As the local geek I get all sorts of (Linux) questions asked, like "How can you delete the nth line with Sed?" or "Is there a way to search for the following in a file?" (the latter being a request to construct a regular expression for grep). And while I'm pretty sure, you can find answers for such questions quickly with $SEARCH_ENGINE I find myself generally typing the answer into the IM session. This, and just seeing Bernd's post about Vim on Planet Debian, prompted me to start a little, irregular series of posts, to which I can point people, whenever I get asked such questions.

I start this off with a tip for Vim, a very powerful text editor. The problem is simple: you get (source code) files with trailing whitespaces (sometimes accumulated in "empty" lines). This makes diffing (and merging) difficult. Thus the question is: how do I prevent that from happening? How do I notice, that I have whitespaces at the end of a line? The solution consists of two lines in your .vimrc:

:highlight TrailWhitespace ctermbg=red guibg=red
:match TrailWhitespace /\s\+$\| \+\ze\t/

If you just want this functionality if syntax highlighting is also active, then you should use

:autocmd Syntax * syn match TrailWhitespace /\s\+$\| \+\ze\t/

instead of the :match line. The regular expression used in both cases matches trailing whitespaces and whitespaces in front of tabs.

Of course there are several other options on how to do this or what you might want to highlight, but that would be beyond the scope of this little post.

Yesterday's blog entry garnered a lot of attention and some responses (in the form of e-mails, which is totally fine). Two of them deserve, as far as I'm concerend, their own little entry in the mini-series I kicked off. So, here we go:

First up was reader Sylvain "ythier" Hitier, who suggested adding

set list lcs=eol:¶,tab:»-,trail:·

to your .vimrc and making use of the :list option. The above line will highlight tabulators, trailing whitespaces and the end of a line. And it'll allow you to differentiate the three.
A warning though: you would want to deactivate this, in case you want to do copy and pasting with your mouse.

[UPDATE] Sylvain just sent me a little key binding for C&P he's using to toggle the highlighting:

function CutPaste()
  setlocal paste! nu! list!
endfunction
map <F10> :call CutPaste()

[/UPDATE]

The second really nice idea came by courtesy of Thilo Six to my attention:

" automatically delete trailing whitespace & Dos-returns    {{{2
fun! <SID>MyDeleteTrailingWhitespace()
  if ! &bin
    let l:l = line(".")
    let l:c = col(".")
    silent! :%s/[\r \t]\+$//
    call histdel("search", -1)
    call cursor(l:l, l:c)
  endif
endfun
autocmd BufWritePre,FileWritePre * call <SID>MyDeleteTrailingWhitespace()

The above snippet will automatically delete trailing whitespace and carriage returns as added by a lot of Windows programs.
Again a little warning: the function is executed automatically, so be sure you have only replacements included in the regular expression you really want to get deleted!

Thanks a lot for all the input I've received!

From time to time I get e-mails about my wine-unstable packages being uninstallable. Most often it turns out, that the problem stems from using e.g. oldstable (Lenny) instead of at least stable (Squeeze). The same problem arises, if you wish to install the wine-unstable packages on a platform for which I don't offer binaries. In either case you need to build the package from source, which isn't as difficult, as many think.

This entry in my ongoing tips mini-series of posts is thus going to detail how you setup pbuilder and afterwards use it to build a package. Please note, that I'm just focusing on pbuilder here and ignore things like cowbuilder.

Preparations

The following steps are needed just once, in general. Some would be needed again, if you want to create a second base.tgz, e.g. for a different distribution.

1. Install pbuilder with your favourite package manager, e.g. aptitude: aptitude install pbuilder
2. Become root
3. Configure pbuilder. Have a look at /etc/pbuilderrc, /usr/share/doc/pbuilder/examples/pbuilderrc and the manual page for pbuilderrc for reference. If you want to change some settings made in /etc/pbuilderrc, create a /root/.pbuilderrc and override any settings you don't like.
4. Create the base.tgz (the chroot environment for your builds). In the easiest case all your settings are made in the configuration files and you just need to run pbuilder --create, which creates the base.tgz in /var/cache/pbuilder, unless you changed the configuration.
   You should also note, that you can override certain settings, like the distribution or the path to (and name of) the base.tgz file created. See the manual page for pbuilder for all options and when they can be used.

Build a package

The following steps are needed for every build:

1. Download the source package, you want to build. No matter how you fetch them (dget, apt-get source, …), you should end up with three files, one of them ending in .dsc.
2. Become root.
3. Update your base.tgz file: pbuilder --update. Again, see the manual page for further options. (You can skip this step, if you just created the base.tgz or you just updated it for another build.)
4. Build the source package: run pbuilder --build /path/to/source-package.dsc (obviously you need to replace the last argument with a real path)
5. If you haven't changed the configuration of pbuilder in that regard, then your binaries end up in /var/cache/pbuilder/result. Install the binaries, you want, from there (with dpkg -i).

Now you should be able to build your own packages from source!

And to answer one question, that some of you might have now ("Why use pbuilder at all and not just build the package with dpkg-buildpackage directly?"): because pbuilder allows you to reliably build in a clean environment. That can safe you a lot of trouble, especially if you modified the source package. And it helps to keep the number of installed packages on your system down.