Jenkins/vars/git.groovy

def withHTTPCredentials(String credentialsId, Closure fn) {
     withCredentials([
        usernamePassword(
            credentialsId: credentialsId, 
            usernameVariable: 'GIT_USERNAME', 
            passwordVariable: 'GIT_PASSWORD'
        )
    ]) {
        def randomUUID = UUID.randomUUID().toString()
        def tmpAskPassScript = pwd(tmp:true) + "/${randomUUID}"
        try {
            writeFile(
                file: tmpAskPassScript,
                text: '''
                #!/bin/sh
                case "$1" in
                Username*) echo $GIT_USERNAME ;;
                Password*) echo $GIT_PASSWORD ;;
                esac
                '''
            )
            sh(script: "chmod +x '${tmpAskPassScript}'")
            withEnv(["GIT_ASKPASS=${tmpAskPassScript}"]) {
                fn.call()
            }
        } finally {
            sh(script: "rm -f '${tmpAskPassScript}'")
        }
    }
}

def withSSHCredentials(String credentialsId, Closure fn) {
    def randomUUID = UUID.randomUUID().toString()
    withCredentials([
        sshUserPrivateKey(
            credentialsId: credentialsId,
            keyFileVariable: 'GIT_SSH_IDENTITY_FILE',
        )
    ]) {
        withEnv(['GIT_SSH_VARIANT=ssh', 'GIT_SSH_COMMAND=ssh -i $GIT_SSH_IDENTITY_FILE -o IdentitiesOnly=yes -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null']) {
            fn.call()
        }
    }
}