Add Check If A File Has Changed In A Script as a Git TIL

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
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-_1671 TILs and counting..._
+_1672 TILs and counting..._
 
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@@ -321,6 +321,7 @@ If you've learned something here, support my efforts writing daily TILs by
 - [Caching Credentials](git/caching-credentials.md)
 - [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)
 - [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-has-changed-in-a-script.md

@@ -0,0 +1,38 @@
+# Check If A File Has Changed In A Script
+
+If I'm at the command line and I want to check if a file has changed, I can run
+`git diff` and see what has changed. If I want to be more specific, I can run
+`git diff README.md` to see if there are changes to that specific file.
+
+If I'm trying to do this check in a script though, I want the command to clearly
+tell the script _Yes_ or _No_. Usually a script looks for an exit code to
+determine what path to take. But as long as `git diff` runs successfully,
+regardless of whether or not their are changes, it is going to have an
+affirmative exit code of `0`.
+
+This is why `git diff` offers the `--exit-code` flag.
+
+> Make the program exit with codes similar to diff(1). That is, it exits with 1
+> if there were differences and 0 means no differences.
+
+With that in mind, we can wire up a script with `git diff` that takes different
+paths depending on whether or not there are changes.
+
+```bash
+if ! git diff --exit-code README.md; then
+  echo "README.md has changes"
+else
+  echo "README.md is clean"
+fi
+```
+
+We can take this a step further and instead use the `--quiet` flag.
+
+> Disable all output of the program. Implies --exit-code. Disables execution of
+> external diff helpers whose exit code is not trusted
+
+This exhibits the same behavior as `--exit-code` and goes the additional step of
+silencing diff output and disabling execution of external diff helpers like
+`delta`.
+
+See `man git-diff` for more details.