gulpfile.js

var gulp = require('gulp'),
    sass = require('gulp-sass');
    
// here you add variables for your paths so you don't have to repeat them
var config = {
    sass: {
        src: './src/assets/sass',
        dest: './public/css'
    },
    bootstrap: {
        src: './node_modules/bootstrap-sass/assets/stylesheets'
    }
};

/**
 * css compilation
 * Usage:
 * gulp sass
 */
gulp.task('sass', function(){
    let files = `${config.sass.src}/**/*.scss`;
    let dest = `${config.sass.dest}`;

    console.log('=== Compiling SASS ===');

    return gulp.src(files)
        .pipe(sass({
            outputStyle: 'expanded',
            precision: 10,
            errLogToConsole: true,
            // that's where the magic happens
            includePaths: config.bootstrap.src
        }).on('error', sass.logError))
        .pipe(gulp.dest(dest));
        
});

/**
 * sass watcher, to automatically compile on file changes
 * Usage:
 *  bin/gulp watch
 */
gulp.task('watch', function () {
    let files = `${config.sass.src}/**/*.scss`;
    gulp.watch(files, ['sass']);
});


gulp.task('default', ['watch']);