HEAD 与 detached HEAD

  $ cat .git/HEAD; git symbolic-ref HEAD; git rev-parse HEAD | cut -c1-12; cat .git/refs/heads/main | cut -c1-12
  ref: refs/heads/main
  refs/heads/main
  e79e434670c8
  e79e434670c8

  $ git checkout -q 9867bddbd4ae45702c6ed062bec552b684c96d4d; cat .git/HEAD | cut -c1-12; git symbolic-ref HEAD 2>&1; git status | head -2; git branch
  9867bddbd4ae
  fatal: ref HEAD is not a symbolic ref
  HEAD detached at 9867bdd
  nothing to commit, working tree clean
  * (HEAD detached at 9867bdd)
    main

  $ git log --oneline --all --graph; git branch --contains HEAD 2>&1
  * 690e7c4 detached 上的提交
  | * e79e434 提交 3
  |/
  * 9867bdd 提交 2
  * 88a540f 提交 1
  * (HEAD detached from 9867bdd)
  切回 main:git 提醒你正在丢下一个提交
  $ git checkout main 2>&1
  Warning: you are leaving 1 commit behind, not connected to
  any of your branches:
  
    690e7c4 detached 上的提交
  
  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> 690e7c4
  
  Switched to branch 'main'
  $ git branch --contains 690e7c485315d1a71ac3e10a948539a48f64a368; echo "(上面没有输出:没有任何分支包含它)"; git reflog | head -5 | cut -c9-
  (上面没有输出:没有任何分支包含它)
  HEAD@{0}: checkout: moving from 690e7c485315d1a71ac3e10a948539a48f64a368 to main
  HEAD@{1}: commit: detached 上的提交
  HEAD@{2}: checkout: moving from main to 9867bddbd4ae45702c6ed062bec552b684c96d4d
  HEAD@{3}: commit: 提交 3
  HEAD@{4}: commit: 提交 2

  $ git switch -c rescued 690e7c485315d1a71ac3e10a948539a48f64a368 2>&1; git log --oneline rescued
  Switched to a new branch 'rescued'
  690e7c4 detached 上的提交
  9867bdd 提交 2
  88a540f 提交 1

  $ git checkout -q main; ls .git | grep -E "^(ORIG_HEAD|FETCH_HEAD|MERGE_HEAD|CHERRY_PICK_HEAD|REVERT_HEAD|REBASE_HEAD)$" || echo "(现在一个都没有)"
  (现在一个都没有)
  $ git reset -q --hard HEAD~1; ls .git | grep -E "_HEAD$"; git rev-parse ORIG_HEAD | cut -c1-12; git reset -q --hard ORIG_HEAD
  ORIG_HEAD
  e79e434670c8
  $ git merge side 2>&1 | head -3; ls .git | grep -E "_HEAD$"; cat .git/MERGE_HEAD | cut -c1-12; git merge --abort; ls .git | grep -E "MERGE_HEAD" || echo "(abort 之后 MERGE_HEAD 没了)"
  Auto-merging a.txt
  CONFLICT (content): Merge conflict in a.txt
  Automatic merge failed; fix conflicts and then commit the result.
  MERGE_HEAD
  ORIG_HEAD
  6aea679fd5ec
  (abort 之后 MERGE_HEAD 没了)
  $ git cherry-pick side 2>&1 | head -2; ls .git | grep -E "_HEAD$"; git cherry-pick --abort
  Auto-merging a.txt
  CONFLICT (content): Merge conflict in a.txt
  CHERRY_PICK_HEAD
  ORIG_HEAD
  $ git remote add o /work/l23 2>/dev/null; git fetch -q o 2>&1; ls .git | grep -E "FETCH_HEAD"
  FETCH_HEAD
  FETCH_HEAD 的前两行(hash 前 12 位 / 是否可合并 / 来源):
    e79e434670c8 not-for-merge branch 'main'
    690e7c485315 not-for-merge branch 'rescued'

  $ cat .git/HEAD; git status | head -3; git rev-parse HEAD 2>&1 | head -1; git log 2>&1 | head -1
  ref: refs/heads/main
  On branch main
  
  No commits yet
  fatal: ambiguous argument 'HEAD': unknown revision or path not in the working tree.
  fatal: your current branch 'main' does not have any commits yet
  $ git symbolic-ref HEAD refs/heads/其它名字; cat .git/HEAD; git status | head -2
  ref: refs/heads/其它名字
  On branch 其它名字
  $ git checkout --orphan fresh 2>&1; cat .git/HEAD; git status | head -3; git log 2>&1 | head -1; ls
  Switched to a new branch 'fresh'
  ref: refs/heads/fresh
  On branch fresh
  
  No commits yet
  fatal: your current branch 'fresh' does not have any commits yet
  a.txt