Where does git store the last checked out branch?

Question

When I do git checkout -, git checks out the last branch I had checked out. Where is that information stored?

Answer 1

It scans .git/logs/HEAD (the reflog for HEAD) for the last line looking like :

checkout: moving from <branchA> to <branchB>

the "last branch" is branchA

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As said in the post linked by @SergioTulentsev :

git checkout - is a shorthand for git checkout @{-1}.

• @{-1} is the branchA of the last checkout: ... line
• @{-2} is the branchA of the previous to last checkout: ... line
• etc ...