Shell Scripting Examples

Generate comma separated dates for last seven days:

# Fetch the date 7 days ago
DATE=$(date -d '7 day ago' '+%Y-%m-%d');

# Generate comma-separated string for last 7 days
for i in {6..1}; do DATE=$DATE","$(date -d "$i day ago" '+%Y-%m-%d'); done

Considering current date is 2016-04-08, DATE variable value will be as follows:

2016-04-01,2016-04-02,2016-04-03,2016-04-04,2016-04-05,2016-04-06,2016-04-07

We can use different date formats on a case to case basis. This thing comes in handy when we have to feed a date range to some reporting tool.

Generate comma separated dates for last month and from today to start of the day:

Java - SQL Parsing

Recently I had a use case where I have to validate a SQL query. Basically I had to look at the JOINS and WHERE clauses in the query.

I thought that why to parse a SQL on my own and extract table names and where clauses from the query. Why to reinvent the wheel and write your own logic when you can re-use existing open source libraries. Keeping that in mind I started searching for a simple open source library to serve the purpose.

Surprisingly I found only a few and those were not serving the purpose then I found Apache-Calcite(https://calcite.apache.org/). Apache-Calcite is a huge project and SQL parsing is just one part of it.

I didn't find a good example though using its SQL parser and had to dig in a bit to get a working example. Following is a working example:

So this is just about extracting table names and where clauses. This SQL parser can do definitely more and can be used to extract other parts of the query as well. Of course you can dig in the java docs and API to cater your requirements. Happy Learning !!!

Unit testing your POJO classes in Java.

Ever bothered about testing your pojo classes in java. Probably NO. We tend to ignore these pojo classes saying why should I test lame methods i.e. setter, getters, toString etc.

However these pojo classes adds to your test coverage report and if you don't have tests written for these classes you'll end up with a reduced test coverage stats for your java project.

What about delegating the testing responsibility to a framework or library which will ensure the quality of your pojo classes. Cool Huh !!!

I have got such a situation in my project and found this cool library POJO-TESTER to serve the purpose.

What about giving it a quick look around? Happy Testing !!!!

Push messages to browser/clients with NodeJS WebSocket

Hi Folks, off late I have been trying to do a POC on pushing messages from server to all the connected clients and found that websocket can be used for it in conjunction with NodeJS. So I am putting it all over to this post.

Following are the prerequisites in order to run the setup:

1. NodeJS Installation (We need npm to install packages) Download Link
2. NPM Packages (websocket, http-server, finalhandler, serve-static).

NPM package installation commands:

• npm install websocket
• npm install http-server
• npm install finalhandler
• npm install serve-static

After successful installation of NodeJS and above mentioned npm packages we can start off writing the code. We need following three files hosted on a folder (All the npm package installation commands will be executed on this folder).

1. frontend.html.
2. frontend.js (NodeJS Frontend).
3. backend.js (NodeJS Backend).

Following is the source code of above files:

frontend.html

frontend.js

backend.js

Following is the directory structure of project:

Limit typing numbers in input type 'number' HTML

Recently I have started front-end development using AngularJS where I had the following requirement:

1. Open a modal window.
2. Paint form on the modal.
3. Form will have a text field of type "number" with following criteria:
1. User can put only positive numbers in the text field nothing else.
2. User can put numbers only from 0-999.

At first above requirement seems to be very easy to implement with min and max properties of input type "number". However there is a catch that min and max restricts only the spinner (increase and decrease) but user can still type numbers which does not belong to 0-999 (Even characters).

To address first criteria we can use onkeypress event and pass it a function which will listen to 0-9 number key presses only and discard the rest:

onkeypress="return (event.charCode == 8 || event.charCode == 0) ? null : event.charCode >= 48 && event.charCode <= 57"

To address second criteria we can use oninput event and pass it a function which will clip/remove the characters after 3rd position:

oninput="this.value.length > 3 ? this.value= this.value.slice(0,3) : this.value"

Note: I have seen suggestions of an alternate approach to accomplish the above using onkeydown event however I have found that if we use onkeydown event it screws up the spinner and also allows text to be pasted and dragged having length more than 3 characters whereas oninput handles all these cases.

Following is the complete input tag:

<input type="number"
       name="sampleTextbox"
       min="0"
       max="999"
       oninput="this.value.length > 3 ? this.value= this.value.slice(0,3) : this.value"
       onkeypress="return (event.charCode == 8 || event.charCode == 0) ? null : event.charCode >= 48 && event.charCode <= 57"/>

Frequency of occurrence of Strings/Words in a list.

I had a problem where I have to keep a track of the frequency of occurrence of a given string in a record (row). I was thinking of doing this using a Map (Probably the best candidate here) where the key is the string and value is the frequency of occurrence.

But there was a catch here, what should we do on the first occurrence of the string as it won't be present in the Map. Below is the solution I came up with:

import java.util.HashMap;

public class MyHashMap {
       private HashMap<String, Integer> myHashMap;

       public MyHashMap(final HashMap<String, Integer> myHashMap) {
           this.myHashMap = myHashMap;
       }

       public void put(String key) {
           Integer integer = myHashMap.get(key);

           if (integer == null) {
                myHashMap.put(key, new Integer(1));
           } else {
                myHashMap.put(key, ++integer);
          }
        }

        @Override
        public String toString() {
            return "MyHashMap [myHashMap=" + myHashMap + "]";
         }

        public static void main(String[] args) {
             MyHashMap m = new MyHashMap(new HashMap<>());

             m.put("Steve");
             m.put("Steve");
             m.put("Simon");

             System.out.println(m);
       }
}

Output:

        MyHashMap [myHashMap={Simon=1, Steve=2}]

Above implementation works fine however you can see we had to do first occurrence handling in put method from our side which I wanted to avoid so I was looking at some other solution. No problem Java 8 Stream API is here and does this thing very smoothly in one liner.

Following is the Java 8 code snippet :

import java.util.ArrayList;

import java.util.List;

import java.util.Map;

import java.util.stream.Collectors;

import java.util.stream.Stream;

public class StreamsDemo {

public static void main(String[] args) {

List<String> list = new ArrayList<>();

list.add("Steve");

list.add("Steve");

list.add("Frank");

list.add("Tom");

list.add("Steve");

Stream<String> stream = list.stream();

Map<String, Long> collect = stream.collect(Collectors.groupingBy(e -> e, Collectors.counting()));

System.out.println(collect);

}

}

Output:

{Frank=1, Tom=1, Steve=3}

Above implementation which uses Java 8 Stream API does the trick as if we are executing a aggregate function COUNT(*) with GROUP BY. Cool !!

Create executable jar with Maven

If you are using Maven and wants to create an executable jar file but not getting an exact solution (Google is providing too many results and nothing is straightforward :-)). So let me get this straight and there is a plugin for it which is maven-assembly-plugin.

Below is the pom.xml which makes use of maven-assembly-plugin to generate executable jar:

<?xml version="1.0" encoding="UTF-8"?>
<project
    xmlns="http://maven.apache.org/POM/4.0.0"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 
       http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>com.example</groupId>
    <artifactId>test-project</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <properties>
        <targetJdk>1.7</targetJdk>
        <project.build.sourceEncoding>UTF-8
                          </project.build.sourceEncoding>
        <project.reporting.outputEncoding>UTF-8
                          </project.reporting.outputEncoding>
    </properties>
    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.5.1</version>
                <configuration>
                    <source>1.7</source>
                    <target>1.7</target>
                </configuration>
            </plugin>
            <plugin>
                <artifactId>maven-assembly-plugin</artifactId>
                <version>2.4</version>
                <configuration>
                    <archive>
                        <manifest>
                            <addClasspath>true</addClasspath>
                            <mainClass>com.test.App</mainClass>
                        </manifest>
                    </archive>
                    <descriptorRefs>
                        <descriptorRef>jar-with-dependencies
                                                </descriptorRef>
                    </descriptorRefs>
                </configuration>
                <executions>
                    <execution>
                        <id>make-jar-with-dependencies</id>
                        <phase>package</phase>
                        <goals>
                            <goal>single</goal>
                        </goals>
                    </execution>
                </executions>
            </plugin>
        </plugins>
    </build>
    <dependencies>
        <dependency>
            ......
            ......
        </dependency>
    </dependencies>
</project>

Now run "Maven install" and following executable jar file will be created : test-project-0.0.1-SNAPSHOT-jar-with-dependencies.jar