What's the difference between the index, cached, and staged in git?

The index/stage/cache are the same thing - as for why so many terms, I think that index was the 'original' term, but people found it confusing, so the other terms were introduced. And I agree that it makes things a bit confusing sometimes at first.

The stash facility of git is a way to store 'in-progress' work that you don't want to commit right now in a commit object that gets stored in a particular stash directory/database). The basic stash command will store uncommitted changes made to the working directory (both cached/staged and uncached/unstaged changes) and will then revert the working directory to HEAD.

It's not really related to the index/stage/cache except that it'll store away uncommitted changes that are in the cache.

This lets you quickly save the state of a dirty working directory and index so you can perform different work in a clean environment. Later you can get back the information in the stash object and apply it to your working directory (even if the working directory itself is in a different state).

The official git stash manpage has pretty good detail, while remaining understandable. It also has good examples of scenarios of how stash might be used.

It's very confusing indeed. The 3 terms are used interchangeably. Here's my take on why it's called each of those things. The git index is:

• a binary file .git/index that is an index of all the tracked files
• used as a staging area for commits
• contains cached SHA1 hashes for the files (speeds up performance)

An important note is that the index/cache/stage contains a list of ALL files under source control, even unchanged ones. Unfortunately, phrases like "add a file to the index" or "file is staged to the index" can misleadingly imply that the index only contains changed files.

Here's a demo that shows that the git index contains list of ALL files, not only the changed files:

# setup
git init

echo 'x' > committed.txt
git add committed.txt
git commit -m 'initial'

echo 'y' > staged.txt
git add staged.txt

echo 'z' > working.txt

# list HEAD
git ls-tree --name-only -r HEAD
# committed.txt

# list index
git ls-files
# committed.txt
# staged.txt

# raw content of .git/index
strings .git/index
# DIRC
# committed.txt
# staged.txt
# TREE

# list working dir
ls -1
# committed.txt
# staged.txt
# working.txt

Additional reading:

https://www.kernel.org/pub/software/scm/git/docs/technical/racy-git.txt

What does the git index contain EXACTLY?