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Git Advanced I

Author: alpha74

Hash Generation:

  • git hash-object <file>:
  • Create a SHA-1 hash for any file.
  • Returns a 40char log string of hash.
  • Same file returns same hash, and vice versa.

References ~ and ^ :

  • Git references are short names for SHA-1 hashes.

  • git show HEAD

    • Shows commit pointed by HEAD(generally the last commit on the branch).
    • Here HEAD is a reference of a SHA-1 hash.

  • ~ :

    • git show HEAD~1
      • Shows detailed commit information of immediate parent of HEAD.
      • ~ is assumed to be ~1. ~~ is same as ~2.
      • ~# : Use different values of # to get different commit information.

  • ^ :

    • Used in case of merge commits, where two branches merge into one.
    • ^ or ^1 : Refers to first parent of commit.
    • ^2 : Refers to second parent of merge commit.
    • ^^ : Refers to first parent's first parent.
    • ^2^ : Refers to second parent's first parent.

TAGS:

  • Tags can be used instead of branch labels or Git IDs in git commands.
  • Tags are simple text messages associated with labels, and they are NOT git objects. Annotated tags are.
  • git tag : Shows all tags in repo.
  • git tag v0.1 : Shows tag with name v0.1.
  • git tag <tagname> [<commit>] : Creates tag with <tagname>. <commit> defaults to HEAD.

ANNOTATED TAGS:

  • git tag -a [-m <msg> | -F <file>] <tagname> [<commit>] : Creates an annotated tag.

  • Annotated tags are git objects which store:

    • Message:

      • [-m <msg>]
      • Optional.
      • Provide a short message in cmd itself.

    • Message from file:

      • [-F <file>] :
      • Optional.
      • Provide a message in a file.

  • Commit:

    • Optional.
    • [<commit>] defaults to HEAD.

  • git tag -a -m "new release" v2.0 :

    • Creates an annotated tag with message "new release" using reference from another tag v2.0.
    • This type of reference is called Symbolic reference.

  • Pushing Tags:

    • Tags are automatically pushed to remote.
    • git push <remote> <tagname> : Pushes tag <tagname> to remote(origin).
    • git push <remote> --tags : Pushes all tags to remote.

BRANCHES & REFLOG:

  • git branch : List all the branches in local repo. Current branch has asterisk on it.

  • git branch <newbranch> : Creates a new branch from HEAD of current branch. Current branch is still the same.

  • git checkout <branch_or_commit> :

    • Checkout to a branch or commit.
    • Checkout commit is used to temporarily view an older version of project.
    • Checkout commit leads to detached HEAD state, as HEAD points directly to SHA-1 label.
    • To edit files here, a new branch should be created.

  • git branch -d <branchname> :

    • Deletes the branch label.
    • Works if no commits are made to that branch, and branch is not merged.

  • git branch -D <branchname> :

    • Deletes branch label even if some commits are made and branch is not merged.

  • git checkout -b <newbranch> : Combines git branch and git checkout. Works only for new branches.

  • git reflog:

    • Returns a local list of recent HEAD commits with SHA-1.
    • Works locally only.
    • git checkout -b <newbranch> <SHA-1> : Creates a branch from reflogged dangling commit.

MERGE:

  • Merges a topic branch in a base branch.

  • Types of merge:

    • Fast Forward (FF):

      • Moves base branch label to tip of topic branch.

      • Moves base branch label from its latest commit to latest commit of topic branch.

      • Only base branch label moves.

      • Possible only if no other commits were made to base branch since branching.

      • Attempting a FF merge is default.

      • Steps to merge topic branch to base branch:

        • git checkout base
        • git merge topic

    • Merge commit:

      • Combines commits at the tips of merged branches.

      • Places result in a new merge commit.

      • A merge commit always has multiple parents.

      • *Steps to merge topic branch to base branch:

        • git checkout base
        • git merge topic
        • In case of merge commit, Git will open default editor to edit the merge message.

      • Steps to create a Merge commit even if a FF commit is possible:

        • git checkout base
        • git merge --no-ff topic

    • Squash merge:

      • It is a type of option in Interactive Rebase.

      • Combines the selected commit with older commit creating a single commit.

      • The work of both commits in included.

      • SEE REBASE & SQUASH MERGE.

      • Delete(vs Squash):

        • No changes from this commit are applied.
        • diff is discarded.
        • Work of this commit is lost.
        • Greater chance of merge conflict.

    • Rebase:

      • SEE REBASE.