I want the default user, ubuntu to be able to run a specific service without being prompted for a password.

Specifically systemctl restart unicorn_my_app.service.

Have followed the instructions here to add user ubuntu to a newly created group, LimitedAdmins, which is confirmed with:

$ getent group LimitedAdmins
LimitedAdmins:x:1001:ubuntu

Created a new file, limitedadmins (using sudo vim) in the /etc/sudoers.d directory containing the following text:

%LimitedAdmins ALL=NOPASSWD: /etc/init.d/unicorn_ofn_america restart, /etc/init.d/unicorn_ofn_america start

I have also tried:

%LimitedAdmins ALL=NOPASSWD: /bin/systemctl/unicorn_ofn_america restart, /bin/systemctl/unicorn_ofn_america start

(And /bin/systemd)

Content of /etc/sudoers/ is the default as confirmed with sudo visudo (or sudo cat /etc/sudoers):

#
# This file MUST be edited with the 'visudo' command as root.
#
# Please consider adding local content in /etc/sudoers.d/ instead of
# directly modifying this file.
#
# See the man page for details on how to write a sudoers file.
#
Defaults    env_reset
Defaults    mail_badpass
Defaults    secure_path="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"

# Host alias specification

# User alias specification

# Cmnd alias specification

# User privilege specification
root    ALL=(ALL:ALL) ALL

# Members of the admin group may gain root privileges
%admin ALL=(ALL) ALL

# Allow members of group sudo to execute any command
%sudo   ALL=(ALL:ALL) ALL

# See sudoers(5) for more information on "#include" directives:

#includedir /etc/sudoers.d

(The hash sign in #includedir is not a comment, but part of the #include directive syntax).

However there's still a password prompt following running systemctl restart unicorn_my_app.service

Service is there in the init.d directory:

$ ls -l /etc/init.d | grep unicorn
-rwxr--r-- 1 ubuntu ubuntu 1874 Oct 29 06:47 unicorn_my_app

Tried chmodding 755 on the app, but don't think that should make a difference, since ubuntu owns it anyway.

Even tried rebooting the system with no difference. Am I missing a step, like a restart/reload)? Configuring something wrong?

I should also mention that I used vim to create the new file within /etc/sudoers.d, as it seems that the visudo command is only for editing /etc/sudoers.

UPDATE

Looks like you can edit additional sudo config files with visudo. See below.

The sudoers file is fairly flexible, and with that comes complexity. What you want here is to permit access to the command /bin/systemctl, with specific parameters:

%LimitedAdmins ALL=NOPASSWD: /bin/systemctl restart unicorn_my_app.service

Basically you just take the exact command line that you would type, hard-code the path name for safety's sake, and put that into your sudoers file (or /etc/sudoers.d). And note that 'start' and 'restart' are completely different as far as sudo is concerned; permitting one won't grant access to the other.

I too thought visudo only worked on /etc/sudoers but happily, I was mistaken.

visudo can be used to modify existing files in /etc/sudoers.d or create new ones. The -f parameter allows this. If the command is invoked like this:

visudo -f /etc/sudoers.d/permissions_for_subset_of_users

you can use visudo's validation capabilities to allow safe editing of sudoers.

Also, if you are using some kind of CI/CD or configuration management, you can use visudo -cf <name_of_file> to run a validation of the configuration. (our lead sysadmin provided that second piece of knowledge).

Reference: https://www.sudo.ws/man/1.8.13/visudo.man.html