How to use fileCopyOperation in jenkins groovy to copy 3 files with the same name from 3 different directories, to 1 directory with 3 different names

At the end of a Jenkins pipeline, I am trying to copy three files with the same name, in three different directories, into one directory, obviously renaming the files to avoid a name clash. The input structure is

repo/
     build-productOne/Release/program
     build-productTwo/Release/program
     build-productThree/Release/program

and the output structure should be

repo/
     output/
            productOne-program
            productTwo-program
            productThree-program

To achieve this, I created the following pipeline entry, after reading this page: https://www.jenkins.io/doc/pipeline/steps/file-operations/

stage('Build') {
    steps {
        sh """
        ...
        make -j -O -C ${WORKSPACE} Release
        """
        script {
            def version = readJSON file: "${WORKSPACE}/some-
            fileOperations(
                [
                    fileCopyOperations(
                        includes: 'build-*/Release/program',
                        flattenFiles: true,
                        renameFiles: true,
                        sourceCaptureExpression: 'build-(.*)/Release/program$',
                        targetNameExpression: '${env.OUTPUT}/blah_$1_program-${version}'
                    )
                ]
            )
        }
    }
}

I was expecting the above code to work, but it fails with the following error message:

08:53:43  File Copy Operation:
08:53:43  null
08:53:43  FATAL: null

Can any kind soul throw any light on this issue?

You need to set targetLocation to your output folder and use targetNameExpression only for the filename. Also make sure your variables like ${version} are defined. For example: groovy Copy Edit fileCopyOperations( includes: 'build-*/Release/program', flattenFiles: true, renameFiles: true, sourceCaptureExpression: 'build-(.*)/Release/program$', targetLocation: "${env.WORKSPACE}/output", targetNameExpression: "\$1-program-${version}" ) Make sure version is defined in your script before this step. The FATAL: null error often happens if these are missing or misconfigured.