Emacs Tip #3: Whitespace Cleanup

Good developers are very careful about the proper use of whitespace in their files - there should be no empty lines at the beginning of a file, no empty lines at the end of a file, no trailing whitespace, no mixture of tabs and spaces, etc.

Emacs, naturally, wants to help with every problem possible and provides a very nice solution in the form of the whitespace-cleanup command. It’s a much more powerful alternative to the older delete-trailing-whitespace command, that simply deletes trailing whitespace, and it’s aware of the whitespace-style variable, used by whitespace-mode.

It usually applies to the whole buffer, but in transient mark mode when the mark is active, it applies to the region. It also applies to the region when it is not in transient mark mode, the mark is active and C-u was pressed just before calling whitespace-cleanup interactively. There is also a similar command called whitespace-cleanup-region.

The problems cleaned up are (borrowed from the official documentation):

1. Empty lines at beginning of buffer.

2. Empty lines at end of buffer. If whitespace-style includes the value empty, remove all empty lines at beginning and/or end of buffer.

3. 8 or more SPACEs at beginning of line. If whitespace-style includes the value indentation: replace 8 or more SPACEs at beginning of line by TABs, if indent-tabs-mode is non-nil; otherwise, replace TABs by SPACEs. If whitespace-style includes the value indentation::tab, replace 8 or more SPACEs at beginning of line by TABs. If whitespace-style includes the value indentation::space, replace TABs by SPACEs.

4. SPACEs before TAB. If whitespace-style includes the value space-before-tab: replace SPACEs by TABs, if indent-tabs-mode is non-nil; otherwise, replace TABs by SPACEs. If whitespace-style includes the value space-before-tab::tab, replace SPACEs by TABs. If whitespace-style includes the value space-before-tab::space, replace TABs by SPACEs.

5. SPACEs or TABs at end of line. If whitespace-style includes the value trailing, remove all SPACEs or TABs at end of line.

6. 8 or more SPACEs after TAB. If whitespace-style includes the value space-after-tab: replace SPACEs by TABs, if indent-tabs-mode is non-nil; otherwise, replace TABs by SPACEs. If whitespace-style includes the value space-after-tab::tab, replace SPACEs by TABs. If whitespace-style includes the value space-after-tab::space, replace TABs by SPACEs.

It might be a good idea to add whitespace-cleanup to your before-save-hook so that every buffer would be cleaned up before it’s saved:

(add-hook 'before-save-hook 'whitespace-cleanup)