Systemd service B to start after another service A only if Service A exists

Question

I have two systemd services A and B . I want B to get executed after A but only if A exists else just execute B .

Answer 1

You need to add in your B.service file:

After=A.service

Or you can add to A.service:

Before=B.service

From SystemD documentation:

Before=, After=

These two settings expect a space-separated list of unit names. They configure ordering dependencies between units. If a unit foo.service contains a setting Before=bar.service and both units are being started, bar.service's start-up is delayed until foo.service has finished starting up. Note that this setting is independent of and orthogonal to the requirement dependencies as configured by Requires=, Wants= or BindsTo=.

Documentation source: https://www.freedesktop.org/software/systemd/man/systemd.unit.html

Answer 2

SerivceB needs to include Wants= directive for ServiceA. Unit section of ServiceB.service file would look something like this:

[Unit]
Description=ServiceB description
Wants=ServiceA.service

From https://www.freedesktop.org/software/systemd/man/systemd.unit.html

Wants= A weaker version of Requires=. Units listed in this option will be started if the configuring unit is. However, if the listed units fail to start or cannot be added to the transaction, this has no impact on the validity of the transaction as a whole. This is the recommended way to hook start-up of one unit to the start-up of another unit.

Note that dependencies of this type may also be configured outside of the unit configuration file by adding symlinks to a .wants/ directory accompanying the unit file. For details, see above.