How to append a text to a file in jenkinsfile

Question

How to append a text to a file in Jenkinsfile injecting Jenkins BUILD_ID

I wish to see

version := \"1.0.25\"

Answer 1

I've used dirty little wrapper function to implement Stefan Crain's answer above:

def appendFile(String fileName, String line) {
    def current = ""
    if (fileExists(fileName)) {
        current = readFile fileName
    }
    writeFile file: fileName, text: current + "\n" + line
}

I really don't like it, but it does the trick and it gets round escaping quotes via slashy strings,e.g.:

def tempFile = '/tmp/temp.txt'
writeFile file: tempFile, text: "worthless line 1\n"
// now append the string 'version="1.2.3"  # added by appendFile\n' to tempFile
appendFile(tempFile,/version="1.2.3" # added by appendFile/ + "\n")