Ignore specific changes to a file in git, but not the entire file

Question

I have a file in a git repository that has a local change on it. I want to have git ignore the local change forever, but not the file. In particular,

• If the file isn't touched besides this change, git add . should never stage it.
• Likewise, git commit -a shouldn't commit it.
• If I ever make an additional change to the file, I should be able to stage and commit that change - but the change I'm ignoring should not be staged and committed.

Is there a way to do this? Doing some research, I read about "smudge/clean cycles," where, if I read correctly,

1. the file would be marked as unchanged,
2. the change I made would be overwritten when I checkout,
3. and then a script would automatically reapply the change and then mark the file as unchanged again.

I am very new to git and scripting, though (I'm an intern with C# and Java experience), so if that's what I need to do, can you please post detailed directions or a link to a tutorial on how to set a smudge/clean cycle up?

Background: I want my working branch to be out of sync with the trunk. There is a low priority bug that only affects development machines, so rather than fix it, we're just commenting out the offending code. Obviously, we don't want this code to be removed from production, where it works just fine.

Answer 1

You can use the skip-worktree bit. Turn it on with:

git update-index --skip-worktree <file>

After that, git will never stage local changes for <file> and will fail (loudly) if git itself has to write to <file> (say, in a merge or a checkout).

If you ever want to stage a future change, you can turn it off, stage the new change, and then turn it back on:

git update-index --no-skip-worktree <file>
git add -p <file>
git update-index --skip-worktree <file>

While not perfect, this might be good enough. It will be up to you to notice that <file> has unstaged changes, since git will no longer tell you that

Note: My original suggestion was to use assume-unchanged. As explained in Git - Difference Between 'assume-unchanged' and 'skip-worktree', it is really skip-worktree that you want. In particular, assume-unchanged is a promise to Git that you won't change the file, and if you violate that promise Git is allowed to erase your changes or commit them! In contrast, Git will not erase or commit your skip-worktree changes.

Answer 2

These answers are good, but may not best solve @Kevin's problem. I had a similar concern, often editing a config file so the app I was working on would access my own private development database instead of the production one. It's only a matter of time before I accidentally check in and push those config changes! I just needed a light weight way to ignore a file. Here's what I learned:

• First, make your needed change to your file. I'll call it my_config.

• Make a patch file of that change with git diff >../somewhere-else/my_config.patch

• Now tell git to ignore that file (without having to change the checked-in .gitignore): git update-index --assume-unchanged my_config

Now, as long as you don't make changes to my_config that you do want to check in, you can work freely. To stop ignoring my_config, do git update-index --no-assume-unchanged my_config. After pulling in somebody else's changes to my_config, you can easily restore your private change with git apply ../somewhere-else/my_config.patch, then ...assume-unchanged again, as above, and get back to work!

Here are some helpful aliases you can put in your ~/.gitconfig:

[alias]
    unchanged = update-index --assume-unchanged
    changed = update-index --no-assume-unchanged
    show-unchanged = !"git ls-files -v | sed -e 's/^[a-z] //p; d'"