How to append a text to a file in jenkinsfile

The pipeline built in writeFile is also very useful here but requires a read+write process to append to a file.

def readContent = readFile 'build.sbt'writeFile file: 'build.sbt', text: readContent+"\r\nversion := 1.0.${env.BUILD_ID}"

env.BUILD_ID is a groovy variable, not a shell variable. Since you used single-quotes (') groovy will not substitute the variables in your string and the shell doesn't know about ${env.BUILD_ID}. You need to either use double-quotes " and let groovy do the substitution

sh "echo version := 1.0.${env.BUILD_ID} >> build.sbt"

or use the variable the shell knows

sh 'echo version := 1.0.$BUILD_ID >> build.sbt'

and since you need the version surrounded with doublequotes, you'd need something like this:

sh "echo version := \\\"1.0.${env.BUILD_ID}\\\" >> build.sbt"

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 tempFileappendFile(tempFile,/version="1.2.3" # added by appendFile/ + "\n")