On the fifty first day, I learned the following things about CI/CD Pipeline.

Build after another project is build (jenkins upstream and downstream)

Now let's take a look that how to link two jobs so that the previous one won't be executed unless the first one is executed.
If the first job(upstream) is executed then the second job(downstream) will start.
First create a job by the name of upstream and then go to the configure to write echo "Hello World" in the execute shell.
Then create the second job by the name of downstream and go to the configuration. Scroll down and in the build triggers option check the option that is build after other projects are build
It will give you an option to link the previous project(upstream) with downstream project.
Write the echo "Hello World" and sleep 5 commands in the execute shell.
Apply and then save the configuration.
Build the upstream job and wait for 5 seconds when the sleep time is over then the second job(downstream) will automatically be executed.
You can design a CI/CD pipeline using upstreaming and downstreaming.

Build after another project is build (failed, unstable job)

Open the newly created job(downstream job) configuration and scroll down to go to the build trigger option and check another option that is Trigger even the build fails.
Apply and save the configuration.
After applying if you made changes in the previous job(upstream job) and write an unexisted in it, the next job will still be executed even if the first one fails.
Now let's apply the unstable functionality in the upstream job.
First open the newly created job(downstream job) configuration and scroll down to go to the build trigger option and check another option that is Trigger even the build is unstable.
Apply and save the configuration.
Once you apply the changes, go to the previous job(upstream job), scroll down and write exit 10 in the Execute shell.
Go to the advanced option of the Execute shell and then write 10 in the Exit code to set build unstable option.
It will show that after 10 seconds, the upstream job will become unstable and downstream job will start executing.

If you want to execute a job and create build after every 2 minutes or any time that you want then you can do it.
To do this, first create a job and then go to the configuration. Scroll down and write date in the Execute shell.
After writing the data in the execute shell, scroll up and go to the build triggers option and check the Build periodically option.
Once it is checked then you can schedule the task by writing the cron job in it. You can get an example by clicking on the question mark.
Write H/2 * * * * to execute the job and build it after every two minutes.

If you want to monitor the changes in your github repository then you can also do this.
First create a job and then go to the configuration. Scroll down and go to the build triggers option and check the Poll SCM option.
Once it is checked then you can schedule the task by writing the cron job in it. You can get an example by clicking on the question mark.
Write H/2 * * * * to execute the job to monitor the changes after every 2 minutes.
The next is to create a github repository and take a link of that repository by clicking on the code button and getting the HTTPS link.
Once the link is copied, go to the jenkins dashboard and go to the source code management in the configuration.
Select the Git option, paste the link there and change the branch from master to main.
Go to the execute shell and write the following data into it.
```
ls
cat README.md
cat index.html
```
After making changes, apply and save the configuration.
Wait for the first build and if you wait for 2 minutes, the build will not happen because no changes are made in the github.
Now make some changes in the github and wait for 2 minutes. After 2 minutes, you will see that the build is created.
If you open the console output, you will see that changes are now present.

Here you can get an explanation in a video. 51/60 Day of DevOps Challenge