Excluding Directories from rgrep in Emacs

(For some reason, I found it difficult to find the solution to this online. I googled for it in the past, but always without success. Hopefully, this will help anyone else who wants to exclude / omit / ignore directories from an rgrep search in Emacs.)

rgrep performs grep recursively on a directory and all the directories contained therein. The nice thing about rgrep in Emacs–versus, say, ack–is that:

1. it prompts you for the starting directory (and ido still works!)
2. it prompts for the file name pattern to search (but defaults to the current extension, so you can just hit <return>)
3. the line isn’t filled up with the raw grep or ack command

The Emacs grep commands already intelligently omit certain directories, but I am spending a lot of time in Rails projects, and rgrep-ing from the project root would dive into the “log” and “vendor” directories. Rails development and testing logs can become very large (which slows down rgrep) and often include a match for the term I am searching (so the search results would consist mainly of log entries). The vendor directory can contain a very large number of files (especially when Rails is frozen into the project), and I usually don’t want search results from 3rd party plugins or the Rails source.

The solution is the grep-find-ignored-directories option:
M-x customize-option
<return>
grep-find-ignored-directories

Add “log” and “vendor” as omitted directories, and then “save for future sessions”. That saves the settings to custom.el in your .emacs.d directory. If you keep your .emacs.d directory under source control (as you can do for free at Github if you make it a public repository), make sure you don’t exclude custom.el.

Note that if you ever do want to search the vendor directory, you can simply set the vendor directory as the “Base directory” in the rgrep prompt.

Unfortunately, this doesn’t appear to be taken into account by find-grep-dired, which is how I perform global search and replace. If anyone knows how to do this for find-grep-dired, please leave a comment. Thanks!

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