スクリプトは、スペースや特殊文字を含むファイル名/パスでは正しく動作しない場合があります。そのような場合に対応するため、例えばファイルパスを二重引用符で囲んでスペースや特殊文字を保持することをお勧めします：

```
#!/bin/bash

# specify the extension of the files you want to look for
extension=".txt"

# loop through all files in the current directory and its subdirectories
find . -name "*$extension" | while read file; do
    # extract the date the file was last modified
    date=$(date -r "$file" +%Y-%m-%d)

    # create a new directory named after the date, if it doesn't already exist
    if [ ! -d "$date" ]; then
        mkdir "$date"
    fi

    # move the file into the directory named after its modification date
    mv "$file" "$date"
done
```
The script may not work properly with filenames/paths that contain spaces or special characters. To handle such cases, it's recommended to use double quotes around file paths to preserve the spaces and special characters, for example:

#!/bin/bash

# specify the extension of the files you want to look for
extension=".txt"

# loop through all files in the current directory and its subdirectories
find . -name "*$extension" | while read file; do
    # extract the date the file was last modified
    date=$(date -r "$file" +%Y-%m-%d)

    # create a new directory named after the date, if it doesn't already exist
    if [ ! -d "$date" ]; then
        mkdir "$date"
    fi

    # move the file into the directory named after its modification date
    mv "$file" "$date"
done