83 lines
3.4 KiB
Markdown
83 lines
3.4 KiB
Markdown
Propellor comes with a lot of properties you can use. But eventually,
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you'll want to write a property of your own.
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This isn't hard. Often propellor has some properties you can use to build
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the property you want. Need to modify the content of a file? Use any of
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the properties in
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[Propellor.Property.File](http://hackage.haskell.org/package/propellor-2.2.1/docs/Propellor-Property-File.htm)
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Need to run some commands? Use [Propellor.Property.Cmd](http://hackage.haskell.org/package/propellor-2.2.1/docs/Propellor-Property-Cmd.html).
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To combine properties, the easiest way is to use `requires`.
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someproperty `requires` otherproperty
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[Propellor.Property.List](http://hackage.haskell.org/package/propellor-2.2.1/docs/Propellor-Property-List.html)
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has a `propertyList` combinator that's also useful.
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[Propellor.Property](http://hackage.haskell.org/package/propellor-2.2.1/docs/Propellor-Property.html)
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has some other functions to modify Properties in useful ways.
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For example, `check` makes a Property call an `IO Bool` to check if the
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Property needs be run.
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## example: User.hasLoginShell
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> As far as I can tell there is no easy way to set a user's
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> login shell. A Property User.hasLoginShell, which ensures
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> that a user has a specified login shell and that said shell
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> is in /etc/shells would be really helpful. Sadly, I lack the
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> skills to put this together myself :( -- weinzwang
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Propellor makes it very easy to put together a property like this.
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Let's start with a property that combines the two properties you mentioned:
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hasLoginShell :: UserName -> FilePath -> Property
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hasLoginShell user shell = shellSetTo user shell `requires` shellEnabled shell
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The shellEnabled property can be easily written using propellor's file
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manipulation properties.
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-- Need to add an import to the top of the source file.
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import qualified Propellor.Property.File as File
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shellEnabled :: FilePath -> Property
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shellEnabled shell = "/etc/shells" `File.containsLine` shell
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And then, we want to actually change the user's shell. The `chsh(1)`
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program can do that, so we can simply tell propellor the command line to
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run:
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shellSetTo :: UserName -> FilePath -> Property
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shellSetTo user shell = cmdProperty "chsh" ["--shell", shell, user]
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The only remaining problem with this is that shellSetTo runs chsh every
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time, and propellor will always display that it's made a change each time
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it runs, even when it didn't really do much. Now, there's an easy way to
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avoid that problem, we could just tell propellor that it's a trivial
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property, and then it will run chsh every time and not think it made any
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change:
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shellSetTo :: UserName -> FilePath -> Property
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shellSetTo user shell = trivial $
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cmdProperty "chsh" ["--shell", shell, user]
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But, it's not much harder to do this right. Let's make the property
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check if the user's shell is already set to the desired value and avoid
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doing anything in that case.
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shellSetTo :: UserName -> FilePath -> Property
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shellSetTo user shell = check needchangeshell $
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cmdProperty "chsh" ["--shell", shell, user]
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where
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needchangeshell = do
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currshell <- userShell <$> getUserEntryForName user
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return (currshell /= shell)
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And that will probably all work, although I've not tested it. You might
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want to throw in some uses of `describe` to give the new properties
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more useful descriptions.
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I hope this has been helpful as an explanation of how to add properties to
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Propellor, and if you get these properties to work, a patch adding them
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to Propellor.User would be happily merged.
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