Use double quotes

dir ("RELEASE/abc/${abc_version}")

in groovy Single quotes are a standard Java String while Double quotes are a templatable String

e.g

a = 10
b = "RELEASE/abc/${a}"
c = 'RELEASE/abc/${a}'
print(b)
print('\n')
print(c)

output will be

RELEASE/abc/10
RELEASE/abc/${a}

You can try it here

This code works for me:

import groovy.io.FileType

def list = []

def dir = new File("path_to_parent_dir")
dir.eachFileRecurse (FileType.FILES) { file ->
  list << file
}

Afterwards the list variable contains all files (java.io.File) of the given directory and its subdirectories:

list.each {
  println it.path
}

If you can run other script as separate process, you can give ProcessBuilder parameter working dir:

def processBuilder=new ProcessBuilder(command)
processBuilder.directory(new File("Working dir"))
def process = processBuilder.start()

or

command.execute(null, new File("Working dir"))

so that process will switch to your new folder and execute it there.

Try this:

System.getProperty("user.dir");

Looks particulary nice in Groovy (untested, taken from this link so code credit should go there):

// Introspection, know all the details about classes :
// List all constructors of a class
String.constructors.each{println it}

// List all interfaces implemented by a class
String.interfaces.each{println it}

// List all methods offered by a class
String.methods.each{println it}

// Just list the methods names
String.methods.name

// Get the fields of an object (with their values)
d = new Date()
d.properties.each{println it}

The general term you are looking for is introspection.

My current plan is to split the string repo/dir/file.java on the forward slash, and create directories until the last element of the split result

Rather than splitting your string manually, you could try using File.mkdirs():

File newDirectoryStructureParent = new File('some/path/to/parent/dir')

def s = 'repo/dir/file.java'
def newContainer = new File(s, newDirectoryStructureParent).getParent()
newContainer.mkdirs()