7.1 Git Tools - Revision Selection

You can obviously refer to any single commit by its full, 40-character SHA-1 hash, but there are more human-friendly ways to refer to commits as well. This section outlines the various ways you can refer to any commit.

Git is smart enough to figure out what commit you’re referring to if you provide the first few characters of the SHA-1 hash, as long as that partial hash is at least four characters long and unambiguous; that is, no other object in the object database can have a hash that begins with the same prefix.

For example, to examine a specific commit where you know you added certain functionality, you might first run the git log command to locate the commit:

In this case, say you’re interested in the commit whose hash begins with 1c002dd…​. You can inspect that commit with any of the following variations of git show (assuming the shorter versions are unambiguous):

Git can figure out a short, unique abbreviation for your SHA-1 values. If you pass --abbrev-commit to the git log command, the output will use shorter values but keep them unique; it defaults to using seven characters but makes them longer if necessary to keep the SHA-1 unambiguous:

Generally, eight to ten characters are more than enough to be unique within a project. For example, as of February 2019, the Linux kernel (which is a fairly sizable project) has over 875,000 commits and almost seven million objects in its object database, with no two objects whose SHA-1s are identical in the first 12 characters.

One straightforward way to refer to a particular commit is if it’s the commit at the tip of a branch; in that case, you can simply use the branch name in any Git command that expects a reference to a commit. For instance, if you want to examine the last commit object on a branch, the following commands are equivalent, assuming that the topic1 branch points to commit ca82a6d…​:

If you want to see which specific SHA-1 a branch points to, or if you want to see what any of these examples boils down to in terms of SHA-1s, you can use a Git plumbing tool called rev-parse. You can see Git Internals for more information about plumbing tools; basically, rev-parse exists for lower-level operations and isn’t designed to be used in day-to-day operations. However, it can be helpful sometimes when you need to see what’s really going on. Here you can run rev-parse on your branch.

One of the things Git does in the background while you’re working away is keep a “reflog” — a log of where your HEAD and branch references have been for the last few months.

You can see your reflog by using git reflog:

Every time your branch tip is updated for any reason, Git stores that information for you in this temporary history. You can use your reflog data to refer to older commits as well. For example, if you want to see the fifth prior value of the HEAD of your repository, you can use the @{5} reference that you see in the reflog output:

You can also use this syntax to see where a branch was some specific amount of time ago. For instance, to see where your master branch was yesterday, you can type:

That would show you where the tip of your master branch was yesterday. This technique only works for data that’s still in your reflog, so you can’t use it to look for commits older than a few months.

To see reflog information formatted like the git log output, you can run git log -g:

It’s important to note that reflog information is strictly local — it’s a log only of what you’ve done in your repository. The references won’t be the same on someone else’s copy of the repository; also, right after you initially clone a repository, you’ll have an empty reflog, as no activity has occurred yet in your repository. Running git show HEAD@{2.months.ago} will show you the matching commit only if you cloned the project at least two months ago — if you cloned it any more recently than that, you’ll see only your first local commit.

The other main way to specify a commit is via its ancestry. If you place a ^ (caret) at the end of a reference, Git resolves it to mean the parent of that commit. Suppose you look at the history of your project:

Then, you can see the previous commit by specifying HEAD^, which means “the parent of HEAD”:

You can also specify a number after the ^ to identify which parent you want; for example, d921970^2 means “the second parent of d921970.” This syntax is useful only for merge commits, which have more than one parent — the first parent of a merge commit is from the branch you were on when you merged (frequently master), while the second parent of a merge commit is from the branch that was merged (say, topic):

The other main ancestry specification is the ~ (tilde). This also refers to the first parent, so HEAD~ and HEAD^ are equivalent. The difference becomes apparent when you specify a number. HEAD~2 means “the first parent of the first parent,” or “the grandparent” — it traverses the first parents the number of times you specify. For example, in the history listed earlier, HEAD~3 would be:

This can also be written HEAD~~~, which again is the first parent of the first parent of the first parent:

You can also combine these syntaxes — you can get the second parent of the previous reference (assuming it was a merge commit) by using HEAD~3^2, and so on.

Now that you can specify individual commits, let’s see how to specify ranges of commits. This is particularly useful for managing your branches — if you have a lot of branches, you can use range specifications to answer questions such as, “What work is on this branch that I haven’t yet merged into my main branch?”