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More Git!

It is important to understand that every git repository is completely standalone and functional by itself, without dependency on other repositories. Main commands we covered in the git fundamentals section (pull, push, fetch) enables transferring/sharing different versions of the code to other repos.

In this section, we will dive deeper into git structure and other git commands and tools to help improve productivity.

Resources & References

There are great resources on git.

Git Structure

Intro to git internals

Git repo is basically a hidden directory (.git) at the top level of your repo. It holds everything that is needed to track the needed changes. If you want to backup your repo, this is the only directory you will need.

$ git init
Initialized empty Git repository in /Users/binnur/tmp/git/a_project/.git/
$ ls -F1 .git
HEAD
config
description
hooks/
info/
objects/
refs/

For more background on the Git internals, check out Git book.

HEAD is a symbolic reference to the branch you are on. Git is a tree structure where each commit creates a new node in that tree, and git commands help navigate that tree.

Git is immutable by design -- every new change creates a new set of nodes, which makes git almost indestructible.

# graphical history tool
$ gitk --all

Branches and Tags

A branch is a pointer to a specific commit. Switching branches walks the tree to derive the history of the repo. git tag is similar to git branch, however it operates like a bookmark to a specific point in time.

# create a new branch
$ git branch <branch-name>
# switch to the new branch
$ git checkout <branch-name>
# create and checkout the branch simultaneously
$ git checkout -b <branch-name>
# checkout master branch
$ git checkout master
# list branches that are tracked
$ git branch
# list all branches, including remotes
$ git branch -a
# pull all branches and tags from remotes
$ git fetch --all
# pull branches from a specific remote
$ git fetch origin

In git, branches basically another node in the tree -- this makes them lightweight. Many teams use feature-driven branching to aid their workflow.

git tag is useful for marking special releases. A tag can be lightweight, just a pointer, or annotated w/ tagger name, description, date, etc.

# list existing tags
$ git tag
# search for a specific tag
$ git tag -l "v1.8*"
# create a tag w/ an annotated message
$ git tag -a v1.0 -m "Release ready for 1st match"
# create a tag w/ annotation and prompt editor for message
$ git tag -a v1.0 -m
# show details on the tag
$ git show v1.0
# lightweight tag -- only stores commit info
$ git tag v1.1

Important by default, git push does not push the tags or the branches to the remotes. You need to explicitly share them.

# push git tags
$ git push origin <tag-name>
# push all existing tags
$ git push origin --tags
# delete a tag --> this doesn't remove the tag from remotes
$ git tag -d <tag-name>
# remote a tag from the remote
$ git push origin :refs/tags/<tag-name>
# you can checkout a tag, though it creates a DETACHED HEAD state
$ git checkout <tag-name>

Git HEAD

By now you may be wondering what "HEAD" is and why it's possible to become detached. HEAD is git's way of referring to your current view of the repository, and corresponds to a single commit hash. Generally, this means HEAD matches the lastest commit on your current branch. A detached HEAD state means that you no longer are viewing the latest commit on a branch, and are instead veiwing an intermediate commit. As such, making additional commits will not directly contribute to the history of one of your branches, and will require more finesse to integrate into your project.

This does not mean that the detached HEAD state is useless. Far from it. It is a great way to rewind history and examine previously working bits of code, in order to find out where it all went wrong.

# Checkout a previously working commit/tag
$ git checkout <commit-sha-or-tag-name>
# From detached HEAD state, your local repository now matches the commit
# You can compile and test code
# Commits you make will be off of the detached HEAD state and wont be kept by default
$ git add .
$ git commit -m "A commit on a detached HEAD"
# Create a new branch from the detached HEAD state in order to keep commits
$ git checkout -b <new-branch-name>
# The commits you made before making the branch are still tracked
# HEAD now points to the end of the new branch (not detached)
# Merge your changes back into master.
$ git checkout master
$ git merge <new-branch-name>

Git checkout performs double duty. When used on a branch, tag, or commit hash it will move your HEAD to the corresponding commit, changing the view of your repository. If used on a file, it will modify the file to the version it was in in the appropriate commit (which defaults to HEAD). The changes will happen in your current branch/state, meaning it will then be possible to commit the old version of the file to your current branch.

# Checkout the version of your file as it was in HEAD
# WARNING: discards all unstaged, uncommitted changes
$ git checkout <file>
# Checkout the version of your file as it was in a tag/commit
$ git checkout <commit-sha-or-tag-name> <file>

For convenience, HEAD^ refers to the commit before HEAD, and HEAD^^ to the commit before that.

Tracking and merging branches

By default, git branches do not know about each other. Tracking creates linkages between the two repos, making it easier to stay in sync between two repos as git will inform you if your branch is ahead or behind the tracked branch.

# track via --track option
$ git checkout --track origin/<branch-name>
# set the tracking on your HEAD branch
$ git branch -u origin/<branch-name>

Branches explicitly need to be pushed to remotes. Similarly, explicitly deleted. Note, you can also use GitHub interface for deleting branches on remotes.

# push a new branch
$ git push -u origin dev
# delete a remote branch
$ git push origin --delete <branch-name>

While git branch creates a fork in the road, git merge allows the forked history to be integrated again. A typical example is to merge a feature branch to master. Process overview:

  • checkout the branch to be updated
  • specify the merge from a target branch
  • optional: delete the target branch
# checkout master branch or verify you are on the receiving branch
$ git checkout master
$ git status
# merge from another branch, such as a feature branch
$ git merge <feature-branch>
# optionally delete the feature branch
$ git branch -d <feature-branch>

If a conflict occurs during merge that git needs you to decide, git will edit the content of the affected files with visual indicators: <<<<<<<, =======, and >>>>>>>. You can search for these indicators to resolve the conflicts.

git rebase & git squash

git rebase is basically merging w/ history -- git takes commits in one branch and attempts to "replay" the differences onto the other branch.

# applying feature branch to master
$ git checkout <feature-branch>
$ git rebase master

git squash condenses the commits, think G-rated version of sausage making, creating a concise and readable history. Interactive rebasing allows selecting and squashing commits.

# interactive shell for rebase
$ git rebase -i <branch>

Reverting changes

There are different ways to undo prior changes and commits.

  • revert takes a specified commit and creates a new commit which inverses the specified commit -- safe operation as creates a new commit

  • reset takes a specified commit and resets to match the state of the repo at that specified commit -- can cause conflicts is history is changed

    Some common operations are below.

# discard changes in working directory
$ git checkout -- <filename>
# discard staged files
$ git reset HEAD

Git Commands & Helful Tools

Git bash completion

If you are using bash shell, there is a nice auto-complete feature you can enable for git.

Follow the instructions for setting it up: git-completion.bash.

More on gitconfig

gitconfig has handy tools/options from customizing git colors to setting aliases. More on gitconfig.

# enable git colors
$ git config --global color.ui auto
$ git config --global alias.co checkout
$ git config --global alias.st status

git diff

Git can show you a list of differences between two commits, or a list of differences between a given commit and the current state using the command git diff.

# diff between what you changed and haven't staged
$ git diff <filename>
# diff between current branch and another branch
$ git diff <branch-name>

git stash

There are times when you are not ready to commit, but you need to change branches -- git stash is to your rescue. It stashes your changes to be returned to at a later time.

# stash your changes
$ git stash
# restore your prior stash -- git default to most recent stash
$ git stash apply
# see a list of your stashes
$ git stash list
# remove a stash
$ git stash drop

git references or refs

Instead of referencing git contents via sha-1 value, such as git log 1a410e, you can create named references to that value.

git branch <branchname> creates refs using git update-ref command to add the sha value with the given branch.

# create and checkout branch
$ git checkout -b a_branch
# print out content of HEAD
$ cat .git/HEAD
ref: refs/heads/a_branch

remove or move a file: git rm && git mv

There will be times when you need to remove a file from your git repo, use git rm or remove command. And, when you need to move or rename a file, use git mv or move command.

# remove unwanted file from the repo
$ git rm <filename>
# move a file/directory
$ git mv <oldname> <newname>