systemctl start/stop/reload/restart/reload-or-restart/enable/disable/cat/list-dependencies app[.service]
systemctl is-enabled/is-active/is-failed app.service
enable copies a symlink from
/etc/systemd/system/*.service to systemd's direcotry
/usr/lib/systemd/system/: units provided by installed packages
/etc/systemd/system/: units installed by the system administrator
In that case add Requires=B and After=B to the [Unit] section of A. If the dependency is optional, add Wants=B and After=B instead. Note that Wants= and Requires= do not imply After=, meaning that if After= is not specified, the two units will be started in parallel.
Type: simple/ forking
systemctl edit will edit an override file, systemctl edit --all will edit the file.
if you service depends on another service, use requires= + after= or wants= + after=