Gradle Task

task cleanAll {
    group = 'minify'
    doFirst {
        tasks.cleanCSS.execute()
        tasks.cleanJS.execute()
    }
}

task cleanCSS {
    group = 'minify'
    doLast {
        String basePath = 'src/main/resources/static/stylesheets/site';
        FileTree tree = project.fileTree(dir: basePath);
        tree.include "**/*.css"
        tree.exclude "**/*.scss"
        tree.exclude "**/*.css.map"

        tree.visit { FileVisitDetails element ->
            if (!element.isDirectory()) {
                element.file.delete()
            }
        }
    }
}

task cleanJS {
    group = 'minify'
    doLast {
        String basePath = 'src/main/resources/static/javascripts/site';
        FileTree tree = project.fileTree(dir: basePath);
        tree.include "**/*.min.js"
        tree.exclude "**/*.js.map"
        tree.exclude "**/*.ts"
        tree.exclude "**/*.d.ts"

        tree.visit { FileVisitDetails element ->
            if (!element.isDirectory()) {
                element.file.delete()
            }
        }
    }
}

task sass {
    group = 'minify'
    doLast {
        String basePath = 'src/main/resources/static/stylesheets/site';
        FileTree tree = project.fileTree(dir: basePath);
        tree.include "**/*.scss"
        tree.exclude "**/*.css"
        tree.exclude "**/*.css.map"

        tree.visit { FileVisitDetails element ->
            if (!element.isDirectory()) {
                String filePath = basePath + "/" + element.getPath();

                int pos = filePath.lastIndexOf(".");
                String newFilePath = "";

                if (pos > 0) {
                    newFilePath = filePath.substring(0, pos) + ".css";
                }

                if (project.file(newFilePath).exists()) {
                    project.file(newFilePath).delete();
                }


                String cmd = String.format("node-sass $filePath $newFilePath");
                java.lang.Runtime.getRuntime().exec(cmd);
            }
        }
    }
}

task csso {
    group = 'minify'
    doLast {
        String basePath = 'src/main/resources/static/stylesheets/site';
        FileTree tree = project.fileTree(dir: basePath);
        tree.include "**/*.css"
        tree.exclude "**/*.min.css"
        tree.exclude "**/*.css.map"
        tree.exclude "**/*.scss"

        tree.visit { FileVisitDetails element ->
            if (!element.isDirectory()) {
                String filePath = basePath + "/" + element.getPath();

                int pos = filePath.lastIndexOf(".");
                String newFilePath = "";

                if (pos > 0) {
                    newFilePath = filePath.substring(0, pos) + ".min.css";
                }

                String cmd = String.format("csso -i $filePath -o $newFilePath");
                java.lang.Runtime.getRuntime().exec(cmd);
            }
        }
    }
}

task uglify {
    group = 'minify'
    doLast {
        String basePath = 'src/main/resources/static/javascripts/site';
        FileTree tree = project.fileTree(dir: basePath);
        tree.include "**/*.js"
        tree.exclude "**/*.min.js"
        tree.exclude "**/*.js.map"
        tree.exclude "**/*.ts"
        tree.exclude "**/*.d.ts"

        tree.visit { FileVisitDetails element ->
            if (!element.isDirectory()) {
                String filePath = basePath + "/" + element.getPath();

                int pos = filePath.lastIndexOf(".");
                String newFilePath = "";

                if (pos > 0) {
                    newFilePath = filePath.substring(0, pos) + ".min.js";
                }

                String cmd = String.format("uglifyjs $filePath -o $newFilePath");
                java.lang.Runtime.getRuntime().exec(cmd);
            }
        }
    }
}

sass.mustRunAfter cleanCSS
csso.mustRunAfter sass
uglify.mustRunAfter cleanJS

task minify{
    group = 'minify'
    doFirst{
        tasks.cleanCSS.execute()
        tasks.cleanJS.execute()
    }
    doLast{
        tasks.sass.execute()
        tasks.csso.execute()
        tasks.uglify.execute()
    }
}

References
https://docs.gradle.org/current/userguide/working_with_files.html
https://docs.gradle.org/current/userguide/tutorial_using_tasks.html
https://docs.gradle.org/current/userguide/custom_tasks.html
https://docs.gradle.org/current/userguide/more_about_tasks.html