gradle: build scala.js (on linux)

gistfile1.txt

tasks.register('buildui') {
    doLast {
        // clone & pull the sbt-extras repository from github
        def sbtExtrasDirPath = 'sbt-extras'
        def sbtExtrasDir = file(sbtExtrasDirPath)
        try{

            if(!sbtExtrasDir.exists()){
                org.ajoberstar.grgit.Grgit.clone(dir: sbtExtrasDir, uri: 'https://github.com/dwijnand/sbt-extras.git')
            }
            Grgit.open(dir: project.file(sbtExtrasDir)).pull(rebase: false)
        } catch (TransportException e){
            logger.warn("Unable to clone & push sbt-extras repository. " +
                    "Failure is imminent if this is the first time this build is executed on this machine.")
        }

        // copy shell script to working dir
        // for now this has to be located in the correct dir
        copy {
            from sbtExtrasDirPath + '/sbt'
            into '.'
        }

        // invoke sbt-extras to build ui
        exec {
            workingDir '.'
            executable 'sbt'
            args 'fastOptJS'
        }

        // remove previously copied file
        delete 'sbt' 
    }
}