I've read the docs on this several times over and I still don't completely get the differences between these different commands. Maybe it's just me, but the documentation could be more lucid:

http://git-scm.com/docs/gitignore

https://help.github.com/articles/ignoring-files

Moreover, a lot of the commentary on this subject seems to use the words "indexed", "committed", "tracked" somewhat loosely, which makes the differences between these three less clear.

My current (admittedly limited) understanding:

Files matched in .gitignore will not be tracked in the future. (Though they may have been tracked previously.) This means that they won't ever show up in a future git status list as changed. However, future changes will still be synced with remote repos. In other words, the files are still "indexed", but they are not "tracked". Because a .gitignore file is in the project directory, the file itself can be versioned.

Files matched in .git/info/exclude will also not be "tracked". In addition, these files will not ever be remotely synced, and thus will never be seen in any form by any other users. These files should be files that are specific to a single user's editor or workflow. Because it is in the .git directory, the exclude file can't itself be versioned.

Files that have had assume-unchanged run on them also don't show up in git status or git diff . This seems similar to exclude , in that these files are neither "indexed" nor "tracked". However, the last version of the file to be committed before assume-unchanged will remain visible to all users in the repo.

My questions:

Is the above interpretation correct? Please correct me.

If a file has already been in a commit, what is the functional different between matching it in .exclude and running assume-unchanged on it? Why would one prefer one approach to another?

My basic use case is that I want to avoid sorting through diffs on compiled files, but I still want those compiled files synced along with the source files. Will a gitignore 'd file still be pushed? If not, how to manage final deployment of the compiled files?

Thanks in advance for any help.

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I'm going to accept this emailed answer from Junio Hamano (the maintainer of Git) because I think it explains some things more lucidly than the official docs, and it can be taken as "official" advice:

The .gitignore and .git/info/exclude are the two UIs to invoke the same mechanism. In-tree .gitignore are to be shared among project members (ie everybody working on the project should consider the paths that match the ignore pattern in there as cruft). On the other hand, .git/info/exclude is meant for personal ignore patterns (ie you, while working on the project, consider them as cruft).

Assume-unchanged should not be abused for an ignore mechanism. It is "I know my filesystem operations are slow. I'll promise Git that I won't change these paths by making them with that bit---that way, Git does not have to check if I changed things in there every time I ask for 'git status' output". It does not mean anything other than that. Especially, it is not a promise by Git that Git will always consider these paths are unmodified---if Git can determine a path that is marked as assume-unchanged has changed without incurring extra lstat(2) cost, it reserves the right to report that the path has been modified (as a result, "git commit -a" is free to commit that change).

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Adding to Junio Hamano's answer, Git 2.3.0 (February 2015) now removes from the gitignore documentation

To ignore uncommitted changes in a file that is already tracked, use ' git update-index --assume-unchanged '.

See commit 936d2c9 from Michael J Gruber ( mjg ):

gitignore.txt : do not suggest assume-unchanged

git-update-index --assume-unchanged was never meant to ignore changes to tracked files ( only to spare some stats ).
So do not suggest it as a means to achieve that.

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Hopefully, not too many sources of information are using tracked, indexed and committed loosely, since they are all different and meaningful.

Indexed means that the file is in the git index. At some point in the past, someone has use git add or an equivalent command on the file. The file is tracked, and might also be committed.

Tracked means that git is watching the file for changes. Any committed file, or any file in the index is tracked.

Committed means that the file is in git's history. There is at least one checkpoint for this file; you can revert to any committed version of the file.

Now to the limit of my own knowledge. I'm not sure about this definition, but this is my understanding; happy to be corrected about this:

when an indexed file is committed, it is no longer in the index. The next time it is modified (or deleted), it is back in the index. The index is the sum of all tracked files that differ from what's committed.

The index is also called the cache, or the staging area.

On to your main question. .git/info/exclude is the same as .gitignore, just a lower precedence and not in the repository (so, not committed and shared). Neither affects already tracked files. Both affect files that are not currently tracked. Updating .gitignore after git add or git commit is too late; git already tracks the file, and .gitignore won't affect that.

Assume-unchanged affects only tracked files, and thus is completely separate to .gitignore. It can temporarily pretend that the file is untracked and ignored (but it doesn't have to and can also do nothing different from normal behaviour). As other answers mention, this is not used for ignoring changes to files, just for potentially avoiding file system operations on slow file systems.

Re: point 3: you should not add compiled files to git. Compile your files to a different directory that your source is in, and ignore that entire directory. Bundle your compiled files into a library and add it to an artifact repository, but don't put them in git.

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