Setting value of setting on command line when no default value defined in build?

sbt

Solution

If the setting has a default value, that default should be set in the plugin. If it does not, the type should probably be `Option[...]` with a default of `None`. Typically, a build should not require a parameter to be passed from the command line just to be loaded.

Finally, if it is primarily the `set` syntax you dislike, you can use system properties and read the system property from your setting. However, this is restricted to Strings and so you lose type safety.

System properties can be set by `-Dkey=value` on the command line (either directly or in your startup script):

sbt -Dconfig.url=http://...

(quoting as needed by your shell). To pull this value into the setting:

MyPlugin.configUrl := 
  url(System.getProperty("config.url", "<default>"))

where `"<default>"` is the value to use if the system property is not set. If you take the `Option` approach,

MyPlugin.configUrl := 
  for(u <- Option(System.getProperty("config.url"))) yield
    url(u)

Problem

I am writing a plugin that requires a specific setting: `configUrl` If I specify that setting in my `build.conf` it would look like this: ``` MyPlugin.configUrl := "http://..../..." ``` I can then use the command line to do this: ``` sbt 'set MyPlugin.configUrl := "http://..../..."' performAction ``` Is there a better way that allows me to not have that setting in build? If I start `sbt` without that setting I get the following error: ``` [error] Reference to undefined setting: [error] [error] *:config-url from *:application-configuration ``` I searched google but could not find a way to provide the setting on the command line, something like this: ``` sbt config-url="http://..../..." ```

Original source