git - filter-branch --index-filter always failing with "fatal: bad source"

Question

So, I am trying to rename a folder in my repository using git filter-branch --index-filter, but I always end up with a "fatal: bad source" error.

The problem is easily demonstrated on a test repository using a file instead of a folder.

Preparation:

$ git init
$ echo whatever >> my_file.txt
$ git add .
$ git commit -m "initial"
$ echo whatever2 >> my_file2.txt
$ git add .
$ git commit -m "second"

Now, I am trying to change my_file.txt to your_file.txt:

$ git filter-branch --index-filter 'git mv my_file.txt your_file.txt' HEAD

But it doesn't work:

Rewrite dac9a2023bdf9dd0159fab46213d9e1342ae9f75 (1/2)fatal: bad source, source=my_file.txt, destination=your_file.txt
index filter failed: git mv my_file.txt your_file.txt

However, the very same git mv command executed normally works without problems:

$ git mv my_file.txt your_file.txt
$ git status
# On branch master
# Changes to be committed:
#   (use "git reset HEAD <file>..." to unstage)
#
#       renamed:    my_file.txt -> your_file.txt
#

I am sure, I am missing something essential here - but what is it?

Answer 1

As twalberg points out in his answer, git mv doesn't just access the index, it also access the disk. That's probably the reason why it doesn't work.

I didn't want to use the slow --tree-filter so I tried to change the sample from the git filter-branch man page that shows how to move the complete repository into a subfolder.

The result is this - and it actually works ;-)

git filter-branch --index-filter '
git ls-files -s | 
sed "s-("*)my_file.txt-1your_file.txt-" | 
GIT_INDEX_FILE=$GIT_INDEX_FILE.new git update-index --index-info && 
mv "$GIT_INDEX_FILE.new" "$GIT_INDEX_FILE"
' HEAD