使用Perl的rename时如下所示：

shopt -s globstar # bash specific, enabled by default in `zsh`
rename 's/\.txt$/\.txt2/' ./**/*.txt

我找到答案了，但几乎读不懂。

for filePath in $(find . -name "*.txt"); do filePathWithoutExtension=$(echo ${filePath} | head -c -4) && mv ${filePath} ${filePathWithoutExtension}txt2; done

之前：

tree
.
├── folder1
│   ├── foo.txt
│   ├── subfolder1
│   │   └── foo.txt
│   └── subfolder2
│       └── foo.txt
├── folder2
│   └── foo.txt
└── foo.txt

4 directories, 5 files

之后：

tree
.
├── folder1
│   ├── foo.txt2
│   ├── subfolder1
│   │   └── foo.txt2
│   └── subfolder2
│       └── foo.txt2
├── folder2
│   └── foo.txt2
└── foo.txt2

4 directories, 5 files