hardware and software configuration

Commands
    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. 

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.

    htop 

 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

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

   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.

 du -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        _x86_64_        (1 CPU)

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

Process and Service Management

Managing processes and services is essential for maintaining system stability.

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 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

 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


Сommands
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)


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

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

 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

 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

 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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