One of the great ways that systemd allows you to modify a unit definition, without necessarily having to change the original unit file definition, is with drop-ins. Drop-ins are super powerful, and it is typical to create them for an individual unit. Let’s take this service unit for example:
svc1.service
1
2
3
4
5
[Unit]
Description=Service 1
[Service]
ExecStart=/bin/echo hello world
The easiest way to create a drop-in for svc1.service is by running systemctl edit svc1.service. In my example, I add this as a drop-in when prompted by the editor (Note: specify the EDITOR variable to change the default editor. For instance, I like to edit text with Vim so I have to do sudo EDITOR=vim systemctl edit svc1.service):
1
2
[Service]
ExecStartPre=/bin/echo running this first
But, once you exit the editor how can you validate that you actually created a drop-in? Run systemctl cat on the unit:
1
2
3
4
5
6
7
8
9
10
11
$ systemctl cat svc1.service
# /etc/systemd/system/svc1.service
[Unit]
Description=Service 1
[Service]
ExecStart=/bin/echo hello world
# /etc/systemd/system/svc1.service.d/override.conf
[Service]
ExecStartPre=/bin/echo running this first
This is a really helpful command. Each of the commented lines (starting with #) show the path of the part of unit that is applied. You can see that our drop-in is below the original unit file with the ExecStartPre directive.
This is great if you just want to add a drop-in for a single unit. But what if you want to add a drop-in for all service units?
This can be accomplished with a top-level drop-in, which is created in a unit dir with the name of <service_type>.d. So if I wanted to create a top-level drop-in for all service units, I would create a dir /etc/systemd/system/service.d and then create my drop-in file there:
10-execstartpre.conf
1
2
[Service]
ExecStartPre=/bin/echo top-level exec start pre
Let’s see what svc1.service looks like now:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
$ systemctl cat svc1.service
# /etc/systemd/system/svc1.service
[Unit]
Description=Service 1
[Service]
ExecStart=/bin/echo hello world
# /etc/systemd/system/service.d/10-execstartpre.conf
[Service]
ExecStartPre=/bin/echo top-level exec start pre
# /etc/systemd/system/svc1.service.d/override.conf
[Service]
ExecStartPre=/bin/echo running this first
You can see now that the top-level drop-in is included in the effective unit definition. Now let’s create a new service:
svc2.service
1
2
3
4
5
[Unit]
Description=Service 2
[Service]
ExecStart=/bin/echo hello world
And without doing anything more for this new service unit, we can see that our top-level drop-in applies:
1
2
3
4
5
6
7
8
9
10
11
$ systemctl cat svc2.service
# /etc/systemd/system/svc2.service
[Unit]
Description=Service 2
[Service]
ExecStart=/bin/echo hello world
# /etc/systemd/system/service.d/10-execstartpre.conf
[Service]
ExecStartPre=/bin/echo top-level exec start pre
Another thing to note, though, is that unit drop-ins can override each other if they have the same name. Let’s create a drop-in for svc2.service that has the same name (10-execstartpre.conf) as the top-level drop-in:
/etc/systemd/system/svc2.service.d/10-execstartpre.conf
1
2
[Service]
ExecStartPre=/bin/echo svc2 exec start pre
Because the drop-in file has the same name, it will override previous drop-ins, including the top-level drop-in:
1
2
3
4
5
6
7
8
9
10
11
$ systemctl cat svc2.service
# /etc/systemd/system/svc2.service
[Unit]
Description=Service 2
[Service]
ExecStart=/bin/echo hello world
# /etc/systemd/system/svc2.service.d/10-execstartpre.conf
[Service]
ExecStartPre=/bin/echo svc2 exec start pre
Hopefully this blog post has showed how to define top-level drop-ins to add or modify configuration for all units of a particular type!