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| # BEFORE READING | ||
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| Quick references: | ||
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| * http://www.fifi.org/doc/debconf-doc/tutorial.html | ||
| * http://en.wikipedia.org/wiki/Debconf_%28software_package%29 | ||
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| In this article, Debconf means the backend database containing users' | ||
| configurations; debconf means SW utilities used to communicating with Debconf. | ||
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| -------------------------------------------------------------------------------- | ||
| # INTRODUCTION | ||
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| Debconf is a backend database, with a frontend that talks to it and presents an | ||
| interface to the user. Debconf should be used whenever your package needs to | ||
| output something to the user, or ask a question. | ||
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| user <--> frontend <--> database | ||
| ^ | ||
| | | ||
| v | ||
| protocol | ||
| .-- a special config script in the control section | ||
| | (debian package) | ||
| |-- postinst scripts | ||
| `-- other scripts | ||
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| 1. These scripts tell the frontend what values they need from the database. | ||
| 2. The frontend asks the user questions to get those values if they aren't set. | ||
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| debconf does not physically configure any packages, but provides a way for users | ||
| to communicate with Debconf. Users' answers to the configuration | ||
| questions asked by debconf are cached in Debconf. | ||
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| ## Debconf-related files | ||
| Debianization files | ||
| * .template | ||
| - Contains questions. | ||
| * .conf | ||
| - A kind of maintainer script and uses Debconf to ask questions. | ||
| - Is executed before the package is unpacked. | ||
| * .postinst | ||
| - Applies configuration changes to the unpacked package. | ||
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| Debconf consists of the files in /var/cache/debconf/: | ||
| * templates.dat | ||
| * config.dat | ||
| * passwords.dat | ||
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| -------------------------------------------------------------------------------- | ||
| # USAGE | ||
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| ## Write Config Script: Shell Scripting | ||
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| /usr/share/debconf/confmodule | ||
| 1. creates a bridge to fronend | ||
| 1. provides db_* functions for conf script so that user can communicate with frontend? | ||
| 1. It seems to communicate with Debconf directly via the Debconf protocol channel prepared by frontend. | ||
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| source exec | ||
| config script <--> confmodule <--> /usr/share/debconf/frontend <--> Debconf | ||
| ^ | | ||
| `----------------------------------------- | ||
| run a new copy (not fork) | ||
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| After confmoudle is sourced, FD3 is directed to FD1, and then FD1 is directed to FD2. | ||
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| FD1 . | ||
| FD2 `-> stderr | ||
| FD3 --> stdout (Debconf protocol channel) | ||
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| The design concepts behind might be: | ||
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| * Commands of Debconf protocol is transtitted via stdout. | ||
| * It is likely that a maintainer tries to display messages via stdout script, but these messages will be treated as Debconf commands. To prevent this issue, messages displayed via echo/printf will be directed to stderr by default. | ||
| * To transmit a Debconf command, you need to send it to FD3 which is directed to stdout. | ||
| * Luckily, confmodule provides wrapper functions to send Debconf commands easily. | ||
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| _db_cmd() in confmodule uses FD3 to communicate with Debconf | ||
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| ## Test Config Script | ||
| Debconf uses 2 rules to find the templates file associated with the config script: | ||
| * Assume the config script's name is foo.conf. | ||
| * Search foo.conf.templates file. | ||
| * Search foo.templates. | ||
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| Unfortunately, I got these errors on Ubuntu Precise... | ||
| (confirmed that this issue does not exist on Debian Jessie): | ||
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| $ sudo bash ./config | ||
| Can't exec "./config": Permission denied at /usr/share/perl/5.14/IPC/Open3.pm line 186. | ||
| open2: exec of ./config failed at /usr/share/perl5/Debconf/ConfModule.pm line 59 | ||
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| # debconf database is not updated? | ||
| $ sudo apt-get purge hello | ||
| $ echo "PURGE" | sudo debconf-communicate hello | ||
| 0 | ||
| $ sudo debconf-show hello | ||
| $ echo "GET hello/password" | sudo debconf-communicate hello | ||
| 0 qwaszx | ||
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| ## Utilities | ||
| * debconf | ||
| - debconf-communicate | ||
| + Interact with Debconf. | ||
| + For supported commands, please refer to the SPECIFICATION section. | ||
| - debconf-set-selections: | ||
| - debconf-show | ||
| + List items (questions and values) owned by a given package. | ||
| - debconf | ||
| + run a debconf-using program | ||
| - debconf-apt-progress | ||
| + install packages using debconf to display a progress bar | ||
| - debconf-copydb | ||
| - debconf-escape | ||
| + Helper when working with debconf's escape capability. | ||
| - dpkg-reconfigure | ||
| + let user to modify configurations of installed packages. | ||
| - dpkg-preconfigure | ||
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| * po-debconf | ||
| - debconf-updatepo | ||
| - debconf-gettextize | ||
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| * ubiquity | ||
| - debconf-get | ||
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| -------------------------------------------------------------------------------- | ||
| # DESIGN CONCEPTS | ||
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| Information in this paragraph is just my personal understanding and is likely | ||
| not correct. I really appreciate it if you find any error and share the correct | ||
| information with me. | ||
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| ## Working Model | ||
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| 1. Prepare questions (template file). | ||
| 2. Ask user these questions and store answers into Debconf (config script). | ||
| * Used to configurate Debconf, is this the reason why config script is called config script? | ||
| 3. Apply user's answers to the program (postinst script). | ||
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| ## Why Debconf can not be modified easily | ||
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| http://feeding.cloud.geek.nz/posts/manipulating-debconf-settings-on/ | ||
| https://lists.debian.org/debian-edu/2014/10/msg00177.html | ||
| https://lintian.debian.org/tags/debconf-is-not-a-registry.html | ||
| http://lwn.net/Articles/100107/ | ||
| * Front-end independence and configurability levels are well-thought-out to provide various options to customize the balance of ease-of-use vs. detailed control. | ||
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| -------------------------------------------------------------------------------- | ||
| # SPECIFICATION | ||
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| [Configuration management protocol, v2.1](http://www.debian.org/doc/packaging-manuals/debconf_specification.html) | ||
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| -------------------------------------------------------------------------------- | ||
| # GLOSSARIES | ||
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| * owner | ||
| * Generally an "owner" is equivalent to a debian package name. |