What is the difference between systemd's 'oneshot' and 'simple' service types?
SystemdSystemd Problem Overview
What is the difference between systemd
service Type
oneshot
and simple
?
This [link][1] states to use simple
instead of oneshot
for timers. I am not able to understand it correctly.
[1]: https://jason.the-graham.com/2013/03/06/how-to-use-systemd-timers/ "link"
Systemd Solutions
Solution 1 - Systemd
The Type=oneshot
service unit:
-
blocks on a start operation until the first process exits, and its state will be reported as "activating";
-
once the first process exits, transitions from "activating" straight to "inactive", unless
RemainAfterExit=true
is set (in which case it becomes "active" with no processes!); -
may have any number (0 or more) of
ExecStart=
directives which will be executed sequentially (waiting for each started process to exit before starting the next one); -
may leave out
ExecStart=
but haveExecStop=
(useful together withRemainAfterExit=true
for arranging things to run on system shutdown).
The Type=simple
service unit:
-
does not block on a start operation (i. e. becomes "active" immediately after forking off the first process, even if it is still initializing!);
-
once the first process exits, transitions from "active" to "inactive" (there is no
RemainAfterExit=
option); -
is generally discouraged because there is no way to distinguish situations like "exited on start because of a configuration error" from "crashed after 500ms of runtime" and suchlike.
Both Type=oneshot
and Type=simple
units:
- ignore any children of the first process, so do not use these modes with forking processes (note: you may use
Type=oneshot
withKillMode=none
, but only do this if you know what you are doing).
Solution 2 - Systemd
From systemd's point of view, Type=simple
is kind of fire and forget. Systemd just forks a process defined in ExecStart=
and goes on its way, even if the process fails to start.