Add Check If A File Is Under Version Control 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).
 
-_1677 TILs and counting..._
+_1678 TILs and counting..._
 
 See some of the other learning resources I work on:
 
@@ -322,6 +322,7 @@ If you've learned something here, support my efforts writing daily TILs by
 - [Change The Start Point Of A Branch](git/change-the-start-point-of-a-branch.md)
 - [Check How A File Is Being Ignored](git/check-how-a-file-is-being-ignored.md)
 - [Check If A File Has Changed In A Script](git/check-if-a-file-has-changed-in-a-script.md)
+- [Check If A File Is Under Version Control](git/check-if-a-file-is-under-version-control.md)
 - [Checking Commit Ancestry](git/checking-commit-ancestry.md)
 - [Checkout Old Version Of A File](git/checkout-old-version-of-a-file.md)
 - [Checkout Previous Branch](git/checkout-previous-branch.md)

git/check-if-a-file-is-under-version-control.md

@@ -0,0 +1,36 @@
+# Check If A File Is Under Version Control
+
+The `git ls-files` command can be used with the `--error-unmatch` flag to check
+if a file is under version control. It does this by checking if any of the
+listed files appears on the _index_. If any does not, it is treated as an error.
+
+In a project, I have a `README.md` that is under version control. And I have
+`node_modules` that shouldn't be under version control (which is why they are
+listed in my `.gitignore` file). I can check the README and a file somewhere in
+`node_modules`.
+
+```bash
+❯ git ls-files --error-unmatch README.md
+README.md
+
+❯ git ls-files --error-unmatch node_modules/@ai-sdk/anthropic/CHANGELOG.md
+error: pathspec 'node_modules/@ai-sdk/anthropic/CHANGELOG.md' did not match any file(s) known to git
+Did you forget to 'git add'?
+```
+
+Notice the second command results in an error because of the untracked
+`CHANGELOG.md` file in `node_modules`.
+
+Here is another example of this at work while specifying multiple files:
+
+```bash
+❯ git ls-files --error-unmatch README.md node_modules/@ai-sdk/anthropic/CHANGELOG.md package.json
+README.md
+package.json
+error: pathspec 'node_modules/@ai-sdk/anthropic/CHANGELOG.md' did not match any file(s) known to git
+Did you forget to 'git add'?
+```
+
+Each tracked file gets listed and then the untracked file results in an error.
+
+See `man git-ls-files` for more details.