Git References

April 6, 2016

git

Everyone knows how to checkout a branch, git checkout master, or how to look at a specific commit, git show a1c8b6d. But few know that branches, tags, and commit SHAs are typically interchangeable in these and many other git commands. Plus there is a whole set of modifiers to reference commits near to specific commits. In this post, we’ll explore all these techniques to become expert git referencers.

References

Below we will explore the different ways we can express a commit in git. While each technique has its own strengths and weaknesses, being able to wield a lot of git’s power and saving a lot of time comes from using these references interchangeably.

SHA1s

This first and most recognizable way to reference a commit is by its SHA1. This is the full or part of the long random looking string of letters and numbers. These are generated based on the contents of the commit, including current state of the repo’s files, commit message, author and committer, and timestamp. If any of those items change, a new SHA1 will be generated. Here are some examples of git commands using the SHA1.

git show abc123
git cherry-pick 321bca
git reset bbbaa4

Branch Names

Branch names are also something that are a big part of a git workflow. Under the hood, branch names are just aliases for commit SHA1s. Literally, you can see the branch names in the .git/refs/heads directory of your repo. Each file there contains one line which has the SHA1 of the commit it references. Here are some common usages of branch names:

git checkout master
git rebase stable
git merge features/new-hottness
git cherry-pick bugs/broken-email-form

An important aspect of branch names is the commit they reference can change. For example, when you create a new commit the branch which you are currently on automatically gets updated to the new SHA1.

Tags

Tags are very similar to branch names in that they are also friendlier aliases for commit SHA1s. However, unlike branches, tags cannot be changed once created. This makes them ideal for marking fixed, important commits in the history of your code base. Releases and versions are a good usage for tags. Here are some example commands using tags:

git tag v1.0.0
git checkout -b 1.1-backport v1.1.0
git show v0.9.0

The HEAD commit is the current tip of the branch you are on. Similar to the pwd program, HEAD tells you where you are. On its own, the useful command is show which will show you the details of the last commit you made.

Modifiers

git also allows you to specify commits by their relation to other commits. In the git data structure the only explicit relationship between commits is child-parent. That is, the child commit knows its parent, but the parent does not know its child/children. Many of the commands which show or span multiple commits, such as log or diff, actually traverse the git tree to aggregate their output.

First Parent

The ~ modifier follows a references first parent. For example, HEAD~ references the first parent of the current commit. The first parent modifier can be repeated: HEAD~~ references the first parent of the first parent of the current commit. Also, a number can be specified for many generations: HEAD~5 follows ancestors back 5 generations. Here are some other uses of the first parent modifier:

// Move the second to last commit into the current branch
git cherry-pick bugs/broken-email-form~ 

// Modify some of the last 5 commits on your branch
git rebase HEAD~6

// Mark your current changes to fixup into two commits ago
git commit -a --fixup HEAD~

Other Parents

Through merges, a commit can have multiple parents. The last modifier of the day specifies any parent of a commit. The caret, ^, modifies the reference to follow its first parent; the same as ‘~‘. However, specifying a number along with the caret follows subsequent parent commits. For example, reference the last commit of a branch before it was merged into the current branch with HEAD^2.

Bringing it all together

Any of the above references can be combined with any number of modifiers to express the exact commit succinctly. Here are some examples:

// See the 3rd to last commit from the last branch merged to the stable branch
git show stable^2~~~

// Pull second to last commit from version 2 into your current branch
git cherry-pick v2.0.0~

// Create a new branch 2 commits back from a known SHA1
git checkout -b branch-from-history abcd122~~

The above examples might seem extremely specific, but knowing and using these techniques can turn wasted time digging through git logs or copy and pasting SHA1s into a quick one-liner. Impress your friends and co-workers, save your own time, and begin thinking of your git history relatively.

Happy referencing!!

– Chris

Image

Christopher R Marshall

@codegoalie

Enjoys programming web applications; especially in Ruby and Go. Also enjoys playing ice hockey as a goalie and playing the guitar.

Categories