rebasing.sh

### Initial situation: branch `develop` has one commit ("Initial Commit")


$ git checkout -b first develop

$ echo 'First content' > README.md

$ git add .

$ git commit -m 'First content'

[first 516f527] First content


### "first" branch now has two commits - "initial commit" and "first content"


$ git checkout -b second develop

$ echo 'Second content' > README.md

$ git add .

$ git commit -m 'Second content'

[second 6c2dfe5] Second content


### "second" branch now has two commits - "initial commit" and "second content"


$ git rebase first

First, rewinding head to replay your work on top of it...
Applying: Second content
Using index info to reconstruct a base tree...
M	README.md
Falling back to patching base and 3-way merge...
Auto-merging README.md
CONFLICT (content): Merge conflict in README.md
Failed to merge in the changes.
Patch failed at 0001 Second content
The copy of the patch that failed is found in:
   /Users/dylan/dev/git-scratchpad/.git/rebase-apply/patch

When you have resolved this problem, run "git rebase --continue".
If you prefer to skip this patch, run "git rebase --skip" instead.
To check out the original branch and stop rebasing, run "git rebase --abort".

$ cat README.md

<<<<<<< 516f527abc28ed9b373cfa05005e8e0aa119ebd4
First content
=======
Second content
>>>>>>> Second content

$ echo 'First content\nSecond content' > README.md

$ git add .

$ git status

rebase in progress; onto 516f527
You are currently rebasing branch 'second' on '516f527'.
  (all conflicts fixed: run "git rebase --continue")

Changes to be committed:
  (use "git reset HEAD <file>..." to unstage)

	modified:   README.md


$ git rebase --continue

Applying: Second content

$ git log

commit 91e91062c322bbae535dfa0bd123699cc757479a
Author: Dylan Pyle <me@dylanpyle.com>
Date:   Mon Oct 5 13:41:13 2015 -0700

    Second content

commit 516f527abc28ed9b373cfa05005e8e0aa119ebd4
Author: Dylan Pyle <me@dylanpyle.com>
Date:   Mon Oct 5 13:40:51 2015 -0700

    First content

commit 422f5e30a9b061d8f7b169c508593df2270ec20d
Author: Dylan Pyle <me@dylanpyle.com>
Date:   Mon Oct 5 13:40:19 2015 -0700

    Initial commit

# End result: The "second" branch only ends up with one copy of each commit;
# We rewrote the history of the second commit, as it changed from a commit 
# based on the content of `develop` to a commit based on the content of
# `develop` and `first`.
#
# Compare the commit hashes at the time we committed them, to the final result:
#
# Before:
#  first = 516f527
#  second = 6c2dfe5
#
# After:
#  first = 516f527
#  second = 91e9106
#
# The commit called "Second content" is not the same commit as the one we 
# initially wrote. GitHub will show them as different from each other, if you're
# comparing two branches. To resolve this, you'll want to force push the second 
# "re-written" commit over the first one in cases where it's appropriate - 
# otherwise you'll end up with two of the same (or similar) commits in your tree

This is great - thanks!

Only question: under what circumstances will this operation require a force push? As it's mostly a destructive operation, notes on etiquette and when a force push is acceptable would be great.