4. Select Advanced Git Usage

While widely used, git is not always well understood and often is used by developers as one would use svn. Below are some advanced usages of git that CELESTE developers should be familiarize themselves with. More documentation is always available at the official Git Documentation.

4.1. Simple Rebasing: git rebase

Git allows developers to rebase branches. To visualize:

Before:

      A---B---C topic
     /
D---E---F---G master

Running git rebase:

git rebase master topic

After:

              A'--B'--C' topic
             /
D---E---F---G master

Developers should be rebasing their changes prior to opening a PR. This will help prevent merge conflicts and the resulting non-linear history that appears when PR is merged in, which makes future code inspection of commits unnecessarily difficult:

# BAD
      A---B---C
     /         \
D---E---F---G---H master

# GOOD
D---E---F---G---A'--B'--C' master

If the commits in the side branch were already pushed prior to rebase, they will need to be force-pushed after the rebase.

4.2. Interactive rebasing: git rebase -i

In addition, git supports interactive rebasing. This allows develoeprs to:

1. Squash related commits together into one commit
2. Edit commit messages
3. Delete non-useful commits

Before:

      A---B---C---D----E---F topic
     /
D---E---F---G master

Running git rebase -i master topic will open an interactive vim shell. Simply edit the file as instructed and save to disk to execute the rebase

operations:

pick 358bc5a commit A in topic branch
squash 1b01e0a commit B in topic branch
pick 755189f commit C in topic branch
pick b95b8a4 commit D in topic branch
squash 084bf71 commit E in topic branch
squash 01e4fde commit F in topic branch

# Rebase 897f795..084bf71 onto 897f795 (5 command(s))
#
# Commands:
# p, pick = use commit
# r, reword = use commit, but edit the commit message
# e, edit = use commit, but stop for amending
# s, squash = use commit, but meld into previous commit
# f, fixup = like "squash", but discard this commit's log message
# x, exec = run command (the rest of the line) using shell
#
# These lines can be re-ordered; they are executed from top to bottom.
#
# If you remove a line here THAT COMMIT WILL BE LOST.
#
# However, if you remove everything, the rebase will be aborted.
#
# Note that empty commits are commented out

After:

      (AB)---C---(DEF) topic
     /
D---E---F---G master

4.3. Applying Exising Commits to a Different Branch: git cherry-pick

Sometimes, we want to apply a desirable change from one working branch to another working branch. In the below example, would like to apply commit C to the master branch.

Before:

      76cada---62ecb3---b886a0 topic
     /
dd2e86---946992---9143a9---a6fd86---5a6057 master

Running git cherry-pick:

git checkout master
git cherry-pick 62ecb3

After:

      76cada---62ecb3---b886a0 topic
     /
dd2e86---946992---9143a9---a6fd86---5a6057--[62ecb3] master