もちろん！ここに、あなたが説明した機能を実装したbashスクリプトがあります：

```
#!/bin/bash

# specify the directory to search and the file extension to look for
search_dir="./"
file_ext=".txt"

# loop through all files in the search directory and its subdirectories
for file in $(find $search_dir -type f -name "*$file_ext"); do
  # extract the date the file was last modified
  file_date=$(date -r "$file" +%Y-%m-%d)
  
  # create a directory named after the modification date, if it doesn't already exist
  if [ ! -d "$file_date" ]; then
    mkdir "$file_date"
  fi
  
  # move the file into the directory named after its modification date
  mv "$file" "$file_date"
done
```

このスクリプトは、findコマンドを使って、指定されたディレクトリ（search_dir）とそのサブディレクトリから、指定された拡張子（file_ext）を持つファイルを探します。見つかった各ファイルについて、dateコマンドを使って更新日を抽出し、mkdirコマンドを使ってその日付にちなんだ名前のディレクトリを作成する（まだ存在しない場合）。最後に、mvコマンドを使ってファイルを更新日付にちなんだディレクトリに移動する。
Sure! Here's a bash script that implements the functionality you described:

```
#!/bin/bash

# specify the directory to search and the file extension to look for
search_dir="./"
file_ext=".txt"

# loop through all files in the search directory and its subdirectories
for file in $(find $search_dir -type f -name "*$file_ext"); do
  # extract the date the file was last modified
  file_date=$(date -r "$file" +%Y-%m-%d)
  
  # create a directory named after the modification date, if it doesn't already exist
  if [ ! -d "$file_date" ]; then
    mkdir "$file_date"
  fi
  
  # move the file into the directory named after its modification date
  mv "$file" "$file_date"
done
```

This script uses the find command to search the specified directory (search_dir) and its subdirectories for files with the specified extension (file_ext). For each file found, it extracts the modification date using the date command, and creates a directory named after that date using the mkdir command (if it doesn't already exist). Finally, it moves the file into the directory named after its modification date using the mv command.