How to preserve all ignored files in git clean -fd?

Git normally doesn't clean ignored files unless the -x flag is specified, but strangely it cleans out when configured as you did (folder/*).

As @VonC pointed out, you should change your .gitignore-file to ignore the directory (data/) rather than its contents (data/*).

It's a subtle difference, but it matters to git.

I've found some more details. Having /tmp/* in gitignore, git clean -fd will remove it. As it was said in other answers, this does not happens with /tmp/ in gitignore.

But once you have any checked-in any file in this directory, git clean -fd will ignore this path. This can be achieved with git add -f or adding !/tmp/.keep to gitignore and checking this file in.