Development/Tutorials/Updating KConfig Files
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This page describes how a developer can use the KConfig update mechanism (kconf_update) available in kdelibs to update a user's already existing config file to accomodate changes made to the format of the default config file.
Why should you use it?
Over time applications sometimes need to rearrange the way configuration options are stored. Since such an update shouldn't influence the configuration options that the user has selected, the application must take care that the options stored in the old way will still be honored.
What used to happen is that the application looks up both the old and the new configuration option and then decides which one to use. This method has several drawbacks:
- The application may need to read more configuration files than strictly needed, resulting in a slower startup.
- The application becomes bigger with code that will only be used once.
kconf_update addresses these problems by offering a framework to update configuration files without adding code to the application itself.
How it works
Applications can install so called "update files" under $KDEDIR/share/apps/kconf_update. An update file has .upd as an extension and contains instructions for transferring/converting configuration information from one place to another.
Updating the configuration happens automatically, either when KDE gets started or when kded detects a new update file in the above mentioned location.
Update files are separated into sections. Each section has an id. When a section describing a configuration change has been applied, the id will be stored in the file kconf_updaterc. This information is used to make sure that a configuration update is only performed once.
If you overwrite an existing update file with a new version that contains a new section, only the update instructions from this extra section will be performed.
File format of the update file
Empty lines or lines that start with '#' are considered comments Commas (,) are used to seperate fields and may not occur as part of any field and all of the keywords are case-sensitive, i.e. you cannot say "key" instead of "Key" for example.
For the rest the file is parsed and executed sequentially from top to bottom. Each line can contain one entry. The following entries are recognized:
With <id> identifying the group of update entries that follows. Once a group of entries have been applied, their <id> is stored and this group of entries will not be applied again.
Specifies that configuration information is read from <oldfile> and written to <newfile>. If you only specify <oldfile>, the information is read from as well as written to <oldfile>. Note that if the file does not exist at the time kconf_update first checks, no related update will be performed (script won't be run at all, etc.).
All entries from <oldfile> are piped into <script>. The output of script is used as new entries for <newfile>. Existing entries can be deleted by adding lines with "# DELETE [group]key" in the output of the script. To delete a whole group use "# DELETEGROUP [group]".
<script> should be installed into $(kde_datadir)/kconf_update, or kconf_update will not be able to find it. It is not portable to install binary applications in $kde_datadir, so you have to stick with interpreted scripts like sh or perl scripts. From KDE 3.2 onwards it's also possible to install kconf_update applications in $(kde_bindir)/kconf_update_bin, which opens the door to kconf_update applications that are written in C++ and use Qt's powerful string API instead.
A workaround for KDE 3.1.x and older is to install a .sh script in $(kde_datadir) that contains a simple exec:
exec "`kde-config --prefix`/bin/kconf_update_bin/my_update_app
This is equivalent to what KDE 3.2 can do directly, but of course the .upd file now points to the .sh script instead of the binary application.
If Script was issued after a "Group" command the behavior is slightly different: All entries from <oldfile>/<oldgroup> are piped into <script>. The output of script is used as new entries for <newfile>/<newgroup>, unless a different group is specified with "[group]". Existing entries can be deleted from <oldgroup> by adding lines with "# DELETE key" in the output of the script.· To delete <oldgroup> use "# DELETEGROUP".
<interpreter> can be something like "perl".
It is also possible to have a Script without specifying <oldfile> or <newfile>. In that case the script is run but it will not be fed any input and its output will simply be discarded.
If specified, the arguments will be passed to <script>. IMPORTANT: It has to be specified before Script=.
Specifies that configuration information is read from the group <oldgroup> and written to <newgroup>. If you only specify <oldgroup>, the information is read from as well as written to <oldgroup>. You can use <default> to specify keys that are not under any group.
Specifies that <oldgroup> is removed entirely. This can be used to remove obsolete entries or to force a revert to default values.
With this entry you can specify options that apply to the next "Script", "Key" or "AllKeys" entry (only to the first!). Possible options are:
- "copy" Copy the configuration item instead of moving it. This means that
the configuration item will not be deleted from <oldfile>/<oldgroup>
- "overwrite" Normally, a configuration item is not moved if an item with the
new name already exists. When this option is specified the old configuration item will overwrite any existing item.
Specifies that configuration information is read from the key <oldkey> and written to <newkey>. If you only specify <oldkey>, the information is read from as well as written to <oldkey>.
Specifies that all configuration information in the selected group should be moved (All keys).
Specifies that all configuration information from all keys in ALL· groups should be moved.
Specifies that <oldkey> is removed from the selected group. This can be used to remove obsolete entries or to force a revert to default values.