Code Spelunking with Ctags and Vim

When I’m programming, I spend a lot of time code spelunking (use the source, Luke!). A lot of times, the best documentation for a system is the source code for it. Knowing how to get to important places in your code and in others’ code makes a huge difference in how productive you are and even in what you can figure out how to do.

And the most important location for any method, function, class, variable, or whatsit is where it’s defined. Usually there’s documentation near. Sometimes there is information about parameters. It’s all very useful.

For finding those places, CTags is indispensable.

What is ctags?

CTags is program that finds the lines of code where things are defined. It knows 41 languages. (And if your favorite language isn’t on the list, you can probably find another program that generates compatible tags files for it.) You run it occasionally, and it indexes your code and stores it in a file named tags. Your editor reads this file and helps you jump through your code.

Installing ctags

The first step in using ctags is to install it.

Windows

The ctags page has a link to a ZIP file with a binary of ctags compiled for Windows. Download this and unzip it somewhere on your %PATH%.

Linux

If you’re using Linux, all major distributions have a package for ctags. See your documentation for details.

Mac

I’ve saved the Mac installation for last, because it’s the most complicated. You weren’t expecting that, were you?

Mac OS X comes with a program named ctags.

Unfortunately, it is not, in fact, Exuberant CTags, and it’s far more limited.

You can get the correct version of ctags using Homebrew:

> brew install ctags

Now you have to make sure that your shell finds the right version:

> which ctags
/usr/bin/

Well, that’s not right. Let’s rearrange our $PATH.

> PATH=/usr/local/bin:$PATH
> which ctags
/usr/local/bin

That’s better. You probably want to put that into your ~/.bash_profile file to make sure you find the right ctags in the future also.

Using ctags in a Text Editor

Now that ctags is installed, let’s put it to work. First, open up a command line or terminal or whatever you call it and change into the main directory of your project. Right now, I’m working on NeatlineFeatures, so I’ll use that for this example:

> cd ~/omeka/plugins/NeatlineFeatures/

Now, let’s run ctags over the code base. We want to to walk through the entire directory tree. But here’s the catch: we really want it to walk over the entire Omeka directory, including the Features plugin:

> ctags -R ../..

This runs ctags over everything in the Omeka directory (../..) and all subdirectories (-R).

This will take a while, and you’ll get some warnings about JavaScript files. Don’t worry about them. CTags has a few problems parsing JavaScript, but that won’t stop it from indexing the other files.

Let’s see what we have:

> ls -lh tags
-rw-r--r--  1 err8n  staff    23M Sep 14 16:00 tags
> wc -l tags
   75917 tags

Wow! More than 76,000 lines and 23MB. That’s a lot of indexing. But then, Omeka’s a large codebase.

What’s in the file? Let’s not worry about that right now. It’s plain text, and there is some metadata and lines detailing identifiers and files and line numbers. It’s actually a little scary, and we don’t have to worry about that anyway.

(If you’re really curious about the file format, look at the format page.)

The tags file can be used by a bunch of different text editors. In fact, there’s more than is on that list. Here are links to integrating tags into some popular editors:

I use Vim, and it comes with support for tags files built in. The rest of this post shows about how to use tags from Vim.

Using with Vim

First let’s start Vim from the directory containing the tags file. If you want to use gVim or MacVim, this may be different (hint: use the :cd command). I’ll just use the command line to start MacVim on one of the models.

> mvim models/NeatlineFeatureTable.php

The Tag Stack

The main point of all this, of course, are the powerful navigation commands. Let’s see what they are.

Vim maintains a stack of locations where we’ve been. This stack starts out empty.

When you jump to a tag, your current location is added to the stack. As you jump ahead, more locations are added to the stack. Here’s how to jump forward and add locations to the stack.

First say I want to look at the code for Omeka_Db_Table. I just move down to where it’s mentioned in the code and hit Control-].

And I’m there. I can open up the class and look in it.

But say this doesn’t tell me what I want. I really want to look at Zend_Db_Table. I don’t see it, so I can’t use Control-]. Instead, from command mode, I type out :tag Zend_Db_Table.

And I’m there.

But it’s empty. I need to jump to Zend_Db_Table_Abstract. I just move my cursor down there and hit Control-].

Say I want to know how fetchAll is defined. I move down there and open it up. But how else is it defined? I put my cursor on fetchAll and hit Control-]. At the bottom of the Vim window, it says tag 1 of 11 or more.

Interesting. How do I get to them?

In normal mode, I just use the command :tselect. Now Vim displays a list of everywhere that fetchAll is defined. I can select the number for which one I want, and Vim moves me there.

Examining the Tag Stack

Now I’ve jumped several times, and I’m a little confused about where I am. How do I find myself again?

In normal mode, use the :tags command. Vim will print out a list of which tags you’ve jumped to and where it is.

At this point, I want to move back to where I was. To do that, I just hit Control-t multiple times. Each time I do, it pops one position off the stack and moves be back to the previous tag location.

Jumping into a new window

Of course, it would be nice to be able to see what I’m working on and the tag too. Yes, I can be demanding. Fortunately, Vim can oblige me.

Unfortunately, it’s not as convenient as Control-], but it’s not bad. (And creating a normal-mode map for this isn’t difficult.)

In normal mode, just give the command :stjump [identifier]. This splits the window, and in the new split, it jumps to the tag. If there’s more than one definition for the tag, it prompts you for which you want, just like :tselect does.

Tag Navigation Cheat Sheet

So here’s what we’ve learned today:

:tag [identifier] Jump to the identifier.
:tags List the tag stack.
Control-] Jump to the tag under the cursor.
:tselect Select which tag location to go to for the current tag.
Control-t Jump back from the current tag.
:stjump [identifier] Jump to the identifier in a new split window.

What does it look like?

CTags in Vim from Scholars’ Lab on Vimeo.

With bonus content!

Next Steps

I’ve just presented the basics. Here’s some more about using tags in Vim.

Learning More

The Vim documentation for tags lists all of the many commands Vim has for working with tags.

Tagbar

Tagbar is a Vim plugin that shows the structure of your code for the file you’re in. It opens a side panel and displays the classes, methods, and other identifiers defined in the current file.

Running Automatically

Finally, Tim Pope has an excellent blog post on how to integrate Git, CTags, and Vim. He also explains how to set up your git repositories to run ctags automatically whenever you commit. It also makes it easy to customize how you run ctags for each project.

I highly recommend this system. It makes ctags even more awesome than it already is.


So let us know, what’s your favorite development productivity or code navigation tool?

My interests include text processing, text mining, and natural language processing, as well as web-development and general programming. Studied medieval English literature and linguistics at UGA. Dissertated on lexicography. Now I program in Haskell and write when I'm not building cool stuff for the Scholars' Lab. Also, husband and parent. Do you notice that sleep…

2 Comments

  1. Concise write-up. I would add that I find “Control-w Control-]” indispensable for jumping to a definition in a new split window.

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