Modify systemd unit file without altering upstream unit file

You have two options:

• Copy the unit file from /lib/systemd/system/ to /etc/systemd/system/.
  And then make your modifications in /etc/systemd/system/pimd.service to completely override the unit file(s) supplied by the package maintainer.

  The command systemctl edit --full <service-name> automates this for you.

• You can alter or add specific configuration settings for a unit, without having to modify unit files by creating .conf files in a drop-in directory /etc/systemd/system/<unit-name>.<unit-type>.d/
  i.e. create a /etc/systemd/system/pimd.service.d/restart.conf

  The command systemctl edit <service-name> performs these steps for you.

See man systemd.unit

The RHEL documentation recommends two ways:

1. Extend the default unit file by creating a configuration directory and file under /etc/systemd/system/[name-goes-here].service.d/config_name.conf

In this case the file would need to contain something like this:

[Service]
Restart=always

This is what systemctl edit [name-goes-here] does, it creates that directory and override.conf within it.

2. Create a copy of the original unit file /usr/lib/systemd/system/ in /etc/systemd/system/ and make changes there.

I would try option one but they are both viable options. Either way remember to run systemctl daemon-reload after making the changes.

RHEL documentation on Creating and Modifying systemd Unit Files

Consider using a script to read the upstream configuration, modify it, and spit it out to drop-in file.

For example, I use Chef and here's a piece of ruby (library) that parses marathon systemd unit file to get original ExecStart from it

require 'inifile'

module Dcos
  def get_execstart_from_unit_file
    marathon_systemd_unit_file = 
    IniFile.load('/etc/systemd/system/dcos-marathon.service')
    return marathon_systemd_unit_file['Service']['ExecStart']
  end
end

Then in the recipe, I create the drop-in file to append an option to ExecStart

chef_gem 'inifile'

exec_start_orig = get_execstart_from_unit_file

systemd_service_drop_in 'dcos-marathon' do
  override 'dcos-marathon.service'
  precursor 'Service' => { 'ExecStart' => nil }
  service do
    exec_start exec_start_orig + ' --env_vars_prefix "DCOS_MARATHON_"'
  end
end