|1.||It is a plain text file.|
2. Lines that begin with a single # character are comments that contain a description of the following configuration setting.
3. Lines that do not begin with the # character contain the configuration setting described by the previous comment lines.
4. Configuration settings are self contained, and completely independent, changing one configuration setting never requires that another, different, configuration setting must be changed too (perhaps any logical conflicts are automatically resolved by the application in a safe, fallback, manner)
The additional information used by sysconftool is encoded as specially-formatted comment lines that begin with two # characters.
An application installs a default configuration file as "filename.dist", when the actual name of the configuration file is really "filename". If there is no existing filename, sysconftool simply copies filename.dist to filename, and calls it a day. Otherwise, sysconftool copies the existing filename to filename.bak and creates a new filename based on the contents of both files.
sysconftool is designed to solve a common problem with application configuration. New versions of applications often include additional functionality that requires new configuration settings. Without the new configuration settings the application will not work, so new configuration files should be installed during the upgrade. However, when that happens, any changes to the existing configuration settings are lost. sysconftool is designed to solve this dillemma, and merge old configuration settings with new ones. sysconftool is designed in a fail-safe way. Whenever theres a doubt as to whats The Right Thing To Do, sysconftool will use the configuration settings from the new file, that are supposedly known to be good, and leave it up to a physical being to sort out any conflicts and make any manual decisions.
The following line should appear at the beginning of filename.dist:
This doesnt have to be the very first line in filename.dist, but it should appear somewhere within the first twenty lines, right before the first configuration setting. "version" should be some kind of an identifier for this particular version of the configuration files. All that sysconftool cares about is that any change to the default configuration, in filename.dist, results in a different version. An excellent way to do this is to simply use the $Id$ RCS/CVS version identification strings, and have this little detail taken care of automatically.
New revisions of an application should not necessarily have a new configuration file version. If the default application configuration settings have not changed from the previous release, version can remain the same. version is copied from filename.dist to filename.
If theres an existing filename, and it includes the same version identifier, sysconftool silently skips over this configuration file, and doesnt do anything, assuming that this configuration file has already been installed. Therefore, running sysconftool more than once (accidentally) will not break anything.
If theres an existing filename, but its version is different, sysconftool backs it up to filename.bak, then creates a new filename. If theres an existing filename, but it doesnt contain a recognizable "##VERSION: version" line, sysconftool assumes that the previous version of the application did not use the sysconftool tool. Thats not a problem. filename is copied to filename.bak, and filename.dist gets installed as the new filename, allowing sysconftool to work with the next version of this configuration file.
Each configuration setting uses the following format in the configuration file:
##NAME: name:revision # # description
sysconftool looks for a line that begins with "##NAME". This line gives the name and the revision of the following setting. name must be unique within its configuration file (the same name can be used by different configuration files, sysconftool works with one file at a time). revision is used by sysconftool to decide when the configuration setting can be safely carried over from an older configuration file, and when it is better to reinstall the default setting from the new configuration file.
One or more comment lines - lines that begin with the # character - may follow "##NAME". The first line that does not begin with # is considered to be the first line that contains the value of the configuration setting, which lasts. The value can be spread over multiple lines. The configuration setting is considered to last until either the end of the file, or until the first following line that begins with another "##NAME".
Aside from that, sysconftool does not care how the configuration setting is represented. It can be "NAME=VALUE", it can be "NAME: VALUE", or "NAME<tab>VALUE", it can even be a base64-encoded binary object, and it can always have leading or trailing blank lines. sysconftool merely looks at which lines begin with the # comment character. After the ##NAME: line, sysconftool looks ahead until the first line that does not begin with #, and thats the first line of the configuration setting. Then, sysconftool looks ahead until the next line that starts with a "##NAME:", which marks the end of this configuration setting.
For this reason it is important that all commented description lines that follow ##NAME: must begin with the # character. If a blank line follows the line with ##NAME: it is assumed to be the start of the corresponding configuration setting. For example, this is correct:
##NAME: flag1:0 # # # This is the first configuration flag #
This is not correct:
# # This is the first configuration flag #
A new configuration file, "filename", is created from its previous version, "filename.bak" and the new default configuration file, "filename.dist", using the following, simple, two-step process.
1. sysconftool begins with filename.dist in hand. This makes sure that sysconftool begins with a good, known, default configuration file.
2. sysconftool then takes each configuration setting in filename.dist, then searches filename.bak. If it finds a configuration setting that has an identical "name" and "version", then the corresponding configuration setting value is taken from filename.bak, replacing the default in filename.dist. After all configuration settings in filename.dist are looked up (and potentially replaced), whats left becomes the new filename.
The above process is a logical description. The actual technical implementation is slightly different (for example, filename is not backed up to filename.bak until the new configuration file has been already created), but is logically equivalent to this process. This process carries a number of consequences that must be considered.
If a new application revision needs a new configuration setting, it will get a new name and version. Since filename.dist is used as a starting point for the new configuration file, the new configuration file will include the new configuration setting. When a configuration setting is removed, it will disappear from the new configuration file for the same exact reason.
sysconftool looks at both name and version. A configuration setting with the same name but different versions are seen by sysconftool as completely different settings. The existence of version allows a finer-grained control of configuration upgrades, as described below.
sysconftool copies setting values with the same name and version from the old configuration file to the new configuration file. However, the associated descriptive comments are not copied, and are taken from the new filename.dist. Therefore, if a new version of the configuration file contains an updated or an embellished description of a particular setting, it will be included in the new configuration file, but the existing configuration value will be preserved! Generally, if a configuration setting does not change its meaning or function, its name and version should remain the same. Its comments can be edited to fix a typo, or revised in a more substantive fashion. Name and version should only be changed if theres a functional change in the configuration setting.
What to do with name and version after a functional change depends on the nature and the magnitude of the change. The nature and the magnitude of the change must be considered not only with respect to the most recent revision of the application, but to all the previous revisions as well. When in doubt, go based upon the largest change in magnitude, in order to guarantee a functional default setting, from filename.dist, and leave it up to a living being to manually figure it out.
If only the default value of a setting should be changed for new application installation, but the existing installations can continue to use the existing value of the setting, both the name and version should be left alone. Existing configuration settings will be preserved, and new installations will get the new default. The descriptive comment of this setting can be updated too (see above).
This should be done only as long as ALL previous values of this configuration setting will ALWAYS be valid in the new application revision. If some possible values of this configuration setting will no longer be valid, version should be changed. sysconftool does not care how name and version are formatted. Both are opaque labels. The only requirements is for the label to be different. The difference between changing version and changing both name and version is this:
If theres an old configuration setting with the same name but different version, sysconftool will still use the new, safe, default value from filename.dist, however sysconftool will also append an additional comment, on its own accord, reminding the reader that this configuration value has been reset, and the reader should consider whether to manually restore the configuration value from the old configuration.
When sysconftool decides to keep an existing setting, with the same name and value, it will also insert a short comment to that effect, reminding the reader to check the default in filename.dist.
Double Precision, Inc.
1. sysconftool(1) [set $man.base.url.for.relative.links]/sysconftool.1.html
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