Add Count Number Of Commits On A Branch as a git TIL

Author: jbranchaud

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://crafty-builder-6996.ck.page/e169c61186).
 
-_1565 TILs and counting..._
+_1566 TILs and counting..._
 
 See some of the other learning resources I work on:
 - [Ruby Operator Lookup](https://www.visualmode.dev/ruby-operators)
@@ -304,6 +304,7 @@ See some of the other learning resources I work on:
 - [Configuring The Pager](git/configuring-the-pager.md)
 - [Copy A File From Another Branch](git/copy-a-file-from-another-branch.md)
 - [Count All Files Of Specific Type Tracked By Git](git/count-all-files-of-specific-type-tracked-by-git.md)
+- [Count Number Of Commits On A Branch](git/count-number-of-commits-on-a-branch.md)
 - [Create A New Branch With Git Switch](git/create-a-new-branch-with-git-switch.md)
 - [Delete All Untracked Files](git/delete-all-untracked-files.md)
 - [Determine The Hash Id For A Blob](git/determine-the-hash-id-for-a-blob.md)

git/count-number-of-commits-on-a-branch.md

@@ -0,0 +1,48 @@
+# Count Number Of Commits On A Branch
+
+The `git rev-list` command will show all commits that fit the given revision
+criteria. By adding in the `--count` flag, we get a count of the number of
+commits that would have been displayed. Knowing this, we can get the count of
+commits for the current branch like so:
+
+```bash
+$ git rev-list --count HEAD
+4
+```
+
+This finds and counts commits from `HEAD` (usually the top of the current
+branch) all the back in reverse chronological order to the beginning of the
+branch (typically the beginning of the repository). This works exactly as
+expected for a the `main` branch.
+
+What about when we are on a feature branch though?
+
+Let's say we've branched off `main` and made a few commits. And now we want the
+count.
+
+```bash
+$ git rev-list --count HEAD
+7
+```
+
+Unfortunately, that is counting up the commits on the feature branch but it
+keeps counting all the way back to the beginning of the repo.
+
+If we want a count of just the commits on the current branch, then we can
+specify a range: from whatever `main` was when we branched to the `HEAD` of
+this branch.
+
+```bash
+$ git rev-list --count HEAD
+3
+```
+
+This is the same as saying, I want all commits on `HEAD`, but exclude (`^`) the
+commits on `main`:
+
+```bash
+git rev-list --count HEAD ^main
+3
+```
+
+See `man git-rev-list` for more details.