Reading file from Workspace in Jenkins with Groovy script

I want to add a Build step with the Groovy plugin to read a file and trigger a build fail depending on the content of the file.

How can I inject the workspace file path in the groovy plugin ?

myFileDirectory = // Get workspace filepath here ???
myFileName = "output.log"
myFile = new File(myFileDirectory + myFileName)

lastLine = myFile.readLines().get(myFile.readLines().size().toInteger() - 1)
if (lastLine ==~ /.Fatal Error.*/ ){
    println "Fatal error found"
    System.exit(1)
} else{
   println "nothing to see here"
}

I realize this question was about creating a plugin, but since the new Jenkins 2 Pipeline builds use Groovy, I found myself here while trying to figure out how to read a file from a workspace in a Pipeline build. So maybe I can help someone like me out in the future.

Turns out it's very easy, there is a readfile step, and I should have rtfm:

env.WORKSPACE = pwd()
def version = readFile "${env.WORKSPACE}/version.txt"

If you are trying to read a file from the workspace during a pipeline build step, there's a method for that:

readFile('name-of-file.groovy')

For reference, see https://jenkins.io/doc/pipeline/steps/workflow-basic-steps/#readfile-read-file-from-workspace.

Based on your comments, you would be better off with Text-finder plugin.

It allows to search file(s), as well as console, for a regular expression and then set the build either unstable or failed if found.

As for the Groovy, you can use the following to access ${WORKSPACE} environment variable:
def workspace = manager.build.getEnvVars()["WORKSPACE"]