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IMP Manual  for IMP version 2.5.0
git submodules and subtrees

IMP includes several pieces of code that are actually maintained in separate git repositories on GitHub. These include the RMF library and the IMP::pmi module. There are two main ways to achieve this with git: git submodule and git subtree. Modern versions of IMP use git subtree (we previously used submodule; see the issue on GitHub for a discussion). subtree is a little harder for developers to work with than submodule, but is much easier for end users (no need to run any setup scripts after getting IMP, forks of the IMP repository work, and GitHub's "Download ZIP" feature works).

This blog post covers several useful commands for working with git subtree.

As an example, a typical workflow for working with the IMP::pmi module is:

  • git clone IMP as per usual
  • Work on the PMI module in modules/pmi
  • Use regular git commands git add, git commit etc. to record changes
  • Periodically push them to the PMI repository (not the main IMP repository) by running (in the top level IMP directory)

    git subtree push --prefix=modules/pmi develop

    • Important: be careful not to also push these commits to IMP. You may need to use git reset HEAD~N (where N is the number of commits you made) to remove them from IMP (or, if you made both PMI commits and regular IMP commits, use git rebase -i to delete just the PMI commits). Use git subtree pull (below) to incorporate PMI changes into IMP.
    • git subtree push is very slow, since it has to examine every commit in the IMP repository.
    • If this fails with fatal: bad object, then it needs you to do a tools/git/ or git subtree pull (see below) first.
  • To incorporate changes from the PMI repository into IMP, use the utility script


    • This will squash all of the PMI changes into a single IMP commit, which you can then git push into the IMP repository in the normal way. (There is a git subtree pull command which is supposed to achieve the same thing, but the syntax is more complicated, and we find that often the commits it generates are enormous and touch every file in IMP.)

Note that since git subtree commands can be very slow, if you will be working a lot with a subtree it is probably faster to checkout its own repository directly and then use regular git push and git pull to work inside it. (For example, you could checkout the PMI repository into ~/pmi, then temporarily replace the subtree in IMP with a symlink to the real repository with something like cd ~/imp/imp/modules; mv pmi pmi.old; ln -sf ~/pmi pmi. Move back pmi.old when you're done, then use to incorporate the PMI changes into IMP.)