Category: Reference

Here are my notes from watching the video Git for Ages Four and Up. It contains a really good expanation of how Git works under the hood. The speaker (Michael G. Schwern <schwern@pobox.com>) created a new repository, and then created a new file, added some text to it, saved it, closed it, then added it to the Staging Area, and then committed it.

git add

• Adds a file your working directory, to the staging area (the Index), as a BLOB.

git commit

• Creates a “Commit” object from that BLOB.
  • That somehow points to the files that are stored in Git.
    • The commit contains all of the files in the repot – not just the ones that were just staged.
• And attaches two labels (“References”) to it.
  • HEAD
    • The name of the most recent Commit.
    • Always points to the tip of the branch that you currently have checked-out.
  • MASTER
    • The name of the currently checked-out Branch.
    • The one that advances upon “git commit”.

When you create a branch

• All Git does is attach a new branch label to the Commit currently pointed to by HEAD.

When you perform a “git checkout”

• Git merely switches which branch it’s going to advance when you perform a “git commit”.
  • We say the branch becomes active.
• And it moves HEAD to the commit object that has that branch label.
• Git populates your Working Directory with a snapshot of the branches tip commit..

Commit ID

• Content + Author + Date + Log + Previous Commit ID.
• A hexadecimal UUID (Universally Unique Identifier).
• Is a checksum (Hash)
• Git calculates this value by running the bits in the BLOB through a function, which produces a thirty eight digit hexadecimal value.

Staging Area (Index)

• Allows you to build up a commit.
• You can add multiple files to the index, and then commit them (i.e., all files as one commit object).

The basic Git workflow

• You isolate your work from everyone else’s (spawn a new branch to work in).
• Do some work (modify one or more files).
• Update from everyone else (pull from origin).
• Commit your work to your local enlistment (commit).
• Finally, you share your work with everyone else (push to origin).

Merge

• git checkout your local master (feature/api_doc).
  • You’ll be bringing your work back into this branch.
• Git performed a Fast Forward.
  • Since no merging of content (all modifications added new content).
    • A merge object wasn’t created.
  • All Git did was move the branch label to the commit from the ‘from’ branch.
  • Git also moved the HEAD label, because you have that branch checked out.

git reset –hard HEAD^

• This undoes the last commit.
• But all it does is move reference labels!
• Moves the current branch label (Master), back one commit to HEAD-1.
• –hard does a checkout, so that you end up where you want to be.

git log –graph –decorate –all

git commit does not mean share (inflict).

• Commit is a local operation.

Merge

• Git can merge in many different ways.
• It picks the most convenient way.
• Fast Forward is the simplest.

Working with others

• This is where remote repositories comes in.
  • remote commands.
    • Push, Fetch, and Pull.

Clone

• You normally start working on a clone of an existing repository.
• When you clone a repository, git attaches labels to the objects in your local repository to mark the tracking branches.
• origin/MASTER
• Git clones only origin/MASTER (the default branch).

Directed Acyclic Graph

• Graph – a collection of nodes and edges.
• Directed – the edges all point to the previous node.
• Acyclic – there are no cycles. I.e., the path from one commit to any other is unique.

Pull

• Is actually a Fetch followed by a Merge.
• When you’re starting out, it’s often easier to do a pull as two steps (a fetch followed by a merge).

We’re going to create a third repository

• So we can have someone other than us, doing work.

We’ll create a new branch called “bugfix”

• And we’ll create a new file in it called “baz”.

git push origin bugfix

• Sometimes you have a space between “origin” and “bugfix”, and sometimes you have a slash between them.
  • The slash version refers to the label (origin/bugfix) on your repository. This is a reference to a tracking branch.
  • The space version refers to the remote (origin) branch (bugfix).

Push

• Does two things.
  • Git checks to see if the remote repository has the commit object your pushing.
    • If it doesn’t, then it checks to see if it has the ancestor commit object(s).
    • If it does, then
      • Git sends over the commit object (plus a branch label, if it’s on a new branch).
    • If it doesn’t, then
      • Git send all of the ancestors + the commit object.
• This is where the cool thing about IDs comes in (this is why git is so amazingly fast).
  • Every ID is unique.
  • Every commit is unique.
  • Commits never change.
  • This means that every commit can be uniquely identified by its ID.
  • Since IDs contain the ID of the previous commit,
    • Every commit’s history can be uniquely identified by its ID.
    • Two devs push to the same repo without pulling…
      • If I have ae123, and you have ae123,
        • Then we know everything from ae123 on down is exactly the same.

Tag

• Tags are good to use when you continually jump back to a particular snapshot.
• E.g., git tag v1.0 bfce7
• All Git does is add a label (reference) to the commit object you want to tag.
• Excep that tag labels never move.

git reflog

• This is your rescue command.
• You can use this command when something gets screwed up, to find a particular commit ID.
• Git shows you the commits you’ve been working on lately.

You can combine git add and git commit into one step

E.g., git commit -a foo.

Rebase

• This is a local operation.
• Example use: instead of fixing a typo and then committing the file again, you can perform a rebase instead.
• Git rebase doesn’t rewrite history. It writes new history (you can’t actually change history in Git).
  • Instead, git creates a whole new line of history.
• Interactive rebase.
  • Go back two commits.
    • git rebase -i HEAD^^
    • Git says “I’m going to replay these things as if they were patches, one on top of the other.
      • Nothing happened! (because nothing changed).
    • You can “squash” a later commit into a previous commit (so the typo fix you made is written on top of the version with the typo).
      • I.e., you combine two (in this case) commits into a new commit.
      • The original two commits become detached!

Note: Once you have pushed, don’t rebase or you’ll screw up everyone else’s history! So never rebase until after you’ve pulled!

Command	Description
git add [file]	adds [file] to the index (stages the file).
git add -i	runs the interactive staging script.
git branch	lists branches.
git branch -a	lists all branches (both local and remote).
git branch [branch]	creates a new branch.
git branch -d [branch]	deletes a branch.
git branch –contains	shows which branches contained the named commit (HEAD if not specified).
git branch –merged	shows which branches are already merged into the branch you’re currently on.
git branch –no-merged	shows all of the branches that contain work that you haven’t yet merged in.
git branch –set-upstream-to origin/[branch]	sets this branch to track a remote branch. Remote branch must exist first.
git branch -v	shows the last commit on each branch.
git branch -vv	shows the tracking branches you have setup.
git cat-file -p HEAD	shows details (files) about the HEAD snapshot.
git checkout – <file>	unmodifies a modified file. Discards your changes. Reverts your file back to last commit.
git checkout [branch]	switches to another branch.
git checkout -b [branch]	creates and checks out a new branch.
git checkout -b [branch] [remote]/[branch]	creates and checks out a new branch, and also sets up a remote tracking branch.
git checkout –track [remote]/[branch]	creates and checks out a new branch, and also sets up a remote tracking branch.
git checkout –track origin/[branch]	creates a local tracking branch from a remote branch.
git clone <URL>-	clones a repository.
git commit –amend	adds more files to the last commit (that you still haven’t pushed).
git commit -m ‘message text’	commits staged files.
git config –list	lists your Git configuration.
git diff	shows changes in the working tree that are not yet staged.
git diff HEAD	shows changes in the working tree since your last commit.
git diff –cached	shows changes between the index and your last commit.
git diff –check	identifies possible white space errors, and lists them.
git diff –staged	same as git diff –cached.
git fetch [remote]	downloads all of the changes on the server that you don’t yet have.
git fetch –all	fetches all remotes.
git grep -n [search term]	finds the files that contain the search term. The -n switch prints the line numbers of matches.
git grep –count [search term]	finds the files that contain the search term, and how many times in each file.
git help <verb>	shows help on a particular command.
git init	creates a new, empty git repository.
git log	shows the history of commits, in revere chronological order.
git log –graph	shows a graphical representation (in ASCII) of your commit history (showing branches).
git log -p -2	generates a patch between the last two commits.
git log –pretty=oneline	shows commits one line at a time, pretty.
git log –since=2.weeks	shows the commits from the last two weeks.,
git log –stat	shows details for each commit.
git log [branch1]..[branch2]	shows what’s in branch1 that hasn’t yet been merged into branch2.
git log [remote]/[branch]..	shows the commits in your current branch, that aren’t in [origin]/[branch].
git log refA refB ^refC	shows all commits that are reachable from refA or refB, but not refC.
git log [branch1]…[branch2]	shows the commits that are not common to both branches.
git log -S [search term]	shows when the search term was introduced.
git log -L :[function name]:[file name]	shows the history of a function or line of code.
git ls-files -m	show files. In this case (-m) modified files. Also -s for staged files.
git ls-tree -r HEAD	lists the files in the HEAD tree, along with each one’s commit ID.
git merge @{upstream}	if this is a tracking branch, merges in remote tracking branch.
git merge [branch]	merges branch into current branch.
git merge –abort”	aborts the current merge.
git mergetool	runs a three-way merge (between local, remote, and common ancestor).
git mv <file_from> <file_to>	renames a file.
git pull [remote]	fetches and merge.
git push [remote] [branch name]	pushes to remote branch.
git push [remote] –delete [branch]	deletes a remote branch.
git push [remote] [tag name]	pushes a particular tag upstream.
git push [remote] –tags	pushes all tags.
git push –set-upstream [remote] [branch]	pushes the current branch, and creates the same branch on the remote, and tracks it.
git remote show [remote]	lists the details about a particular remote.
git remote	lists your remotes.
git reset <file>	unstages a file (essentially copies <file> from HEAD to index.
git reset [commit]	Moves HEAD to point to [commit].
git reset HEAD~	Removes the last commit. But also unstages files in the Index.
git reset –hard HEAD~	Removes the last commit. Cleans the index and working tree (nothing changed, nothing staged). Note: This is the dangerous version of the reset command – because you’ll lose your work in progress.
git reset –mixed HEAD~	Same as git reset HEAD~.
git reset –soft HEAD~	Undoes the last commit, without changing the working tree or the index.
git rev-parse branch	shows the commit that a particular branch points to.
git rm <file>	removes a file from the working tree and the index.
git show	shows details of the last commit.
git show HEAD@{n}	shows the commit that HEAD pointed to “n” commits ago.
git show [branch]@{yesterday}	shows the commit on a particular branch that HEAD pointed to yesterday.
git show HEAD@{2.months.ago}	shows the commit that HEAD pointed to two months ago.
git show HEAD^	shows the parent of HEAD.
git show d921970^2	shows the second parent of the specified commit. This only works for merge commits.
git show HEAD~n	shows the nth first parent. Equivalent to HEAD^^^ (for n = 3).
git stash	saves your staged and modified files on the stack.
git stash list	lists your stashes.
git stash apply	applies the last stash on your current index and working directory.
git stash apply index	also restages any files that are staged, but updates them with the stash.
git stash drop stash@{n}	deletes the specified stash.
git stash pop	applies a stash, and then deletes it from the stack.
git status	shows the status of your working tree.
git status -s	the short version of git status.
git tag v1.4	tags HEAD as v1.4.
git tag	lists all of the tags.

Command	Description
add	Add file contents to the index
bisect	Find by binary search the change that introduced a bug
branch	List, create, or delete branches
checkout	Checkout a branch or paths to the working tree
clone	Clone a repository into a new directory
commit	Record changes to the repository
diff	Show changes between commits, commit and working tree, etc
fetch	Download objects and refs from another repository
grep	Print lines matching a pattern
init	Create an empty git repository or reinitialize an existing one
log	Show commit logs
merge	Join two or more development histories together
mv	Move or rename a file, a directory, or a symlink
pull	Fetch from and merge with another repository or a local branch
push	Update remote refs along with associated objects
rebase	Forward-port local commits to the updated upstream head
reset	Reset current HEAD to the specified state
rm	Remove files from the working tree and from the index
show	Show various types of objects
status	Show the working tree status
tag	Create, list, delete or verify a tag object signed with GPG

You can exit a listing by pressing Ctrl+c.