Suppose I have 3 tracked files:

```
A.txt
B.txt
C.txt

```

I make changes to B.txt and do:

```
$ git add B.txt

```

Later on I make more changes to both A.txt and B.txt and I want to re-add those changes to B.txt to the index, but not A.txt. If I do this:

```
$ git add -u .

```

This will add both `A.txt` and `B.txt`. However, I want a quick way to add only tracked, modified files to the index that have already been previously added to the index. This would mean that only `B.txt` should be added.

Note: Doing `git add B.txt` isn't an appropriate solution here since my example is contrived. The more complex example has tons of files in my index, but I modify them again, and I want a quick way to update only those files in the index with the new working tree changes.