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content/posts/pull_files_into_submodule.md

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+---
+title: "Separate files from git repo into a submodule"
+date: 2023-02-22T20:31:34-05:00
+topics: ["git", "software development"]
+---
+
+I recently had a situation where a library I was working on, originally as part of one project, was going to be needed for another project.
+The ideal way to handle this situation, is to have the library files as their own git repo which is then added to the projects as a submodule.
+This way any changes required to the submodule for the needs of each project can be shared easily.
+It took me much longer than I would've liked to, but I finally managed to find the solution and wanted to share it with anyone else who might need it. 
+
+
+Assume I have a git repo as below:
+
+```
+test
+├── README.md
+├── main.cpp
+├── test.cpp
+└── test.h
+```
+
+If I wanted to pull out the test.cpp and test.h files on their own with all the history for only those commits, the command is:
+
+```
+$ git filter-branch --force  --prune-empty --index-filter \
+ 'git rm --cached --ignore-unmatch $(git ls-files | grep -v "test.h\|test.cpp")'
+```
+
+A couple notes should be mentioned. 
+First when you run the git filter-branch command it will give you a message saying that you shouldn't use filter-branch and should instead use filter-repo. 
+I don't know how to do this with filter-repo and didn't have the time to figure it out.
+The `--prune-empty` flag deletes all the commits that aren't associated with the files of interest so that you don't have to do a rebase.
+Finally to specify the files you want, you need to pass them to the grep command as:
+
+```
+grep -v "file1\|file2\|file3"
+```
+
+The `-v` flag inverts the match, returning all files to the `git rm` command which don't match the files you specify.
+The file names must be separated with `\|` for matching multiple different tokens.
+Once this command completes, you should be left with just the files of interest and the associated history. 
+All that's left is simply setting a new remote url and then push, i.e.
+
+```
+$ git remote set-url origin submodule.git.url
+$ git push
+```
+
+**Final Important Note:** If this goes wrong you may get worried as all of your git history is wiped out. 
+To fix this you can use `git reflog`.
+If you run `git reflog` after you mess up the `git filter-branch` you should see something like this (dummy commits from my fake repo):
+
+```
+$ git reflog
+790c883 (HEAD -> master) HEAD@{0}: filter-branch: rewrite
+3b3c8b8 HEAD@{1}: commit: Update test library
+93cfbd4 HEAD@{2}: commit: Add main function
+18259bc HEAD@{3}: commit: README update
+aba8323 HEAD@{4}: commit: init test files
+```
+
+You can then reset your git repo to a state before your filter branch command by running:
+
+```
+$ git reset --hard HEAD@{1}
+```
+
+Hopefully this helps someone out!