See this page as a slide show

CT320: Configuration Information

blix

Consider the blix command (I made it up), which blixes things. It requires configuration information.

• What things should be blixed?
• How often should the blixing take place?
• If this high- or low-priority blixing?
• Which uses are permitted to blix?
• What is the default blixing level?

A configuration file

The straightforward solution is to have a configuration file, /etc/blix.conf. It’s installed as part of the blix package, and contains default values, such as:

    Frequency: 30m		# Every 30 minutes
    Priority: medium
    Level: 3			# 1 is lowest level, 5 is highest level
    Users: root			# Only root is permitted to blix

To allow user sanchez to blix, change the last line to:

    Users: root,sanchez		# Sanchez, too!

Problem

What happens when a new revision of the blix package comes out? A new parameter has been added to /etc/blix.conf. However, you’ve edited /etc/blix.conf:

    Frequency: 30m
    Priority: medium
    Level: 3
    Users: root,sanchez

    Frequency: 30m
    Priority: medium
    Level: 3
    Users: root
    Compression: xz

Conflict! When you update, you’ll have to merge that file manually. Ack!

Solution #1

You could have separate system & user config files:

• /etc/blix.system.conf
• /etc/blix.user.conf

/etc/blix.system.conf would be reserved for the system defaults, and /etc/blix.user.conf would be available for user editing.

That sounds great, but …

Problem

What if another package wants to add some blixing information? Say that the vim package wants to add a line to the blix configuration file to specify how often ~/.vimrc should be blixed?

Whichever file it modifies, we’ll have the same old problem at the next update.

Maybe we should have a whole bunch of configuration files!

A Whole Bunch of Configuration Files

That’s the best solution: instead of a single configuration file /etc/blix.conf, have a directory /etc/blix.d. The blix program will treat all files in /etc/blix.d as configration file. If the order matters, we can put prefixes in the filenames to force processing order.

• /etc/blix.d/40-vim (part of the vim package)
• /etc/blix.d/50-blix (defaults provided as part of the blix package)
• /etc/blix.d/90-user (created by the user)

The filenames will be unique, as long as two packages don’t use the same name.