Rebasing

Rebase changes modifies a series of commits to start from a later commit then they originated from. The common example is when you branch off the master branch and the master branch moves ahead in parallel to your branch work.

                             # master -> D254644; HEAD -> master
git checkout -b abranch      # abranch -> master -> D254644; HEAD -> abranch
git add .                    # G154547 commit-tree created; G154547.parent -> D143424
git commit -m "abranch work" # abranch -> G154547
git checkout master          # HEAD -> master
git add .                    # J154510 commit-tree created; J154510.parent -> D143424
git commit -m "master work"  # master -> J154510
git checkout abranch         # HEAD -> abranch
git rebase master            # G154547 changes are calculated off of J154510 into new L141211
                             # L141211.parent -> J154510; abranch -> L141211
git checkout master          # HEAD -> master
git merge abranch            # master -> L141211

This rebase changes the way the master merged in the branch. What was bound to be a composite merge is now a simple fast-forward merge. It is as if you branched for master after those changes were made to master.

Interactive Rebasing

                             # master -> D254644; HEAD -> master
git checkout -b abranch      # abranch -> master -> D254644; HEAD -> abranch
git add .                    # G154547 commit-tree created; G154547.parent -> D143424
git commit -m "abranch 1"    # abranch -> G154547
git add .                    # J154510 commit-tree created; J154510.parent -> G154547
git commit -m "abranch 2"    # abranch -> J154510
git rebase -i

This brings up an interactive interface.

pick G154547 abranch 1
pick J154510 abranch 2

# Rebase G154547..J154510 onto G154547
#
# 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

I just want to squash my history together.

pick G154547 echo 1
s J154510 echo 2

and save

I'm prompted by a commit message screen

and save

Now my commits are collapsed into 1 commit.