Rewrite history git filter-branch create / split into submodules / subprojects

I am currently importing a cvs project into git.
After importing, i want to rewrite the history to move an existing directory into a seperate submodule.

Suppose i have a structure like this:

file1
file2
file3
dir1
dir2
library


Now i want to rewrite the history so that the directory library is always a git submodule. Say, split out specified directories into their own submodules / subprojects

This is my currently code:

File rewrite-submodule (which is called)

cd project
git filter-branch --tree-filter $PWD/../$0-tree-filter --tag-name-filter cat -- --all

File rewrite-submodule-tree-filter

    #!/bin/bash

    function gitCommit()
    {
        unset GIT_DIR
        unset GIT_WORK_TREE
        git add -A
        if [ -n "$(git diff --cached --name-only)" ]
        then
            # something to commit
            git commit -F $_msg
        fi
    }

    _git_dir=$GIT_DIR
    _git_work_tree=$GIT_WORK_TREE
    unset GIT_DIR
    unset GIT_WORK_TREE
    _dir=$PWD

    if [ -d "library" ]
    then
        _msg=$(tempfile)
        git log ${GIT_COMMIT}^! --format="%B" > $_msg
        git rm -r --cached lib
        cd library
        if [ -d ".git" ]
        then
            gitCommit
        else
            git init
            gitCommit
        fi
        cd ..
        export GIT_DIR=$_git_dir
        export GIT_WORK_TREE=$_git_work_tree
        git submodule add -f ./lib
    fi

    GIT_DIR=$_git_dir
    GIT_WORK_TREE=$_git_work_tree
    

This code creates the .gitmodules file, but not the submodule commit entry (the line Subproject commit <sha1-hash>, output by git diff) in the main repository and the files in directory library are still versioned in the main repository and not in the subproject repository.

Thanks in advance for any hint

The .gitmodules look like this:

    [submodule "library"]
        path = library
        url = ./library
    

I resolved my own question, here is the solution:

git-submodule-split library another_library

Script git-submodule-split:

    #!/bin/bash

    set -eu

    if [ $# -eq 0 ]
    then
        echo "Usage: $0 submodules-to-split"
    fi

    export _tmp=$(mktemp -d)
    export _libs="$@"
    for i in $_libs
    do
        mkdir -p $_tmp/$i
    done

    git filter-branch --commit-filter '
    function gitCommit()
    {
        git add -A
        if [ -n "$(git diff --cached --name-only)" ]
        then
            git commit -F $_msg
        fi
    } >/dev/null

    # from git-filter-branch
    git checkout-index -f -u -a || die "Could not checkout the index"
    # files that $commit removed are now still in the working tree;
    # remove them, else they would be added again
    git clean -d -q -f -x

    _git_dir=$GIT_DIR
    _git_work_tree=$GIT_WORK_TREE
    _git_index_file=$GIT_INDEX_FILE
    unset GIT_DIR
    unset GIT_WORK_TREE
    unset GIT_INDEX_FILE

    _msg=$(tempfile)
    cat /dev/stdin > $_msg
    for i in $_libs
    do
        if [ -d "$i" ]
        then
            unset GIT_DIR
            unset GIT_WORK_TREE
            unset GIT_INDEX_FILE
            cd $i
            if [ -d ".git" ]
            then
                gitCommit
            else
                git init >/dev/null
                gitCommit
            fi
            cd ..
            rsync -a -rtu $i/.git/ $_tmp/$i/.git/
            export GIT_DIR=$_git_dir
            export GIT_WORK_TREE=$_git_work_tree
            export GIT_INDEX_FILE=$_git_index_file
            git rm -q -r --cached $i
            git submodule add ./$i >/dev/null
            git add $i
        fi
    done
    rm $_msg
    export GIT_DIR=$_git_dir
    export GIT_WORK_TREE=$_git_work_tree
    export GIT_INDEX_FILE=$_git_index_file

    if [ -f ".gitmodules" ]
    then
        git add .gitmodules
    fi

    _new_rev=$(git write-tree)
    shift
    git commit-tree "$_new_rev" "$@";
    ' --tag-name-filter cat -- --all

    for i in $_libs
    do
        if [ -d "$_tmp/$i/.git" ]
        then
            rsync -a -i -rtu $_tmp/$i/.git/ $i/.git/
            cd $i
            git reset --hard
            cd ..
        fi
    done
    rm -r $_tmp

    git for-each-ref refs/original --format="%(refname)" | while read i; do git update-ref -d $i; done

    git reflog expire --expire=now --all
    git gc --aggressive --prune=now