GIT Interview Questions 2

1. GIT Commit command
   The command that is used to write a commit message is “git commit -a”
   Now explain about -a flag by saying -a on the command line instructs git to commit the new content of all tracked files that have been modified. Also, mention you can use “git add <file>” before git commit -a if new files need to be committed for the first time.
   
   
2. How can you fix a broken commit?
   In order to fix any broken commit, use the command “git commit --amend”. When you run this command, you can fix the broken commit message in the editor.
   
   

3.  Basic Git commands:

   Command	Function
   git rm [file]	deletes the file from your working directory and stages the deletion.
   git log	list the version history for the current branch.
   git show [commit]	shows the metadata and content changes of the specified commit.
   git tag [commitID]	used to give tags to the specified commit.
   git checkout [branch name] git checkout -b [branch name]	used to switch from one branch to another. creates a new branch and also switches to it.

   
   
4. In Git how do you revert a commit that has already been pushed and made public?
   There can be two approaches to tackle this question and make sure that you include both because any of the below options can be used depending on the situation:
   
   Remove or fix the bad file in a new commit and then push it to the remote repository. This is the most obvious way to fix an error. Once you have made necessary changes to the file, then commit it to the remote repository using the command: git commit -m “commit message”
   
   Also, you can create a new commit that undoes all changes that were made in the bad commit. To do this use the command
   
   git revert <name of bad commit>
   
   
5. What is the difference between git pull and git fetch?
   Git pull command pulls new changes or commits from a particular branch from your central repository and updates your target branch in your local repository.
   
   Git fetch is also used for the same purpose but it works in a slightly different way. When you perform a git fetch, it pulls all new commits from the desired branch and stores it in a new branch in your local repository. If you want to reflect these changes in your target branch, git fetch must be followed with a git merge. Your target branch will only be updated after merging the target branch and fetched branch. Just to make it easy for you, remember the equation below:
   
   Git pull = git fetch + git merge
   
   
6. What is git stash?
   Often, when you’ve been working on part of your project, things are in a messy state and you want to switch branches for some time to work on something else. The problem is, you don’t want to do a commit of half-done work just so you can get back to this point later. The answer to this issue is Git stash.
   
   Stashing takes your working directory that is, your modified tracked files and staged changes and saves it on a stack of unfinished changes that you can reapply at any time.
   
   
7. What is the function of ‘git stash apply’?
   If you want to continue working where you had left your work then ‘git stash apply‘ command is used to bring back the saved changes onto your current working directory.
   
   
8. What is git stash drop?
   Git ‘stash drop’ command is used to remove the stashed item. It will remove the last added stash item by default, and it can also remove a specific item if you include it as an argument.
   
   Now give an example.
   
   If you want to remove a particular stash item from the list of stashed items you can use the below commands:

• git stash list: It will display the list of stashed items like:
• stash@{0}: WIP on master: 049d078 added the index file
• stash@{1}: WIP on master: c264051 Revert “added file_size”
• stash@{2}: WIP on master: 21d80a5 added number to log
  
  If you want to remove an item named stash@{0} use command git stash drop stash@{0}.
  
  

1. Can you explain the Gitflow workflow?
   To record the history of the project, Gitflow workflow employs two parallel long-running branches – master and develop:
   
   Master – this branch is always ready to be released on LIVE, with everything fully tested and approved (production-ready).
   
   Hotfix – these branches are used to quickly patch production releases. These branches are a lot like release branches and feature branches except they’re based on master instead of develop.
   
   Develop – this is the branch to which all feature branches are merged and where all tests are performed. Only when everything’s been thoroughly checked and fixed it can be merged to the master.
   
   Feature – each new feature should reside in its own branch, which can be pushed to the develop branch as their parent one.
   
   
2. What is Git fork? What is the difference between fork, branch, and clone?
   A fork is a copy of a repository. Normally you fork a repository so that you are able to freely experiment with changes without affecting the original project. Most commonly, forks are used to either propose changes to someone else’s project or to use someone else’s project as a starting point for your own idea.
   
   git cloning means pointing to an existing repository and make a copy of that repository in a new directory, at some other location. The original repository can be located on the local file system or on remote machine accessible supported protocols. The git clone command is used to create a copy of an existing Git repository.
   
   In very simple words, git branches are individual projects within a git repository. Different branches within a repository can have completely different files and folders, or it could have everything the same except for some lines of code in a file.
   
   
3. What is the difference between rebasing and merge in Git?
   In Git, the rebase command is used to integrate changes from one branch into another. It is an alternative to the “merge” command. The difference between rebasing and merge is that rebase rewrites the commit history in order to produce a straight, linear succession of commits.
   
   Merging is Git’s way of putting a forked history back together again. The git merge command helps you take the independent lines of development created by git branch and integrate them into a single branch.
   
   
4. What is git cherry-pick?
   The command git cherry-pick is normally used to introduce particular commits from one branch within a repository onto a different branch. Another common use is to forward- or back-port commits from a maintenance branch to a development branch. This is in contrast with other ways such as merge and rebase which normally apply many commits onto another branch.
   
   Consider:
   
   git cherry-pick <commit-hash>
   
   
5. How do you squash the last N commits into a single commit?
   There are two options to squash the last N commits into a single commit include both of the below-mentioned options in your answer
   If you want to write the new commit message from scratch use the following command
   
   git reset –soft HEAD~N &&git commit
   
   If you want to start editing the new commit message with a concatenation of the existing commit messages then you need to extract those messages and pass them to Git commit for that I will use
   
   git reset –soft HEAD~N &&git commit –edit -m”$(git log –format=%B –reverse .HEAD@{N})”
   
   
6. How do I rename a local Git branch?
   Here are the steps to rename the branch:
   
   Switch to the branch which needs to be renamed
   
   git branch -m <new_name>
   git push origin :<old_name>
   git push origin <new_name>:refs/heads/<new_name>

Difference between MVC and Viper architecture

https://themindstudios.com/blog/mvp-vs-mvc-vs-mvvm-vs-viper/

What is MVVM?

MVVM - Model View ViewModel

Advantage : 

Better Separation of Concerns

In a project built with the Model-View-Controller pattern, you are often faced with the question which code goes where. Code that doesn’t fit or belong in the model or view layer is often put in the controller layer. This inevitably leads to fat controllers that are difficult to test and manage.

The MVVM pattern presents a better separation of concerns by adding view models to the mix. The view model translates the data of the model layer into something the view layer can use. The controller is no longer responsible for this task.

Improved Testability

View controllers are notoriously hard to test because of their relation to the view layer. By migrating data manipulation to the view model, testing becomes much easier. Testing view models is easy. Because a view model doesn’t have a reference to the object it is owned by, it easy to write unit tests for a view model.

Another, and often overlooked, benefit of MVVM is improved testability of view controllers. The view controller no longer depends on the model layer, which makes them easier to test.

Transparent Communication

The responsibilities of the view controller are reduced to controlling the interaction between the view layer and the model layer, glueing both layers together.

The view model provides a transparent interface to the view controller, which it uses to populate the view layer and interact with the model layer. This results in a transparent communication between the four layers of your application.

There are some rules you know when using MVVM

Rule1 : The view does not know about the controller it is owned by. Remember that views are supposed to be dumb. They only know how to present the data they are given to the user.

Rule 2 : The controller does not know about the model. This is what separates MVC from MVVM.

Rule 3 : The model does not know about the view model it is owned by.

Rule 4 : The view model owns the model. When using the Model-View-Controller pattern, the model is usually owned by the controller.

Rule 5 : The view controller owns the view. This relationship remains unchanged when using the Model-View-ViewModel pattern.

Rule 6 : And, finally, the controller owns the view model. It interacts with the model layer through one or more view models.

Download task when app is inactive

 https://developer.apple.com/documentation/foundation/url_loading_system/downloading_files_in_the_background

For the background download, there is beginBackgroundTaskWithExpirationHandler: that is specifically designed to do that. When you use it, you will get few more minutes to execute whatever you need (after that limit, your application will get terminated no matter what).

You can write following methods:

func beginBackgroundTask() -> UIBackgroundTaskIdentifier {
    return UIApplication.sharedApplication().beginBackgroundTaskWithExpirationHandler({})
}

func endBackgroundTask(taskID: UIBackgroundTaskIdentifier) {
    UIApplication.sharedApplication().endBackgroundTask(taskID)
}

When you want to use it, you just simple begin / end the task when starting / finishing the download call:

// Start task
let task = self.beginBackgroundTask()

// Do whatever you need
self.someBackgroundTask()

// End task
self.endBackgroundTask(task)

Hope it helps!

What is SSL Pinning and How to implement it?

SSL stands for Secure Socket Layer, which is a protocol for creating an encrypted connection between client and server. It ensures that all data pass in network will be private and integral.

How SSL works? When client establishes the connection with server (called SSL handshake):

1. Client connects to server and requests server identify itself.
2. Server sends certificate to client (include public key)
3. Client checks if that certificate is valid. If it is, client creates a symmetric key (session key), encrypts with public key, then sends back to server
4. Server receives encrypted symmetric key, decrypts by its private key, then sends acknowledge packet to client
5. Client receives ACK and starts the session

Using SSL, the client will allow the connection only from trusted sources that have the valid certificate. And it looks good for most cases. But what if someone stands between client and server, and acts like they are the real server? Let's call client is C, server is S and the attacker is A.

In step 1, instead of sending packet to S, A can catch the packet and pretend it as S. What if instead of receiving certificate from S, client C will receive fake certificate from A and believe it's valid. A can make C think that it is communicating with S, but actually all connection flows will be directed to attacker A.

Hence, SSL pinning can be the solution to prevent Man-In-The-Middle (MITM) attack. SSL pinning will ensure that client connect with designated server. The main key of SSL pinning that server certificate will be saved in app bundle. Then, when client receives certificate from server, it then compares 2 certificates to make sure that they are the same before establishing the connection.

URLSession

For NSURLSession, the main method to handle SSL pinning is URLSession:didReceiveChallenge:completionHandler:delegate. Set your class to conform URLSessionDelegate and paste this function to your class:

func urlSession(_ session: URLSession, didReceive challenge: URLAuthenticationChallenge, completionHandler: @escaping (URLSession.AuthChallengeDisposition, URLCredential?) -> Void) {

    if (challenge.protectionSpace.authenticationMethod == NSURLAuthenticationMethodServerTrust) {

        if let serverTrust = challenge.protectionSpace.serverTrust {

            var secresult = SecTrustResultType.invalid

            let status = SecTrustEvaluate(serverTrust, &secresult)

            

            if (errSecSuccess == status) {

                if let serverCertificate = SecTrustGetCertificateAtIndex(serverTrust, 0) {

                    let serverCertificateData = SecCertificateCopyData(serverCertificate)

                    let data = CFDataGetBytePtr(serverCertificateData);

                    let size = CFDataGetLength(serverCertificateData);

                    let cert1 = NSData(bytes: data, length: size)

                    let file_der = Bundle.main.path(forResource: "name-of-cert-file", ofType: "cer")

                    

                    if let file = file_der {

                        if let cert2 = NSData(contentsOfFile: file) {

                            if cert1.isEqual(to: cert2 as Data) {

                                completionHandler(URLSession.AuthChallengeDisposition.useCredential, URLCredential(trust:serverTrust))

                                return

                            }

                        }

                    }

                }

            }

        }

    }

    

    // Pinning failed

    completionHandler(URLSession.AuthChallengeDisposition.cancelAuthenticationChallenge, nil)

}

This function will “requests credentials from the delegate in response to an authentication request from the remote server.” We will compare the certificate from server with the one that saved in app bundle. If 2 certificates are identical, the authentication will let it pass and client can connect to server.

Types of SSL Pinning(What to Pin)?

Pin the certificate: You can download the server’s certificate and put this in your app bundle. At runtime, the app compares the server’s certificate to the one you’ve embedded.

One disadvantage of SSL pinning is that you have to save the certificate in the app. Whenever the certificate is updated, we need to release new version of app. But this also leads to another problem: what we do with older version?

• Maintain the old certificate for a time, until we make sure all users have downloaded new version already.

Pin the public key: You can retrieve the certificate’s public key and include it in your code as a string. At runtime, the app compares the certificate’s public key to the one hard-coded hash string in your code.