Git tips and tricks

This page collects various git tips and tricks which may be useful.

Merging history from two repos

Scenario

A new version of a service or a new deployment of an existing service has been developed in a new repository. We have moved over to using that repository as the source of truth but want the history of the old respository present in the new one.

Example

The IdP frontend container for Shibboleth is now the canonical source of configuration. We want to merge configuration from the previous Ansible configuration.

Recipe

Move to your local clone of https://gitlab.developers.cam.ac.uk/uis/devops/iam/authentication/shibboleth/idp-frontend and add a new remote:

cd path/to/idp-frontend-container
git checkout master  # checkout the master branch
git pull             # make sure we're up to date
git remote add other git@gitlab.developers.cam.ac.uk:uis/devops/iam/authentication/raven/ansible-shibboleth.git
git fetch other

The other repository's master branch is now available at other/master. Create a new branch which will be used to open a MR with the new history:

git checkout -b branch-for-mr

We want to merge the histories of the old repo into the new but not change any of the existing files. This is an example of a merge strategy. Git calls this strategy "ours". Perform the merge:

git merge --strategy ours other/master

Check that no changes have been made by computing a diff against master:

git diff master  # should produce no output

Check that the other repo's history is present:

git log  # should see some comits from the other repo

If all is good, push to upstream and open a MR:

git push -u origin branch-for-mr