Installing universal-ctags on Ubuntu so Emacs/Vim can use it

universal-ctags.org

Installing universal-ctags on Ubuntu

This is my preferred way to go about it.

1. Download and build universal-ctags

These instructions are described in some detail in the universal-ctags install instructions on GitHub. But here’s what I ran:

% cd ~/src
% hub clone universal-ctags/ctags universal-ctags && cd universal-ctags
% ./autogen.sh
% ./configure

I initially wanted to use --program-prefix=universal- to result in the final code being installed into /usr/local/bin/universal-ctags/ instead of just as ctags. But it turned out that breaks because of how universal-ctags handles etags compatibility: it must be called as ${PROGRAM_PREFIX}etags. But via the alternatives system, it will actually be called as plain old etags, so it needed to be built with no --program-prefix argument.

Continuing the build:

% make

2. Install universal-ctags

Simply:

% sudo make install

3. Add universal-ctags to Ubuntu ctags alternatives

Here, we want to tell Ubuntu that it can use universal-ctags as a ctags variant. The update-alternatives command is used for this. First, let’s have a look at what’s already been installed as known alternatives:

% update-alternatives --display ctags
ctags - auto mode
  link best version is /usr/bin/ctags.emacs24
  link currently points to /usr/bin/ctags.emacs24
  link ctags is /usr/bin/ctags
  slave ctags.1.gz is /usr/share/man/man1/ctags.1.gz
/usr/bin/ctags.emacs24 - priority 27
  slave ctags.1.gz: /usr/share/man/man1/ctags.emacs24.1.gz

This may look complex, but it’s just telling us that a version of ctags has been automatically selected from the single known alternative — ctags.emacs24, installed as part of Emacs.

As earlier mentioned, if you examine the manpage for universal-ctags, you’ll note that to run in Emacs mode, it expects to be called as etags. So let’s see what’s currently in use as etags:

% update-alternatives --display etags
etags - auto mode
  link best version is /usr/bin/etags.emacs24
  link currently points to /usr/bin/etags.emacs24
  link etags is /usr/bin/etags
  slave etags.1.gz is /usr/share/man/man1/etags.1.gz
/usr/bin/etags.emacs24 - priority 27
  slave etags.1.gz: /usr/share/man/man1/etags.emacs24.1.gz

Much the same thing.

So now, we have to inform Ubuntu’s alternatives system about the new command.

% sudo update-alternatives --install /usr/bin/ctags ctags /usr/local/bin/ctags 50 --slave /usr/share/man/man1/ctags.1.gz ctags.1.gz /usr/local/share/man/man1/ctags.1

Here, we tell it that /usr/local/bin/ctags is an acceptable version of ctags, and that its manpage can be found at /usr/local/share/man/man1/ctags.1.gz. Note the 50 — that argument sets the “priority” when autoselecting an alternative. The above output told us that Emacs’ version was set to priority 27, so 50 has been arbitrarily chosen as a higher priority, forcing universal-ctags to be preferred.

We need to do the same with etags, as so:

% sudo update-alternatives --install /usr/bin/etags etags /usr/local/bin/ctags 50 --slave /usr/share/man/man1/etags.1.gz etags.1.gz /usr/local/share/man/man1/ctags.1

Now we can see the links are right:

% echo =ctags =etags
/usr/local/bin/ctags /usr/bin/etags
% readlink -f =etags =ctags
/usr/local/bin/ctags
/usr/local/bin/ctags

Note

The syntax =ctags tells zsh to replace the string with the full path name to the ctags command. In bash, $(which ctags) would be equivalent.

The most obvious difference between ordinary ctags and etags is where they put the tagfile: the former uses tags, the latter TAGS. So we can test this as so to ensure that universal-ctags is being called correctly; if etags creates a TAGS file instead of a tags file, we’re good:

% ls tags TAGS
ls: cannot access 'tags': No such file or directory
ls: cannot access 'TAGS': No such file or directory
% ctags -R .
% ls tags TAGS
ls: cannot access 'TAGS': No such file or directory
tags
% etags -R .
% ls tags TAGS
TAGS  tags

Now, all that’s required is to use your tags as normal.