I recently discovered zmv, a builtin feature of zsh that makes batch file renaming, a breeze.

To use it you need to add this to your zsh configuration file (.zshrc). 

autoload zmv

My blog contains several Markdown files with a .markdown extension and I want to use .md instead. Here is what the markdown files looks like. 

ls **/*.markdown  
    ./_drafts/2012-05-06-learning-ruby-and-ruby-on-rails.markdown
    ./_drafts/2015-07-19-welcome-to-jekyll.markdown
    ./_posts/2012-02-10-baked-with-octopress.markdown
    ./_posts/2015-08-15-batch-rename-files-with-zsh.markdown
    ./_posts/2015-08-15-octopress-3-0.markdown
    ./about/index.markdown
    ./contact/index.markdown
Note I used the double star zsh notation to recursively list the files which is shorter than find . -name '*.markdown'.

This is where zmv comes in handy to recursively change the extension from .markdown to .md. Once more the double stars ** match the current directory or whatever directory beneath it. 

zmv '(**/)(*).markdown' '$1$2.md'

Voilà, the files are renamed! 

ls **/*.md
    ./_drafts/2012-05-06-learning-ruby-and-ruby-on-rails.md
    ./_drafts/2015-07-19-welcome-to-jekyll.md
    ./_posts/2012-02-10-baked-with-octopress.md
    ./_posts/2015-08-15-batch-rename-files-with-zsh.md
    ./_posts/2015-08-15-octopress-3-0.md
    ./about/index.md
    ./contact/index.md

You can also use the -n option to ask zmv to print what it would do without actually doing it. This gives you an opportunity to check that everything is ok before running the command to prevent you from doing a lot of back and forth only because of a typo in the command ;-).

zmv  -n '(**/)(*).markdown' '$1$2.md'
    mv -- _drafts/2012-05-06-learning-ruby-and-ruby-on-rails.markdown _drafts/2012-05-06-learning-ruby-and-ruby-on-rails.md
    mv -- _drafts/2015-07-19-welcome-to-jekyll.markdown _drafts/2015-07-19-welcome-to-jekyll.md
    mv -- _posts/2012-02-10-baked-with-octopress.markdown _posts/2012-02-10-baked-with-octopress.md
    mv -- _posts/2015-08-15-batch-rename-files-with-zsh.markdown _posts/2015-08-15-batch-rename-files-with-zsh.md
    mv -- _posts/2015-08-15-octopress-3-0.markdown _posts/2015-08-15-octopress-3-0.md
    mv -- about/index.markdown about/index.md
    mv -- contact/index.markdown contact/index.md

I only scratched the surface of this feature. If you want to read more I recommend you Seth Brown’s post.