Friday, March 6, 2009

Colors for Terminal Apps

This has been in my plans for a long time, but i had trouble learning how to do colored output in terminals correctly. I asked people, i googled, looked at terminfo, ncurses, but still could not find a satisfactory answer. So i started to look at other non-curses terminal apps, and i stopped already at the first one: git.

#include <unistd.h>
#include <stdlib.h>
#include <string.h>

bool has_colors()
if (::isatty(STDOUT_FILENO))
char *term = ::getenv("TERM");
if (term && ::strcmp(term, "dumb"))
return true;
return false;

This (translated to a C/C++ hybrid) is all that git (and now also zypper :O) does to detect the color capability of the stdout. stout must be a terminal, and the TERM environment variable must not be "dumb". The idea is that most color-capable terminals understand the ISO 6429 (former ANSI) color control sequences, and if the above check fails for your terminal, you can configure git (and soon zypper :O) to never or always use colors (or set the TERM to "dumb").

Regarding terminfo, i found it hard to learn and an overkill for just printing colored text (of course, once learned it can be used in other ways, too, e.g. to move the cursor, turn off input echoing when writing passwords, etc.). Maybe some other time.

On the other hand, ncurses are nothing for an existing terminal application which already does lots of output the usual way - you would have to rewrite all the output handling using ncurses. With ncurses it is that you write your app either with or without it from the start. But you cannot switch to it later, nor mix it with standard C I/O (printf() or C++ streams (cout <<). At least that's my impression from what i've learned...

So, zypper is now (since 1.1.0) a bit less boring, with colors :O) The idea is to print normal messages and progress in darker color, highlight important messages like results, warnings, errors, and color some special stuff, e.g. in install summary, default options in prompts, and so on. And it should be configurable, so that anybody can adapt it to his or her terminal background and text color. I'll write more about that once it's done.

Wednesday, March 4, 2009

Augeas for libzypp and zypper?

The need for a configuration file for zypper (plus my wish for a nice CLI for handling it) and a visit to Raphaƫl Pinson's presentation at FOSDEM made me look at Augeas closely.

Augeas enables you to read and edit your config files in a way which preserves comments and formatting details, allowing humans and programs coexist peacefully when it comes to editing configuration files. More than that, you can map some special structures of the file to special parts of the augeas tree, rather than using the generic INI file parser. See the zypp.conf for example (zypper.conf will have the same structure):

## Option description.
## Default: etc.
# disabledoption = value

## Another option description

another.option = value

With Augeas you can write a lens (the file contents description) which will map such file into a tree like this:

/files/etc/zypp/zypp.conf/main/1/description[1] = "Option description."
/files/etc/zypp/zypp.conf/main/1/description[2] = "Default: etc."
/files/etc/zypp/zypp.conf/main/1/disabledoption = "value"
/files/etc/zypp/zypp.conf/main/2/description[1] = "Another option description."
/files/etc/zypp/zypp.conf/main/2/another.option = "value"

Note the commented node. Augeas has an API for looking for the nodes, getting values from them, setting the values, and finally saving the tree back to the config file.

What does this mean for zypper? Implementing zypper conf --list is a piece of cake, as well as zypper conf --set another.option value. Commenting or uncommenting the option is a matter of adding or removing the commented node. Getting the option description for zypper conf --help some.option is easy (and maybe it is possible to map the multi-line ## description to a single 'description' node!).

Here is a draft of lens for zypper (it will need comments and further improvements). To try it out you can crete a /etc/zypp/zypper.conf file with contents like the above example, and use the augtool like this:

$ augtool -I /the/dir/with/the/zypper.aug/file
augtool> print /files/etc/zypp/zypper.conf/
/files/etc/zypp/zypper.conf/anon/description[1] = "Configuration file for zypper"
/files/etc/zypp/zypper.conf/anon/description[3] = "Boolean values are 0 1 yes no on off true false"
/files/etc/zypp/zypper.conf/main/1/description[1] = "Whether to use colors"
/files/etc/zypp/zypper.conf/main/1/description[3] = "Default value: autodetected"
/files/etc/zypp/zypper.conf/main/1/colors = "yes"

Type help for other commands, use TAB key for completion, and check also the Augeas home page for more info.

What does this mean for libzypp? Basically the above, with respect to libzypp's files. Currently, you either edit your repos.d/*.repo, locks, credentials, and other files by hand or you leave it to libzypp's frontends, or you at least take care not to write comments in those files, because they will all get removed upon the next change done by libzypp (what we do there is to rewrite the entire files from in-memory data upon saving). This can be easily avoided using Augeas and we can add an editing API for zypp.conf. Plus the bonuses described above.

The library package (libaugeas0) would add 300 kiB of installed size to libzypp's/zypper's dependencies, (which is not little), plus an optional 200 kiB of the official lenses (augeas-lenses), if we decide to use them.

todo: Look at Augeas' error reporting. The user can of course break the structure of the file in a way that the lens can't map it to the tree anymore. If parsing of the file will fail, we need to report why, so that the user can fix it.