Creating a Git repo

mkdir path/to/repo
cd path/to/repo
git init
ls -al .git # The entire repository is stored here

Creating a blob object

1. Create your README.md file, using the editor

   # Git workshop
   
   Customize your text here...
   

2. Start the Python console, then run the following

   import os
   import hashlib
   import zlib
   
   content = open("README.md", "rb").read()
   
   # Generate header
   content_type = "blob"
   content_size = len(content)
   header = f"{content_type} {content_size}".encode("utf8")
   
   # Create blob object
   blob_object = header + b"\0" + content
   
   # Compute object SHA1
   h = hashlib.sha1()
   h.update(blob_object)
   sha1 = h.hexdigest()
   
   # Compute filepath
   dirpath = f".git/objects/{sha1[0:2]}"
   filename = sha1[2:]
   os.makedirs(dirpath)
   
   # Compress & store into Git database
   compressed = zlib.compress(blob_object)
   file = open(f"{dirpath}/{filename}", "wb")
   file.write(compressed)
   file.close()
   
   # Print the Blob SHA1
   print(sha1)

3. Read the object from Git database

   Exit the Python console, then run the following command

   git cat-file -p <sha1>
   

   It's supposed to print your README.md file content to output

Creating a tree object

1. Start the Python console, then run the following

   import os
   import hashlib
   import zlib
   
   # Create tree content
   content = b"100644 README.md\0" + bytes.fromhex("< Blob SHA1 >") # Enter the hex value here
   
   # Generate header
   content_type = "tree"
   content_size = len(content)
   header = f"{content_type} {content_size}".encode("utf8")
   
   # Create blob object
   tree_object = header + b"\0" + content
   
   # Compute object SHA1
   h = hashlib.sha1()
   h.update(tree_object)
   sha1 = h.hexdigest()
   
   # Compute filepath
   dirpath = f".git/objects/{sha1[0:2]}"
   filename = sha1[2:]
   os.makedirs(dirpath)
   
   # Compress & store into Git database
   compressed = zlib.compress(tree_object)
   file = open(f"{dirpath}/{filename}", "wb")
   file.write(compressed)
   file.close()
   
   # Print the Tree SHA1
   print(sha1)

2. Read the object from Git database

   Exit the Python console, then run the following command

   git cat-file -p <sha1>
   

   It's supposed to print the tree

Creating a commit object

1. Start the Python console, then run the following

   import os
   import hashlib
   import zlib
   from datetime import datetime
   
   # Create commit content
   content = "tree < Tree SHA1 >\n"
   content += f"author < Author name > {int(datetime.now().timestamp())} +0100\n"
   content += f"committer < Committer name > {int(datetime.now().timestamp())} +0100\n"
   content += "\n"
   content += "This is the commit message\n"
   
   # Generate header
   content_type = "commit"
   content_size = len(content)
   header = f"{content_type} {content_size}".encode("utf8")
   
   # Create commit object
   commit_object = header + b"\0" + content.encode("utf8")
   
   # Compute object SHA1
   h = hashlib.sha1()
   h.update(commit_object)
   sha1 = h.hexdigest()
   
   # Compute filepath
   dirpath = f".git/objects/{sha1[0:2]}"
   filename = sha1[2:]
   os.makedirs(dirpath)
   
   # Compress & store into Git database
   compressed = zlib.compress(commit_object)
   file = open(f"{dirpath}/{filename}", "wb")
   file.write(compressed)
   file.close()
   
   # Print the commit SHA1
   print(sha1)

You have just created a commit without using git commit!

git show <sha1>

Creating a branch

echo <sha1> > .git/refs/heads/<branch-name>

Exercise

1. Create a new commit that adds a new file inside a subdirectory

   Simulate the following

   echo 'Hello' > dir/test.txt
   git add dir/test.txt
   git commit -m "Commit message"