git references acquaintance (HEAD, branch, tag)

git saving a reference in the document, the principle is very simple

References principle

引用I refer to is 提交记录the reference
提交记录used 哈希值to uniquely identify
each 引用represented by a file, save the file其引用的提交记录的哈希值

Reference classification

• Branch
  • Variable, at different times can point不同的提交记录
  • Local branch
    • The corresponding .git/refs/heads/目录files
    • Each 本地仓库multiple本地分支
  • Remote branch
    • The corresponding .git/refs/remotes/<远端仓库名>/目录files
    • Each 本地仓库may correspond to a plurality 远端仓库, and each 远端仓库can have a plurality of远端分支
• tag
  • The corresponding .git/refs/tags/目录files
  • Immutable, re-create deleted unless otherwise always points特定的提交记录
  • Each can have multiple git repositorytag
• HEAD
  • The corresponding .git/HEADfile
  • variable
    • Usually points to a 本地分支, that references to references
    • It may be pointed directly 某个提交记录, referred to HEAD detached, i.e. separated from the head pointer state
    • Can also point tag, git processed into such a situationHEAD detached
    • Can also point 远端分支, git processed into such a situationHEAD detached
  • Each is only a git repositoryHEAD
  • It represents the current 工作区detection file (or files before modification) which belongs to 提交记录the
  • git checkout 指令It is directed at changing HEAD
    • git checkout 本地分支名
    • git checkout 提交记录哈希值, detached
    • git checkout 远端分支名, detached
    • git checkout tag名, detached

experiment

$ git checkout master
Switched to branch 'master'

$ cat .git/HEAD
ref: refs/heads/master

$ cat .git/refs/heads/master
89d496d44f93d107a7eb404890cd15a14ba8845d

After checkout master, HEAD points to master, master pointing 89d496

$ git checkout milestone
Note: checking out 'milestone'.
You are in 'detached HEAD' state. 
HEAD is now at eecc5fe milestone

$ cat .git/HEAD
eecc5fe060e5b86957f931fd931beae4f206d4eb

After checkout tag milestone, HEAD pointing eecc5f, detached HEAD