[git] How to unstage large number of files without deleting the content

Use git reset HEAD to reset the index without removing files. (If you only want to reset a particular file in the index, you can use git reset HEAD -- /path/to/file to do so.)

The pipe operator, in a shell, takes the stdout of the process on the left and passes it as stdin to the process on the right. It's essentially the equivalent of:

$ proc1 > proc1.out
$ proc2 < proc1.out
$ rm proc1.out

but instead it's $ proc1 | proc2, the second process can start getting data before the first is done outputting it, and there's no actual file involved.