SBT run task after all subprojects - sbt

Question

I want to write a task that is run after all subproject tasks are complete.
Eg, if I do
sbt a b then after task a is complete on ALL subprojects I want to execute task b. I don't want to do (a b) on each project.
Is that possible?
In fact, I'll take it so that I modify the build.sbt directly. I don't necessarily have to specify it at the command line.

Answer 1

I wrote a blog post on the subject: sequencing tasks with sbt-sequential.
addCommandAlias
Here's an example. We'll define a custom task a in sub1 and sub2 and b in root. The simplest way to achieve sequential execution is using addCommandAlias, so we'll just do that.
lazy val a = taskKey[Unit]("a")
lazy val b = taskKey[Unit]("b")
lazy val root = (project in file(".")).
aggregate(sub1, sub2).
settings(addCommandAlias("ab", ";a;b"): _*).
settings(
b := {
println("b")
}
)
lazy val sub1 = (project in file("sub1")).
settings(a := println("a - sub1"))
lazy val sub2 = (project in file("sub2")).
settings(a := println("a - sub2"))
You can run this from shell as sbt ab.
$ sbt ab
[info] Loading global plugins from ...
[info] Loading project definition from ...
[info] Set current project to root (in build ...)
a - sub2
a - sub1
[success] Total time: 0 s, completed Nov 22, 2014 8:36:18 PM
b
[success] Total time: 0 s, completed Nov 22, 2014 8:36:18 PM
Def.taskDyn
Here's another example. This time using Def.taskDyn, which is also featured in the blog post.
I'm constructing a ScopeFilter from the aggregate and then I'm dispatching task a to them.
lazy val a = taskKey[File]("a")
lazy val b = taskKey[Seq[File]]("b")
lazy val root = (project in file(".")).
aggregate(sub1, sub2).
settings(
b := (Def.taskDyn {
val proj = thisProject.value
val filter = ScopeFilter(inProjects(proj.aggregate: _*))
Def.task {
val allValues = a.all(filter).value
println(allValues.mkString(","))
allValues
}
}).value
)
lazy val sub1 = (project in file("sub1")).
settings(a := new File("a"))
lazy val sub2 = (project in file("sub2")).
settings(a := new File("b"))
You can run this from shell as sbt b.
$ sbt b
[info] Loading global plugins from ...
[info] Loading project definition from ...
[info] Set current project to root (in build ...)
a,b
[success] Total time: 0 s, completed Nov 23, 2014 9:42:16 PM