MNNIT CC 2021-22 Classes

Git Class - II

Git Class - II recording: Here

April 21st, 2021

“No matter how many mistakes you make or how slow you progress, you are still way ahead of everyone who isn’t trying.” - Tony Robbins

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• Git diff

  git diff is a multi-use git command that when executed runs a diff function on git data sources. These data sources can be commits, branches, files and more. The git diff command is often used along with git status and git log to analyze the current state of a git repo.

  • git diff: compares the staging area with working directory.
  • git diff --cached or git diff --staged: compares the most recent commit with staging area.
  • git diff <commit_hash_1> <commit_hash_2>: Results in the difference of the two commits.

  Reading git diff output

  diff --git a/file1 b/file1
  index aba2e68..e024a2a 100644
  --- a/file1
  +++ b/file1
  @@ -2,11 +2,8 @@
  #include<stdlib.h> 
  #include<stdio.h> 
       
  -// Merges two subarrays of arr[]. 
  -// First subarray is arr[l..m] 
  -// Second subarray is arr[m+1..r] 
  -void merge(int arr[], int l, int m, int r) 
  -{ 
  +
  +void merge(int arr[], int l, int m, int r){ 
       int i, j, k; 
       int n1 = m - l + 1; 
       int n2 =  r - m; 
  @@ -24,15 +21,12 @@ void merge(int arr[], int l, int m, int r)
       i = 0; // Initial index of first subarray 
       j = 0; // Initial index of second subarray 
       k = l; // Initial index of merged subarray 
  -    while (i < n1 && j < n2) 
  -    { 
  -        if (L[i] <= R[j]) 
  -        { 
  +    while (i < n1 && j < n2){ 
  +        if (L[i] <= R[j]){ 
            arr[k] = L[i]; 
            i++; 
       } 
  -        else
  -        { 
  +        else{ 
            arr[k] = R[j]; 
            j++; 
       } 
       
  

  Above shows the difference between two versions of c++ code.
  You can see multiple differences denoted by hunks.
  A hunk example: @@ -2,11 +2,8 @@ can be broken down into 3 main parts:-
  -/+ :: removal and addition respectively. first number :: start line. second number :: number of lines after the start line (including it). For a detailed discussion, refer this article or this article

• Branching

  In simple terms, a branch in git is a unique series of code changes with a unique name. Each repo can have one or more branches.
  A branch represents an independent line of development. You can think of them as a way to request a brand new working directory, staging area, and project history.

  In a little advance yet relatively simple term they are labels to your commits.

  

  • Create a branch: git branch "<branch-name>"
    Eg: git branch "little-feature"

  • Switching branches: git checkout branch-name

  • Creating a new branch and switching to it: git checkout -b "<branch-name>"
    eg: git checkout -b "little-feature" creates a new branch litte-feature and switches to it, in short it is a combination of above two commands.

  For a detailed discussion, refer this

  HEAD

  • The branch name that we define for our branches is actually an alias of a pointer to the last commit in that branch.

  • HEAD is a special pointer to a commit just like name of any branch. But it is special because it is the "HEAD" pointer (and not the currently checked out branch) which decides the state of your current Working Directory. Eg: You can make your "HEAD" pointer point to some previous commit in the same branch, so the state of your Working Directory will change to that commit which is pointed by "HEAD" and NOT to the commit to which "branch name" pointer is pointing i.e. the last commit in this branch.

  • git checkout <commit_hash> is used to make your "HEAD" point to a specific commit.

  DETACHED HEAD vs ATTACHED HEAD

  • In simple terms whenever your "HEAD" is not linked with any Branch it is called DETACHED HEAD and vice-versa.
  • "HEAD" being linked with a branch simply means whatever progess(commits) you make from your current state of project, "HEAD" as well as "branch-name" pointer will move to the latest commit together.
  • When "HEAD" is not linked to any of the branch (i.e. DETACHED HEAD State), then only the "HEAD" pointer will move to the latest commit not the "branch-name" pointer. Git will not make this change as part of any branch because there is no branch which is linked to "HEAD" pointer.
  • In a nutshell, whenever we make a commit, it is the "HEAD" pointer that moves to this newly created commit and if "HEAD" is linked to a branch, then the "branch-name" pointer will also move along with "HEAD".
    

  2 Ways of using git checkout <branch-name/commit-hash> command

  • git checkout <commit-hash> is used to make your "HEAD" point to a specified commit. Using this way will always make your "HEAD" DETACHED regardless whether any branch starts from (or ends at) that commit. Also this will make your current working directory state to reach at that specified commit (version).
  • git checkout <branch-name> is used to make your "HEAD" point to last commit in the branch "branch-name". Using this will always make your "HEAD" ATTACHED/LINKED to branch "branch-name"
    

  For a detailed discussion, refer this

• Resources for further study

  • Class Presentation
  • Git Practice Resources
  • Git Cheat Sheet
  • Progit Book
  • Udacity Course: How to Use Git and GitHub?
  • Git Internals
  • Git Visualised using animations

• A meme

  

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