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Oh, the Git! - detached HEAD

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detached HEAD is a common situation, sometimes useful, sometimes dangerous. It doesn’t point to any branches, so it will be cleaned by git.

The current commit history is as follows:

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$ git log --oneline --all --graph
* 5ea3cec (HEAD -> master) Modify README.md of src
* 2b6e826 Same file, same blob
* 1ff08f2 Modify README.md.
* 8c19a38 Add Copyright notice.
* 318c11a Copy css to lib.
| * 1134f9e (dev) Make graph more readability
| * 71c40d3 Modify README.md in dev branch.
|/
* ce4297f Add image.
| * 2cb23ac (dev-1.0) README for dev-1.0 branch.
|/
* 6fc4b44 (tag: kikoff_tag) Copy doc README.md
* 726c6c0 Add source README.md
* c8ff9c5 Add README

Create detached HEAD

Create a detached HEAD by git checkout cmd.

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$ git checkout 8c19a38
Note: checking out '8c19a38'.

You are in 'detached HEAD' state. You can look around, make experimental
changes and commit them, and you can discard any commits you make in this
state without impacting any branches by performing another checkout.

If you want to create a new branch to retain commits you create, you may
do so (now or later) by using -b with the checkout command again. Example:

  git checkout -b <new-branch-name>

HEAD is now at 8c19a38 Add Copyright notice.

The note of git is important, it provides a lot information.

Check the current HEAD

We can see the current HEAD pointer points to ‘8c19a38’ by the following.

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$ cat .git/HEAD
8c19a3856e27ff8e29171e49ccccdc042f1de32e

$ git log --oneline --all --graph
* 5ea3cec (master) Modify README.md of src
* 2b6e826 Same file, same blob
* 1ff08f2 Modify README.md.
* 8c19a38 (HEAD) Add Copyright notice. # HEAD pointer, detached HEAD
* 318c11a Copy css to lib.
| * 1134f9e (dev) Make graph more readability
| * 71c40d3 Modify README.md in dev branch.
|/
* ce4297f Add image.
| * 2cb23ac (dev-1.0) README for dev-1.0 branch.
|/
* 6fc4b44 (tag: kikoff_tag) Copy doc README.md
* 726c6c0 Add source README.md
* c8ff9c5 Add README

Add a commit

  • Apply the following patch.
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diff --git a/README.md b/README.md
index a687b4c..16b8b3f 100644
--- a/README.md
+++ b/README.md
@@ -1,3 +1,6 @@
 Demonstration
 ===
+
+detached HEAD.
+
  • Generate commit 2457088 by git add and git commit.
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$ git add .;git commit -m "detached HEAD"
[detached HEAD 2457088] detached HEAD
 1 file changed, 3 insertions(+)

# Check new HEAD.
$ git log --oneline --all --graph
* 2457088 (HEAD) detached HEAD
| * 5ea3cec (master) Modify README.md of src
| * 2b6e826 Same file, same blob
| * 1ff08f2 Modify README.md.
|/
* 8c19a38 Add Copyright notice.
* 318c11a Copy css to lib.
| * 1134f9e (dev) Make graph more readability
| * 71c40d3 Modify README.md in dev branch.
|/
* ce4297f Add image.
| * 2cb23ac (dev-1.0) README for dev-1.0 branch.
|/
* 6fc4b44 (tag: kikoff_tag) Copy doc README.md
* 726c6c0 Add source README.md
* c8ff9c5 Add README

Switch branch

When we switch to master branch, git prompts us to create a branch for detached HEAD.

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$ git checkout master
Warning: you are leaving 1 commit behind, not connected to
any of your branches:

  2457088 detached HEAD

If you want to keep it by creating a new branch, this may be a good time
to do so with:

 git branch <new-branch-name> 2457088

Switched to branch 'master'

Check commit 2457088

When I switch branches, commit 2457088 is gone, this is the dangerous part.

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$ git log --oneline --all --graph
* 5ea3cec (HEAD -> master) Modify README.md of src
* 2b6e826 Same file, same blob
* 1ff08f2 Modify README.md.
* 8c19a38 Add Copyright notice.
* 318c11a Copy css to lib.
| * 1134f9e (dev) Make graph more readability
| * 71c40d3 Modify README.md in dev branch.
|/
* ce4297f Add image.
| * 2cb23ac (dev-1.0) README for dev-1.0 branch.
|/
* 6fc4b44 (tag: kikoff_tag) Copy doc README.md
* 726c6c0 Add source README.md
* c8ff9c5 Add README