Linux Platform - Yash-777/LearnJava GitHub Wiki

Firewall Check

$ telnet github.com 443
Trying 140.82.118.3...
Connected to github.com.       [Firewal enabled]
telnet: connect to address 10.55.72.14: Connection timed out  [Firewal disabled/not-enabled]

How To Kill Process in Linux or Terminate a Process in UNIX / Linux Systems](https://www.cyberciti.biz/faq/kill-process-in-linux-or-terminate-a-process-in-unix-or-linux-systems/)

Syntax

kill -[signal] PID
kill -15 PID
kill -9 PID
kill -SIGTERM PID
kill [options] -SIGTERM PID

[Yash777@devappsr001 bin]$ ps -ef|grep java

[Yash777@devappsr001 bin]$ kill -l
 1) SIGHUP       2) SIGINT       3) SIGQUIT      4) SIGILL       5) SIGTRAP
 6) SIGABRT      7) SIGBUS       8) SIGFPE       9) SIGKILL     10) SIGUSR1
11) SIGSEGV     12) SIGUSR2     13) SIGPIPE     14) SIGALRM     15) SIGTERM

[Yash777@devappsr001 bin]$ kill -9 2675
-bash: kill: (2675) - Operation not permitted

-- Switch to a group having permission to kill the process
[Yash777@devappsr001 bin]$ sudo su - tomcat

[Yash777@devappsr001 bin]$ kill -9 2675

Give group the right to send kill signals

The problem is limiting damage. You can get quite close with sudo. Consider this sudoers entry:

%group1 ALL = (user1) pkill -HUP <name of process>
Then members of group1 can do:

sudo -u user1 pkill -HUP <name of process>

PortQryUI - User Interface for the PortQry Command Line Port Scanner. support.microsoft

Portqry.exe is a command-line utility that you can use to help troubleshoot TCP/IP connectivity issues. Portqry.exe runs on Windows 2000-based computers, on Windows XP-based computers, and on Windows Server 2003-based computers. The utility reports the port status of TCP and UDP ports on a computer that you select.

Portqry.exe reports the status of a TCP/IP port in one of the following three ways:

  • Listening : A process is listening on the port on the computer that you selected. Portqry.exe received a response from the port.
  • Not Listening: No process is listening on the target port on the target system. Portqry.exe received an Internet Control Message Protocol (ICMP) "Destination Unreachable - Port Unreachable" message back from the target UDP port. Or if the target port is a TCP port, Portqry received a TCP acknowledgement packet with the Reset flag set.
  • Filtered: The port on the computer that you selected is being filtered. Portqry.exe did not receive a response from the port. A process may or may not be listening on the port. By default, TCP ports are queried three times, and UDP ports are queried one time before a report indicates that the port is filtered.

PuTTY is an SSH and telnet client

WinSCP Free SFTP, SCP and FTP client for Windows Documentation

WinSCP is an open source free SFTP client, FTP client, WebDAV client, S3 client and SCP client for Windows. Its main function is file transfer between a local and a remote computer. Beyond this, WinSCP offers scripting and basic file manager functionality.

WinSCP functionality

Main Features

  • Graphical user interface
  • Integrated text editor
  • All common operations with files
    1. Binary and text transfer modes
    2. File masks for selecting files
    3. Operation masks for changing filenames
  • Scripting and task automation
  • Translated into many languages
  • Advanced transfer settings

In a class-based object-oriented language, in general, state is carried by instances, methods are carried by classes, and inheritance is only of structure and behavior.

Method signature: It consists of method name and parameter list (number/type/order of the parameters). methodName(parametersList y). An instance method in a subclass with the same signature and return type as an instance method in the super-class overrides the super-class's method.

Java OOP concepts
Class  - Collection of a common features of a group of object [static/instance Fields, blocks and Methods]
Object - Instance of a class (instance fields)
Abstraction - Process of hiding complex info and providing required info like API, Marker Interfaces ...
Encapsulation(Security) - Class Binding up with data members(fields) and member functions.
Inheritance  (Reusability by placing common code in single class)
 1. Multilevel - {A -> B -> C} 2. Multiple - Diamond problem {A <- (B) -> C} [Java not supports] 3. Cyclic {A <-> B} [Java not supports]
 * Is-A   Relation - Class A extends B
 * Hash-A Relation - Class A { B obj = new B(); } - (Composition/Aggregation)
Polymorphism (Flexibility) 1. Compile-Time Overloading    2. Runtime Overriding  [Greek - "many forms"]


int[] arr = {1,2,3}; int arrLength = arr.length;   // Fixed length of sequential blocks to hold same data type
String str = "Yash"; int strLength = str.length(); // Immutable Object value can't be changed.

List<?> collections = new ArrayList<String>(); int collectionGroupSize = collections.size();
Map<?, ?> mapEntry = new HashMap<String, String>();
Set<?> keySet = mapEntry.keySet(); // Set of Key's
Set<?> entrySet = mapEntry.entrySet(); // Set of Entries [Key, Value]

// Immutable Objects once created they can't be modified. final class Integer/String/Employee
Integer val = Integer.valueOf("100"); String str2 = String.valueOf(100); // Immutable classes
final class Employee { // All Wrapper classes, java.util.UUID, java.io.File ...
    private final String empName; // Field as Final(values can be assigned only once) Only getter functions.
    public Employee(String name) { this.empName = name; }
}

Native Java Code for Hashtable.h, Hashtable.cpp SQL API.

You can check your current JDK and JRE versions on your command prompt respectively,

  • JDK javac -version [C:\Program Files\Java\jdk1.8.0_121\bin] o/p: javac 1.8.0_121
  • JRE java -version [C:\Program Files\Java\jdk1.8.0_121\bin] o/P: java version "1.8.0_102"

JAVA_HOME - Must be set to JDK otherwise maven projects leads to compilation error. [ERROR] No compiler is provided in this environment. Perhaps you are running on a JRE rather than a JDK? C:\Softwares\OpenJDK\, 7-zip

Fatal error compiling: invalid target release: JRE and JDK must be of same version 1.8.0.XXX

Disable TLS 1.0 and 1.1security-libs/javax.net.ssl: TLS 1.0 and 1.1 are versions of the TLS protocol that are no longer considered secure and have been superseded by more secure and modern versions (TLS 1.2 and 1.3).

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