More details on git diff

kbroman · kbroman · commit 1c5b5026ee7d · 2013-05-13T09:35:58.000-05:00
diff --git a/index.md b/index.md
@@ -21,7 +21,8 @@ minimal guide to get started.
 - [Contribute to someone's repository](pages/fork.html): fork.
 - [Handling merge conflicts](pages/merge_conflicts.html).
 - [Oops; that last commit message was wrong](pages/amend_commit_msg.html).
-- [Exploring the code and its history](pages/exploring_code.html).
+- [Exploring the code and its history](pages/exploring_code.html):
+  tag, diff
 - [Branching and merging](pages/branching.html).
 - [Other (much more thorough) resources](pages/resources.html).
 
diff --git a/pages/exploring_code.md b/pages/exploring_code.md
@@ -4,11 +4,70 @@ title: Exploring code and its history
 ---
 
 For my own repositories, I like to use [gitx](http://gitx.frim.nl/) on
-my Mac.
+my Mac to explore the commit history.
 
 But, of course, you can also just poke around
 [github](http://github.com) itself.
 
 And you can do a lot of things from the command line, say to look at
 the combined changes to a file that were made over the last year.
-See the [github page about `git diff`](http://learn.github.com/p/diff.html).
+See the
+[github page about `git diff`](http://learn.github.com/p/diff.html).
+
+### Tags
+
+Each commit is assigned a &ldquo;hash tag&rdquo; which is a unique
+sequence of letters and numbers, like
+[4d9fe11e56652bd19e19e28eac3906f09d5a3074](https://github.com/kbroman/github_tutorial/commit/4d9fe11e56652bd19e19e28eac3906f09d5a3074).
+When you refer to these hash tags, you can just use an initial substr,
+like `4d9fe`, that is unique to your repository.
+
+For my [R/qtl package](http://github.com/kbroman/qtl), I like to tag
+particular commits by the version number of the package, then I can
+use my assigned tag in place of the less memorable hash tag.
+
+To assign a tag, use something like
+
+    git tag -a -m "Tagging version 1.28-5" 1.28-5
+
+To push the tags to github, you need to use
+
+    git push --tags
+
+To delete a tag, use
+
+    git tag -d 1.28-5
+
+and then you need to remove the tag from github
+
+    git push origin :refs/tags/1.28-5
+
+### More uses for diff
+
+To see all of the changes since the last commit, type
+
+    git diff
+
+To see all of the changes since a given commit, type
+
+    git diff [commit]
+
+Where in place of `[commit]` you use the initial part of a hash tag,
+like `9f4668c`, or a tag you've created, like `1.28-1`.
+
+To see all of the changes to a given file since a given commit, type
+
+    git diff [commit] [file]
+
+For example,
+
+    git diff 1.22-21 R/scanone.R
+
+To see all of the changes between two revisions, type something like
+
+    git diff 1.22-21 1.23-16
+
+If you use [gitx](http://gitx.frim.nl/), you can use it to view the
+differences using a pipe:
+
+    git diff 1.22-21 1.23-16 | gitx
